Accuracy and reproducibility of a new contrast clearance method for the determination of glomerular filtration rate

Measured Glomerular Filtration Rate: The Query for a Workable Golden Standard Technique

Inulin clearance has, for a long time, been considered as the reference method to determine measured glomerular filtration rates (mGFRs). However, given the known limitations of the standard marker, serum creatinine, and of inulin itself, and the frequent need for accurate GFR estimations, several other non-radioactive (iohexol and iothalamate) and radioactive (51Cr-EDTA, 99mTc-DTPA, 125I iothalamate) exogenous mGFR filtration markers are nowadays considered the most accurate options to evaluate GFR. The availability of 51Cr-EDTA is limited, and all methods using radioactive tracers necessitate specific safety precautions. Serum- or plasma-based certified reference materials for iohexol and iothalamate and evidence-based protocols to accurately and robustly measure GFR (plasma vs. urinary clearance, single-sample vs. multiple-sample strategy, effect of sampling time delay) are lacking. This leads to substantial variation in reported mGFR results across studies and questions the scientific reliability of the alternative mGFR methods as the gold standard to evaluate kidney function. On top of the scientific discussion, regulatory issues are further narrowing the clinical use of mGFR methods. Therefore, this review is a call for standardization of mGFR in terms of three aspects: the marker, the analytical method to assess concentrations of that marker, and the procedure to determine GFR in practice. Moreover, there is also a need for an endogenous filtration marker or a panel of filtration markers from a single blood draw that would allow estimation of GFR as accurately as mGFR, and without the need for application of anthropometric, clinical, and demographic characteristics.

Urinary Excretion of Iohexol as a Permeability Marker in a Mouse Model of Intestinal Inflammation: Time Course, Performance and Welfare Considerations

Simple Summary

In mammals, different diseases are associated with intestinal changes that may cause an increase in gut permeability. Intestinal permeability tests allow the evaluation of intestinal damage in humans, veterinary patients and laboratory animal models. When used in mouse models, these tests require that animals are singly housed in metabolic cages with a wire-grid floor to collect urine samples. This raises welfare concerns. Iohexol meets several criteria for an ideal intestinal permeability marker and has recently been used in several species. Here, we examined the performance of an intestinal permeability test using iohexol administered by mouth and following excretion over 24 h in urine. As a model, we chose immunodeficient mice with intestinal inflammation induced by adoptive transfer of effector/memory T cells. We collected urine samples at seven time points to profile the urinary excretion of iohexol, in addition to intestinal tissue samples for histological assessment. We conclude that a 6 h cumulative urine sample may be sufficient to evaluate small intestinal permeability in this mouse model and increased urinary excretion of iohexol is correlated with increased severity of duodenitis. The welfare of mice housed in metabolic cages could be improved by reducing the cage periods from 24 to 6 h.

Abstract

Intestinal permeability (IP) tests are used to assess intestinal damage in patients and research models. The probe iohexol has shown advantages compared to ⁵¹Cr-EDTA or absorbable/nonabsorbable sugars. During IP tests, animals are housed in metabolic cages (MCs) to collect urine. We examined the performance of an iohexol IP test in mice. Rag1-/- (C57BL/6) mice of both sexes were divided into controls or treatment groups, the latter receiving injections of effector/memory T cells to induce intestinal inflammation. After two, four and five weeks (W), a single dose of iohexol was orally administered. Urine was collected seven times over 24 h in MCs. Iohexol concentration was measured by ELISA. Intestinal histological damage was scored in duodenal sections. In control and treated mice of both sexes, urinary excretion of iohexol peaked at 4 h. From W2 to W4/W5, urinary iohexol excretion increased in treated mice of both sexes, consistent with development of duodenitis in this model. Positive correlations were observed between the urinary excretion of iohexol in W4/W5 and the histological severity of duodenitis in treated male mice. We conclude that a 6 h cumulative urine sample appears sufficient to evaluate small IP to iohexol in this mouse model, improving animal welfare by reducing cage periods.

Estimation of the glomerular filtration rate in older individuals with serum creatinine-based equations: A systematic comparison between CKD-EPI and BIS1

AIM

CKD-EPIcr (Chronic Kidney Disease Epidemiology Collaboration creatinine) and BIS1 (Berlin Initiative Study) are two serum creatinine-based formulae for the estimation of glomerular filtration rate (GFR). However, their comparative accuracy in older people has not been well established. Our aim was to conduct a systematic comparative study of the accuracy of estimation of GFR in older people with these two formulae.

METHODS

We conducted a systematic search in Pubmed, EMBASE and Central databases on the validity of the CKD-EPIcr and BIS1 formulae in people aged 60 or more years. The search ranged from 2009 and 2012 for CKD-EPIcr and BIS1, respectively, until May 2017. The validity criterion for comparing the formulae was to have a P30 accuracy level equal to or greater than 80%.

RESULTS

Of 1295 identified studies, 16 met our inclusion criteria. Out of 16 studies reporting the accuracy of the CKD-EPIcr formula, only 5 (31.3%) had P30 scores ≥80% (mean P30 was 77.1 ± 7.711 range 55.5-91.7), and out of 9 studies on accuracy using the BIS1 formula, 6 (66.7%) were ≥80% (mean P30 was 83.88 ± 9.37, range 67.0-95.8).

CONCLUSION

Our results suggest that for the estimation of the GFR in older people, BIS1 formula may be more accurate than CKD-EPIcr.

The diagnostic value of serum creatinine and cystatin c in evaluating glomerular filtration rate in patients with chronic kidney disease: a systematic literature review and meta-analysis

BACKGROUND

Serum biomarkers, such as serum creatinine (SCr) and serum cystatin C (SCysC), have been widely used to evaluate renal function in patients who have chronic kidney disease (CKD).

OBJECTIVE

This article aims to assess the value of determining SCr and SCysC levels in patients that have long-term kidney disease. Approaches: MEDLINE, EmBase, the Cochrane Library and other databases were searched using both MeSH terms and text words to collect research that assessed the diagnostic value of using SCr and SCysC to evaluate Glomerular Filtration Rate (GFR) in patients with CKD. Data were converted into fourfold tables. Summary Receiver Operating Characteristic Curves and meta-analyses were accomplished via Meta-Disc version 1.4.

RESULTS

In total, 21 relevant articles involving 3112 study subjects were included in our review. Results showed that the collective sensitivity for SCr and SCysC was 0.77 (95% CI: 0.69-0.84) and 0.87 (95% CI: 0.82-0.91), respectively. The pooled specificity for SCr and SCysC was 0.91 (95% CI: 0.86-0.94) and 0.87 (95% CI: 0.82-0.91), respectively. Subgroup analyses demonstrated that when GFR cut-off values are set to 60 (ml/min/1.73 m²), the pooled sensitivity is 0.94 (95% CI: 0.90-0.96) for SCysC and 0.75 (95% CI: 0.68-0.82) for SCr.

CONCLUSIONS

The diagnostical accuracy for impaired kidney function favors SCysC. Confidence intervals for the pooled sensitivity and specificity for SCr and SCysC overlap. However, SCysC is more sensitive for estimating GFR than SCr when GFR cut-off values are set to 60 (ml/min/1.73 m²).

Experimental Studies Comparing Iohexol and 51Cr-Edta for Glomerular Filtration Rate Measurements

The total plasma clearance as well as the renal clearance of iohexol were evaluated for determination of the glomerular filtration rate (GFR) in 16 anesthetized pigs. The iohexol levels in urine and plasma were measured by X-ray fluorescence. The total plasma clearance of 1 and 4 ml/kg b.w. of iohexol 300 mg I/ml was compared to the simultaneously as well as nonsimultaneously calculated total plasma clearance of 51Cr-EDTA. The total plasma clearance of 51Cr-EDTA and of iohexol turned out to be equal and independent of the injected dose of iohexol. The injected dose did not have any effect on the renal clearance of iohexol either. It is concluded that iohexol and 51Cr-EDTA are similar as marker substances for determination of the GFR.

Determination of the Relative Glomerular Filtration Rate of Each Kidney in Man

Iohexol and 99mTc-DTPA were used in 43 patients to determine the relative glomerular filtration rate (GFR), i.e., the GFR of each kidney in percent of total GFR. The amount of any GFR marker accumulating in Bowman's space, tubuli and renal pelvis within a few minutes after i.v. injection, before any marker had left the kidney via the ureter, was defined as proportional to the GFR of that kidney. The renal accumulation of iohexol was determined by CT using 10 slices of 8-mm thickness 1 to 4 minutes after injection. The renal accumulation of 99mTc-DTPA was determined with a gamma camera within 2 minutes after injection. The correlation coefficient between the two methods was 0.98. Due to the higher radiation dose from CT than from 99mTc-DTPA injection, relative GFR determination with CT should be performed when there is also a diagnostic need to reveal morphology.

Glomerular Filtration Rate Estimated after Multiple Injections of Contrast Medium during Angiography

In twenty-six patients referred for angiography, clearance of contrast medium was determined with x-ray fluorescence analysis after multiple injections of contrast medium. A formula for correction of the injected amount, which takes into consideration the different times of contrast medium injections, approximating the total injected amount into one injection, was used. A single injection clearance of 51Cr-EDTA was determined at the same time. The results showed a good correlation between the clearance of contrast medium after multiple injections and the 51Cr-EDTA clearance after a single injection (r=0.945). The correlation between contrast medium clearance calculated without correction for the different injection times, and “Cr-EDTA clearance was the same (r=0.946), due to short angiography time and rather low clearance values in our patients. It is concluded that total plasma clearance of contrast medium can easily be estimated after multiple injections. In this way patients with a risk of developing post-angiographic renal failure can be found.

Estimating Kidney Function during Urography

Blood samples were taken from 21 subjects at 2 to 4 hours after simultaneous injection of contrast medium (metrizoate) for urography and 51Cr-EDTA. Clearance calculations were performed using the single injection (single slope) technique. The plasma concentrations of 51Cr-EDTA and contrast medium were measured by gamma counting and X-ray fluorescence analysis, respectively. A good correlation was found between the clearance of 51Cr-EDTA and clearance of contrast medium (r=0.94).

Simultaneous Urography and Determination of Glomerular Filtration Rate

The total plasma clearance of iohexol at urography and 51Cr-EDTA was compared in 31 patients with di- or tetraparesis. A reference 51Cr-EDTA clearance was also performed 24 hours prior to the urography. The GFR was calculated from one, 2 or 4 plasma samples collected 180, 210, 240 and 270 min after the injection. An X-ray fluorescence analyzer was used for the analysis of iohexol in plasma as well as the contrast medium clearance calculations. It was shown that single or multiple sample clearance of iohexol and 51Cr-EDTA were equivalent methods for measurement of the GFR. The GFR was not affected by iohexol in a dose routinely used for urography. It was concluded that the patient comfort is improved if 51Cr-EDTA clearance is replaced by contrast medium clearance in association with urography.

Iohexol plasma clearance for measuring glomerular filtration rate in clinical practice and research: a review. Part 1: How to measure glomerular filtration rate with iohexol?

While there is general agreement on the necessity to measure glomerular filtration rate (GFR) in many clinical situations, there is less agreement on the best method to achieve this purpose. As the gold standard method for GFR determination, urinary (or renal) clearance of inulin, fades into the background due to inconvenience and high cost, a diversity of filtration markers and protocols compete to replace it. In this review, we suggest that iohexol, a non-ionic contrast agent, is most suited to replace inulin as the marker of choice for GFR determination. Iohexol comes very close to fulfilling all requirements for an ideal GFR marker in terms of low extra-renal excretion, low protein binding and in being neither secreted nor reabsorbed by the kidney. In addition, iohexol is virtually non-toxic and carries a low cost. As iohexol is stable in plasma, administration and sample analysis can be separated in both space and time, allowing access to GFR determination across different settings. An external proficiency programme operated by Equalis AB, Sweden, exists for iohexol, facilitating interlaboratory comparison of results. Plasma clearance measurement is the protocol of choice as it combines a reliable GFR determination with convenience for the patient. Single-sample protocols dominate, but multiple-sample protocols may be more accurate in specific situations. In low GFRs one or more late samples should be included to improve accuracy. In patients with large oedema or ascites, urinary clearance protocols should be employed. In conclusion, plasma clearance of iohexol may well be the best candidate for a common GFR determination method.

Glomerular filtration rate (GFR) during and after STEMI: a single-centre, methodological study comparing estimated and measured GFR

OBJECTIVES

To validate the performance of the most commonly used formulas for estimation of glomerular filtration rate (GFR) against measured GFR during the index hospitalisation for ST-elevation myocardial infarction (STEMI).

SETTING

Single centre, methodological study.

PARTICIPANTS

40 patients with percutaneous coronary intervention-treated STEMI were included between November 2011 and February 2013. Patients on dialysis, cardiogenic shock or known allergy to iodine were excluded.

OUTCOME MEASURES

Creatinine and cystatin C were determined at admission and before discharge in 40 patients with STEMI. Clearance of iohexol was measured (mGFR) before discharge. We evaluated and compared the Cockcroft-Gault (CG), the Modification of Diet in Renal Disease (MDRD-IDMS), the Chronic Kidney Disease Epidemiology (CKD-EPI) and the Grubb relative cystatin C (rG-CystC) with GFR regarding correlation, bias, precision and accuracy (P30). Agreement between eGFR and mGFR to discriminate CKD was assessed by Cohen's κ statistics.

RESULTS

MDRD-IDMS and CKD-EPI demonstrated good performance to estimate GFR (correlation 0.78 vs 0.81%, bias -1.3% vs 1.5%, precision 17.9 vs 17.1 mL/min 1.73 m(2) and P30 82.5% vs 82.5% for MDRD-IDMS vs CKD-EPI). CKD was best classified by CKD-EPI (κ 0.83). CG showed the worst performance (correlation 0.73%, bias -1% to 3%, precision 22.5 mL/min 1.73 m(2) and P30 75%). The rG-CystC formula had a marked bias of -17.8% and significantly underestimated mGFR (p=0.03). At arrival, CKD-EPI and rG-CystC had almost perfect agreement in CKD classification (κ=0.87), whereas at discharge agreement was substantially lower (κ=0.59) and showed a significant discrepancy in CKD classification (p=0.02). Median cystatin C concentration increased by 19%.

CONCLUSIONS

In acute STEMI, CKD-EPI showed the best CKD-classification ability followed by MDRD-IDMS, whereas CG performed the worst. STEMI altered the performance of the cystatin C equation during the acute phase, suggesting that other factors might be involved in the rise of cystatin C.

Uric acid lowering to prevent kidney function loss in diabetes: the preventing early renal function loss (PERL) allopurinol study

Diabetic kidney disease causes significant morbidity and mortality among people with type 1 diabetes (T1D). Intensive glucose and blood pressure control have thus far failed to adequately curb this problem and therefore a major need for novel treatment approaches exists. Multiple observations link serum uric acid levels to kidney disease development and progression in diabetes and strongly argue that uric acid lowering should be tested as one such novel intervention. A pilot of such a trial, using allopurinol, is currently being conducted by the Preventing Early Renal Function Loss (PERL) Consortium. Although the PERL trial targets T1D individuals at highest risk of kidney function decline, the use of allopurinol as a renoprotective agent may also be relevant to a larger segment of the population with diabetes. As allopurinol is inexpensive and safe, it could be cost-effective even for relatively low-risk patients, pending the completion of appropriate trials at earlier stages.

Glomerular filtration rate in evaluation of the effect of iodinated contrast media on renal function

OBJECTIVE

The purpose of this study was to use measured glomerular filtration rate (GFR), the reference standard of renal function, to assess the deleterious effect of iodinated contrast media on renal function. Such an effect has been traditionally defined as a greater than 0.5-mg/dL increase in serum creatinine concentration or a 25% or greater increase 24-72 hours after the injection of iodinated contrast medium.

SUBJECTS AND METHODS

This pilot investigation was focused on the consequences of clinically indicated IV injection of iodinated contrast media; intraarterial injection was excluded. One hundred thirteen patients with normal serum creatinine concentrations were enrolled in an approved protocol. At random, as chosen by one of the investigators, patients underwent imaging with one of three monomeric agents (iopamidol 300, iopromide 300, iohexol 300) and one dimeric agent (iodixanol 320). Measured GFR was determined immediately before CT and approximately 3 and 72 hours after the contrast injection for the CT examination. Iodinated contrast medium, a glomerular filtrate with no tubular excretion or reabsorption, was the GFR marker. Measured GFR was determined by x-ray fluorescence analysis with nonisotopic iodinated contrast media.

RESULTS

Monomeric and dimeric contrast agents in diagnostic CT volumes (based on bodyweight and imaging protocol) did not induce a significant change in measured GFR (95% confidence by Wilcoxon test), suggesting that use of the evaluated contrast media will not lead to more than a 12% variation.

CONCLUSION

The three monomeric agents studied and the one dimeric agent were equivalent in terms of lack of a significant effect on measured GFR when administered to patients with a normal GFR.

Routine determination of GFR in renal transplant recipients by HPLC quantification of plasma iohexol concentrations and comparison with estimated GFR

Estimated glomerular filtration rate (eGFR) methods are not sufficiently reliable in renal transplant recipients (RTR) and should be replaced by iohexol plasma clearance measurement. However, this method has poor availability in health centers. The aim of our study was to develop a high-performance liquid chromatography (HPLC) method for plasma iohexol measurement in routine practice and to evaluate its plasma clearance as a reference of GFR. We developed an HPLC method using UV detection. We evaluated sample storage conditions to provide recommendations for routine practice. Then, we compared GFRbased on plasma iohexol clearance (GFR-iohexol) to eGFR using modification of diet in renal disease, Cockcroft and Gault, and CDK-EPIequations in 40 RTR. The method was validated over a concentration range of 15-300 μg/l. Excellent linearity (r > 0.998), inter- and intraday precision (CV < 3.3%), and accuracy (>96.8%) were complied with ICH guidelines. We also demonstrated excellent samples stability (9 days). Although eGFR methods are not references in RTR, we found a correct concordance between eGFR and GFR-iohexol in our population. To conclude, our method is simple, rapid, accurate, and reliable for routine clinical and research use especially in RTR.

Measuring the glomerular filtration rate in different age groups using iohexol, the protocol from the Belgian iohexol study

OBJECTIVES

Measuring the exact glomerular filtration rate (GFR) is difficult. Iohexol can be used instead of inulin or labeled EDTA or DTPA. In recent years, different studies have validated GFR-estimating equations in adults. Validation of these estimations in adolescents and elderly is lacking. With this study, we aim to develop a simplified (only 1-3 blood collections) iohexol protocol to measure the true GFR for patients of all ages and try to develop GFR-estimating equations for adolescents and the elderly.

DESIGN AND SETTING

Participants of different ages will be recruited: 50 adolescent (14-18 years) and 30 adults (20-65 years), 60 elderly (65-80 years) and 60 very elderly (80+ years old) stratified based on their GFR. Biometric data, serum creatinine and cystatin C will be measured. After injecting 5 mL iohexol, 9 blood samples will be taken between 20 and 360 min. First, the GFR will be calculated by using the double exponential decay method and different GFRs based on 1-3 blood samples, which will be compared with the GFR of the abovementioned 9 samples. Second, the GFR will be calculated by using new and existing equations and compared to the true GFR.

DISCUSSION

The availability of a reliable GFR measurement is important in situations such as screening patients for kidney donation or when taking potentially nephrotoxic treatments. This study will allow us to develop a simplified protocol for measuring the true GFR in all ages and will allow us to validate existing equations and develop new eGFR equations for adolescents and the elderly.

Diffusion-weighted MRI with parallel imaging technique: apparent diffusion coefficient determination in normal kidneys and in nonmalignant renal diseases

The purpose of the study was to assess the capability and the reliability of apparent diffusion coefficient (ADC) measurements in the evaluation of different benign renal abnormalities. Twenty-five healthy volunteers and 31 patients, divided into seven different groups (A-G) according to pathology, underwent diffusion-weighted magnetic resonance imaging (DW MRI) of the kidneys using 1.5-T system. DW images were obtained in the axial plane with a spin-echo echo planar imaging single-shot sequence with three b values (0, 300, and 600 s/mm²). Before acquisition of DW sequences, we performed in each patient a morphological study of the kidneys. ADC was 2.40±0.20×10⁻³ mm² s⁻¹ in volunteers. A significant difference was found between Groups A (cysts=3.39±0.51×10⁻³ mm² s⁻¹) and B (acute/chronic renal failure=1.38±0.40×10⁻³ mm² s⁻¹) and between Groups A and C (chronic pyelonephritis=1.53±0.21×10⁻³ mm² s⁻¹) (P<.05). An important difference was also observed among Group D (hydronephrosis=4.82±0.35×10⁻³ mm² s⁻¹) and Groups A, B, and C (P<.05), whereas no differences were found between Groups B and C (P>.05). A considerable correlation between glomerular filtration rate and ADC was found (P=.04). In conclusion, significant differences were detected among different patient groups, and this suggests that ADC measurements can be useful in differentiating normal renal parenchyma from most commonly encountered nonmalignant renal lesions.

Methods to evaluate renal function in elderly patients: a systematic literature review

CONTEXT

multiple studies of elderly patients show that the prevalence of chronic renal failure in people aged 65 years and older is dependent on the method used to calculate the glomerular filtration rate. We performed a systematic literature search with research question: What is the best method that could be applicable in clinical practice for evaluating renal function in the elderly? Studies using inulin, Cr-51-EDTA, Tc-DTPA or iohexol assays as the gold standard were included.

METHODS

we searched the PubMed and EMBASE databases. Articles found were screened first by title and abstract and then by five criteria. Retained articles were scored using an adapted version of QUADAS.

RESULTS

twelve articles had an identified population or subpopulation aged 65 years and older. The studies were heterogeneous with regard to the population investigated and the statistical procedures used to compare the methods and equations with the gold standard. The Cockcroft-Gault (CG) and MDRD equations and the serum cystatin C concentration produced the highest correlations with the gold standard.

CONCLUSIONS

no accurate method to evaluate renal function in the elderly was found. Serum cystatin C concentration and the CG and MDRD formula might be valuable parameters, although there is insufficient evidence.

Determination of split renal function by 3D reconstruction of CT angiograms: a comparison with gamma camera renography

OBJECTIVE

The objective of our study was to examine the correlation between CT-based and radionuclide renography-based measures of split renal function in a healthy population of live potential kidney donors using 3D models generated from CT angiography.

MATERIALS AND METHODS

The records of 173 renal donor candidates who had undergone CT and radionuclide renography between March 1, 2005, and February 28, 2006, were retrospectively evaluated; of those 173 patients, 152 met study inclusion criteria. A blinded investigator using 3D models that were created semiautomatically from the unenhanced, arterial, and excretory phase data made measurements of CT renal volumes and attenuations. The mean renal attenuation and volume were used to calculate the net accumulation of contrast material and split renal function for comparison with radionuclide renography. Split function from CT was calculated in the arterial and excretory phases as well as based on split renal volume and the Patlak method.

RESULTS

All four CT-based methods for the calculation of split renal function showed correlation with no significant difference from radionuclide renography (p > 0.05, Student's t test). Pearson's correlation coefficients varied from 0.36 to 0.63 (p < 0.001 for each). Difference scores revealed that the excretory and renal volume splits had the narrowest range and showed a linear, nonzero relationship to the renography splits. Bland-Altman analysis confirmed that the majority of difference scores between each CT method and the radionuclide renography were within the 95% CI of the differences.

CONCLUSION

Split renal function based on 3D CT models can provide a "one-stop" evaluation of both the anatomic and the functional characteristics of the kidneys of living potential kidney donors. The excretory phase data and the split renal volume data show the best correlation and the smallest difference scores compared with radionuclide renography data.

Evaluation of renal function in transplantation

Kidney dysfunction causes a myriad of adverse influences on posttransplant outcomes necessitating accurate assessment of kidney function for patient management. This evaluation assists in guiding treatment decisions, with the ultimate aim of allaying renal function decline. In clinical practice, renal function is typically estimated from serum creatinine levels, creatinine-based estimation equations or creatinine clearance; however, each of these methods has demonstrated limitations when used in the kidney transplant setting. Equally important is the emerging recognition of the incidence and impact of kidney dysfunction in recipients of nonrenal solid organ transplantation. The performance of commonly used estimation equations and methods for measuring kidney function in renal and liver transplant patients are overviewed here along with their potential roles in clinical transplantation.

Split renal function measured by triphasic helical CT

PURPOSE

To present a method for calculating split renal function solely from routine triphasic helical computed tomography (CT).

SUBJECTS AND METHODS

We retrospectively included 26 adult patients who received renal scintigraphy and triphasic CT within 4 weeks in the years 2003 and 2004. All scans were performed using a standard abdominal protocol. Split renal function was calculated as relative single-kidney glomerular filtration rate (GFR) using a simplified "two-point Patlak plot" technique. As a reference method, split renal function was determined from renal scintigraphy using the standard technique.

RESULTS

Linear correlation between the two methods was r=0.91, split renal function (CT)=0.0266+0.9573 x split renal function (scintigraphy).

CONCLUSION

Split renal function can be measured accurately by minimally extended triphasic CT.

Measurement of single-kidney glomerular filtration rate using a contrast-enhanced dynamic gradient-echo sequence and the Rutland-Patlak plot technique

PURPOSE

To determine the accuracy of single-kidney glomerular filtration rate (GFR) determination using contrast-enhanced dynamic magnetic resonance imaging (MRI) and the Rutland-Patlak plot technique.

MATERIALS AND METHODS

Twenty-eight adult patients were included. As reference method, the GFR was measured by plasma clearance using a small bolus injection of iopromide. A three-dimensional gradient-echo (GRE) sequence with a flip angle of 50 degrees was used for MRI; this showed a good linear relationship between gadolinium (Gd)-DTPA concentration and signal change when measured up to a Gd-DTPA concentration of 10 mmol/liter. A slab containing both kidneys and the abdominal aorta was measured 30 times in approximately 3.5 minutes. During this measurement, 15 mL of Gd-DTPA, 0.5 mol/liter diluted to a volume of 60 mL, was injected over 60 seconds. A Rutland-Patlak plot was calculated from the signal changes in the aorta and the renal parenchyma. Single-kidney GFR was calculated for different time windows from the Rutland-Patlak plot slope.

RESULTS

The best correlation compared to the reference method was found with the GFR calculated from the slope of the Rutland-Patlak plot 40-110 seconds postaortic rise. Pearson's correlation coefficient was r = 0.86, SD was 14.8 mL/minute. In many of the patients, a decrease of the renal signal was observed in the excretory phase, which was probably caused by very high Gd-DTPA concentrations in the collecting tubules.

CONCLUSION

Single-kidney GFR can be calculated from dynamic contrast-enhanced MRI. We found a promising correlation of global GFR calculated by MRI compared to the reference method. In any future study, the amount of Gd-DTPA should by reduced to avoid artificial signal drop in the excretory phase induced by the T2* effect.

Glomerular filtration rate measured by using triphasic helical CT with a two-point Patlak plot technique

PURPOSE

To determine the accuracy of the two-point Patlak plot in the calculation of glomerular filtration rate (GFR).

MATERIALS AND METHODS

Fifty patients without acute renal disorder were included. GFR was calculated by using a two-point Patlak plot technique. The computed tomography (CT) protocol consisted of a plain examination followed by two contrast material-enhanced examinations in the arterial and portovenous phase. Each examination included the entire kidneys and was performed after injection of 120 mL iopromide and 300 mg of iodine per milliliter given per 75 kg of body weight. All examinations were performed with a standard abdominal protocol. Section thickness was 4 x 2.5 mm, and table advance was 12.5 mm. Bolus triggering commenced 10 seconds after the start of contrast medium injection. Twelve dynamic scans were obtained with reduced tube current every 3 seconds to obtain sufficient arterial input function data. Correction for hematocrit level was made by using the unenhanced attenuation of the aorta. As a reference method, plasma clearance of the contrast medium injected for CT was calculated from three iodine plasma concentration measurements obtained 3, 4, and 5 hours after injection. Linear correlation was performed.

RESULTS

GFR was calculated from CT data in 48 patients. Two patients were excluded because of breathing errors. Mean GFR was 80 mL/min (range, 17-153 mL/min) as measured with iopromide plasma clearance and 82 mL/min (range, 28-148 mL/min) as measured with CT. Linear correlation between the two methods was r = 0.889; GFR calculated with the two-point Patlak plot was equal to 15 plus 0.83 times GFR (plasma clearance). The mean difference between GFRs as determined with the two methods was -1.2 mL/min (95% CI: -27.1, 24.6).

CONCLUSION

Total GFR can be measured accurately with minimally extended triphasic CT in patients without acute renal disorder by using a two-point Patlak plot technique.

Measurement of glomerular filtration rate by low-dose iopromide plasma clearance

PURPOSE

Glomerular filtration rate (GFR) can be measured by iopromide plasma clearance. As an injection of 120 ml nonionic contrast medium is expensive and especially in patients with nephropathy potentially nephrotoxic, we investigated whether iopromide plasma clearance could be measured using a tenth of that dose as 'low-dose' clearance.

MATERIAL AND METHODS

Fifty adult patients scheduled for CT were recruited. Iopromide 300 mg I/ml was used for GFR measurement. Prior to CT, low-dose clearance was measured by injecting 12 ml iopromide per 75 kg b.w. At 3, 4 and 5 h after injection, plasma samples were obtained and the iodine concentration was measured by X-ray fluorescence analysis. Immediately after the last blood sample, CT was again performed following injection of 120 ml iopromide per 75 kg b.w. A further 3 plasma samples were then obtained 3, 4, and 5 h after CT and used for the determination of high-dose clearance.

RESULTS

Low-dose clearance ranged from 20 ml/min to 141 ml/min (mean 78.3 ml/min). High-dose and low-dose clearance correlated excellently, with clearance (high-dose) = 1.4 + 0.994 clearance (low-dose); the correlation coefficient was r = 0.944, the standard deviation SDxy= 9.3 ml/min.

CONCLUSION

Plasma clearance of iopromide is dose-independent on use of iodine amounts of 3.6 g and 36 g I/75 kg b.w. The GFR can be determined by high-dose and low-dose iopromide plasma clearance with identical accuracy.

Glomerular filtration rate as a putative 'surrogate end-point' for renal transplant clinical trials in children

Only with prospective randomized controlled trials is it possible to evaluate the several immunosuppressive regimens available to renal allograft recipients. Commonly used surrogate markers of clinical outcome, such as patient and graft survival, are constantly improving. Current immunosuppressive protocols have improved 1-yr graft survival to over 90%. The small differences in graft survival among the various immunosuppressive regimes require large patient cohorts in order to establish statistical significance. Such studies are often difficult to conduct in a timely manner, particularly in children. This necessitates the search for better surrogate markers sensitive enough to detect differences in smaller cohorts and in a shorter period of time. While the degree of fibrosis in transplant biopsies might well predict long-term graft survival, protocol biopsies are expensive, invasive, and unpopular among clinicians. In native kidneys, glomerular filtration rate (GFR) closely correlates with disease progression and interstitial fibrosis and appears to be well positioned as a less invasive surrogate marker for long-term outcome. Nonetheless, the ideal marker for GFR remains obscure. Serum creatinine has several major drawbacks, making it a poor predictor of GFR. This review discusses the several methods used to estimate or measure GFR with emphasis on 125I-iothalamate clearance and serum cystatin C (cys-C). Of all the serum markers, cys-C is the most reliable and the most promising. However, cys-C and other endogenous markers cannot replace the diagnostic sensitivity and reliability of radiolabeled markers of GFR such as 125I-iothalamate in renal transplant clinical trials. Unfortunately, clearance of most radiolabeled markers of GFR including 125I-iothalamate remain costly and time consuming.

Measurement of renal function in pre-ESRD patients

The measurement of renal function in pre-dialysis patients is important in order to determine the appropriate time to begin renal replacement therapy, to forecast the start, and to compare, in groups of patients, the efficiency of different treatments that limit renal disease progression. The most reliable methods, such as inulin clearance or measurement by radioisotopes, are too awkward for the usual clinical follow-up of patients. Although much simpler and almost as reliable, the use of iohexol radiologic contrast does not allow the frequent monitoring of the patient either. The determinations of the plasmatic creatinine and its clearance or the estimate of the glomerular filtration rate by means of equations derived from the creatinine are the methods most often used in order to measure renal function, although not without problems in pre-dialysis. In order to try to overcome such problems, more precise equations and procedures, including the measurement of averaged urea-creatinine clearance or creatinine clearance with cimetidine, have been designed that better estimate the glomerular filtration rate. However, none of these methods is totally reliable in pre-dialysis. A new endogen marker, cystatin C, has advantages over creatinine, though more studies are needed in pre-dialysis in order to ascertain its use. The initial proposal of the National Kidney Foundation's Kidney Disease Outcome Quality Initiative (DOQI) guidelines to use weekly Kt/V and nutritional parameters to determine the time for starting renal replacement therapy has widened the prospects of the debate on the measurement of renal function in pre-dialysis, but further work is required to define their role in pre-dialysis patients' follow-up.

Contrast media clearance in a single kidney measured on multiphasic helical CT: results in 50 patients without acute renal disorder

OBJECTIVE

Single kidney contrast media clearance was measured using multiphasic CT in patients without acute renal disorder. The aim of this study was to answer two questions. First, how accurate is CT in measuring contrast media clearance compared with plasma clearance? Second, is the accuracy of CT clearance measurements dependent on the timing of CT scans with respect to the contrast media injection?

SUBJECTS AND METHODS

Fifty adult patients without acute renal disorder were included in this study. Each patient underwent CT for clinical indications. The CT protocol consisted of an unenhanced scan and three contrast-enhanced scans 45, 75, and 105 sec after starting an injection of 120 mL of iopromide using an injection rate of 3 mL/sec. All scans included both kidneys. As a reference, plasma clearance of contrast media was determined as a slope clearance by measuring iodine concentration in eight blood specimens up to 8 hr postinjection.

RESULTS

CT clearance was calculated three times for each patient, including early CT clearance, 45-75 sec postinjection; late CT clearance, 75-105 sec postinjection; and overall CT clearance, 45-105 sec postinjection. An overall CT clearance yielded the best correlation with plasma clearance with a correlation coefficient of r = 0.84 and a regression line of y = 7.5 + 0.94x. The mean difference was -3 mL/min (95% confidence interval, -35 to 29 mL/min).

CONCLUSION

CT clearance calculated from data acquired with a minimally modified diagnostic abdominal CT protocol was well correlated with the reference method in determining contrast media clearance for patients without acute renal disorders. The presented method can be used to calculate single kidney contrast media clearance in patients receiving contrast-enhanced abdominal CT for clinical indications.

Plasma clearance of iodine contrast media as a measure of glomerular filtration rate in critically ill patients

OBJECTIVE

The selection of the optimal method for assessing renal function relies on the accuracy of the technique. Plasma clearance of nonradioactive iodine contrast media (i.e., iohexol or iopromide) has been suggested as a reliable alternative to the renal clearance of inulin for estimating glomerular filtration rate (GFR). The accuracy of this method when used with critically ill patients displaying different levels of renal function in an intensive care unit (ICU) has not, until now, been examined.

DESIGN

The accuracy of double- and multiple-point iohexol or iopromide plasma clearances was compared with that of already established techniques for measuring GFR (creatinine clearance, formula clearance by Cockcroft and Gault) and with that of inulin clearance, which is regarded as the gold standard for the measurement of GFR.

PATIENTS

Values were obtained from 31 ICU patients who exhibited a wide range of renal function (serum creatinine: 0.6-6.7 mg/dL).

MEASUREMENTS

Inulin clearance was performed using the constant-infusion technique. Creatinine clearance was determined from 24-hr urine samples. The clearance formula was calculated according to Cockcroft and Gault's formula. Iohexol or iopromide were applied as a single intravenous dose, and blood samples were taken up to 6 hrs after the injection. Iodine concentrations were determined by radiographic fluorescence.

RESULTS

Plasma clearance of iohexol/iopromide measured after the single injection of contrast media and that of the conventional inulin clearance was almost identical (y = 0.971x + 7.65, r2 =.96; n = 31). Two-point clearance of iohexol/iopromide (double sampling technique) was as reliable as the three-point clearance (three-slope-intercept method, y = 0.995x + 0.62, r2 =.999; n = 18). With respect to inulin clearance, GFR measurements determined by creatinine clearance or according to the formula given by Cockcroft and Gault revealed errors that increased proportionally (y = 1.03x, r2 =.88; n = 27; and y = 0.93x, r2 =.62; n = 31, respectively). It could also be shown that the accuracy of GFR measurements involving plasma clearance of iohexol was not greatly affected by the degree of renal insufficiency or the route by which contrast media were applied.

CONCLUSION

These findings indicate that the determination of plasma clearance of iohexol/iopromide is a simple, rapid, and accurate method that can indeed be used for estimating GFR in ICU patients with normal renal function or even different degrees of renal insufficiency.

Chromium-51 ethylenediamine tetraacetic acid glomerular filtration rate: a better predictor than glomerular filtration rate calculated by the Cockcroft-Gault formula for renal involvement in systemic lupus erythematosus patients

OBJECTIVE

To investigate whether the ethylenediamine tetraacetic acid (EDTA) glomerular filtration rate (GFR) is a better indicator of the degree of renal involvement than serum creatinine concentration or creatinine clearance calculated by the Cockroft-Gault formula.

METHODS

We studied prospectively all systemic lupus erythematosus (SLE) patients with normal or borderline serum creatinine concentration (<110 micromol/l) and urinary sediment abnormalities and/or proteinuria in the last 2 yr. EDTA-GFR, serum creatinine concentration, calculated creatinine clearance (Cockroft-Gault formula) and 24-h urine protein were determined at the same time. Renal biopsies were performed in patients with low values of EDTA-GFR or significant proteinuria.

RESULTS

Twenty-three patients were identified, of whom 22 were females. The average age of the patients was 31.6+/-8.2 yr. Biopsies were assigned to WHO classes as follows: class II, 1 patient; class III, 6 patients; class IV, 10 patients; class V, 6 patients. The average serum creatinine concentration, EDTA-GFR and calculated creatinine clearance were 79.8+/-mol/l, 74.5 ml/min and 97 ml/min respectively. EDTA-GFR showed abnormal values (<80 ml/min) in 15 of the 23 patients (65.2%) while calculated creatinine clearance was abnormal (<80 ml/min) in three of the 23 patients (13%) (P<0.001). Using the Pearson correlation test, we did not find any correlation between EDTA-GFR or creatinine clearance values and the sum of activity and chronicity indices.

CONCLUSION

GFR performed by EDTA-GFR correctly predicted renal involvement in SLE patients, whereas GFR calculated by the Cockcroft-Gault formula may have underestimated renal function. Significant numbers of patients with WHO class III, IV or V lupus nephritis may be missed if biochemical creatinine clearance or serum creatinine concentration alone is used to assess renal disease.

Iohexol as a marker of glomerular filtration rate in patients with diabetes: comparison of multiple and simplified sampling protocols

AIMS

To report on the reproducibility of iohexol glomerular filtration rate (GFR) estimation, to compare the plasma clearance of iohexol with that of[51Cr]EDTA and to evaluate the reliability of reduced sampling schedules in estimating GFR in Type 1 and Type 2 diabetes mellitus.

METHODS

Agreement was assessed in 15 Type 1 and 26 Type 2 diabetics with creatinine ranging from 53 to 564 micromol/l.

RESULTS

The regression between multiple-sample iohexol and[51Cr]EDTA clearances was 0.999 in Type 1 and 0.987 in Type 2 diabetes (P < 0.0001 for both). A seven-sample design and the three-sample approach by Brøchner-Mortensen were validated by comparison with the full-sample schedule in 87 patients (51 Type 1, 36 Type 2). Full-sample GFR was 80.3 +/- 43.8, seven-sample 79.5 +/- 43.9 (r = 0.990) and three-sample 79.8 +/- 45.2 ml.min-1.1.73 m-2 (r = 0.972). The coefficients of variation of GFR were 2.7 +/- 1.4% and 3.8 +/- 1.9% for the full-sample and the seven-sample approaches, respectively, and significantly higher for the three-sample design (6.9 +/- 3.4%, P = 0.0001).

CONCLUSIONS

After iohexol injection, the Brøchner-Mortensen schedule does not provide an accurate estimate of GFR. The seven-sample approach gives acceptable errors and allows a good estimate of GFR throughout a wide range of renal function.

Determination of glomerular filtration rate per unit renal volume using computerized tomography: correlation with conventional measures of total and divided renal function

PURPOSE

Previous studies suggest that functional computerized tomography (CT) can measure glomerular filtration rate (GFR) per unit renal volume. We compared this index with conventionally determined GFR measurements.

MATERIALS AND METHODS

A total of 16 men and 8 women 63.3 +/- 14.9 years old (range 31 to 88) were studied using with contrast enhanced CT. A single slice of kidney was scanned sequentially after bolus injection (0.5 to 1.0 ml. per second(-1)) of 20 ml. iopamidol (300 mg. iodine per ml.(-1)). GFR per volume of kidney was calculated using a Patlak graphical analysis, and this index was multiplied by renal volume on CT to yield global GFR (ml. per minute(-1)). Divided function was also calculated. GFR and divided renal function were calculated in all cases from radioisotope renography with 99m diethylenetetraminepentaacetic acid. In 12 subjects in whom 24-hour urine collection was possible GFR was also calculated from creatinine clearance.

RESULTS

A strong correlation was observed between divided renal function, expressed with respect to the right kidney calculated from CT (52.7 +/- 14.8%, range 19.9% to 97.4%) and by radioisotope renography (51.7 +/- 14.6%, range 18.9% to 92.6%, r = 0.97, p <0.0001). A strong correlation (r = 0.92, p <0.0001) was also seen between global GFR determined by CT (80.1 +/- 43.9 ml. per minute(-1), range 38.2 to 197.9) and creatinine clearance (72.4 +/- 47.5, range 14.6 to 168.5), and was stronger than the correlation between the radioisotope and creatinine clearance method (r = 0.67, p = 0.02) in the same patients.

CONCLUSION

Functional CT using nonionic contrast material can measure GFR normalized to renal volume and is an accurate alternative to conventional methods of renal function evaluation.

Evaluation of effective renal plasma flow with I-127 ortho-iodohippurate and I-123 ortho-iodohippurate in rabbits

RATIONALE AND OBJECTIVES

Radiolabeled ortho-iodohippurate is commonly employed for evaluating effective renal plasma flow (ERPF) by means of either in vivo scintigraphy and/or plasma clearance curves. A new method has recently been developed for measuring levels of stable iodine (iodine-127) in biologic samples, based on the detection of x-ray fluorescence photons. In this study, the authors assessed the potential of the new system to evaluate ERPF by using an iodinated contrast medium with adequate glomerular filtration and tubular secretion properties.

MATERIALS AND METHODS

A commercial system was used to evaluate ERPF after intravenous injection of stable I-127 ortho-iodohippurate. The results were compared with the clearance values of I-123 ortho-iodohippurate, considered the reference standard. Seven rabbits under general anesthesia were given intravenous injections of I-123 ortho-iodohippurate and I-127 ortho-iodohippurate. The corresponding plasma curves were evaluated from 4 to 60 minutes to calculate ERPF as the dose/integral of plasma curve.

RESULTS

The initial distribution volumes of I-123 ortho-iodohippurate (149.4 mL/kg +/- 12.1) and I-127 ortho-iodohippurate (148.8 mL/kg +/- 11.8) were virtually superimposable, thus confirming the chemical identity of the two compounds. The plasma clearance values for I-127 ortho-iodohippurate (11.15 mL/min kg(-1) +/- 1.44) were slightly (not significantly) higher than those for I-123 ortho-iodohippurate (10.49 mL/min kg(-1) +/- 1.41), perhaps because of a relative "mass" load effect of the iodinated medium.

CONCLUSION

The results obtained in this study demonstrate the feasibility of the new system for evaluating ERPF, provided that a compound with adequate glomerular filtration and tubular secretion properties is employed.

Functional CT of the kidney

The iodinated contrast agents used for computed tomography (CT) are filtered at the glomerulus and not reabsorbed by the tubules and have pharmacokinetics comparable to inulin. They can thus measure physiological indices such as contrast clearance per unit volume, which is closely related to glomerular filtration rate per unit renal volume of kidney, after due allowance for the difference between blood and plasma clearance. In this review, we show how dynamic CT can be used to measure both regional and global blood clearance of contrast material. A single slice of kidney is scanned sequentially after bolus intravenous (i.v.) injection of contrast material. Next, time-attenuation curves are constructed and contrast clearance per unit volume is calculated using a Patlak graphical analysis. CT determination of renal volume is made and global contrast clearance can be then also calculated. In normal kidneys, clearance/volume averaged 0.49+/-0.11 ml min(-1) ml(-1) (mean +/- S.D.), and these values agreed with literature data obtained using other techniques. A negative correlation between patient's age and clearance/volume was seen. A strong correlation was observed between creatinine whole blood clearance and the global contrast clearance (the product of renal volume determined by CT and contrast clearance/volume). Dynamic CT can provide quantitative renal physiological information on a regional basis non-invasively.

Use of contrast-enhanced computed tomography to measure clearance per unit renal volume: a novel measurement of renal function and fractional vascular volume

The iodinated contrast agents used for computed tomography (CT) have pharmacokinetics similar to inulin and can measure physiological indices, such as clearance per unit renal volume (alpha/V) and fractional vascular volume (fvv). Clinical experience with these techniques is, however, scanty, and the present study explored their potential in subjects with and without renal dysfunction. In a series of subjects, a single slice of kidney was scanned sequentially after the bolus injection of contrast material. Time-attenuation curves were constructed, and alpha/V and fvv were calculated using a Patlak graphic analysis. In the first part of the study, 50 normal kidneys in 35 subjects (aged 21 to 75 years) were studied. In the second stage, alpha/V was compared with glomerular filtration rate (GFR) measurements in 24 patients with diabetes (aged 28 to 84 years) with or without renal dysfunction. In normal kidneys, alpha/V averaged 0.49 +/- 0.11 mL/min/mL and fvv averaged 35% +/- 12%. These values agree with literature data obtained using other techniques. A negative correlation was seen between age and alpha/V (r = 0.66; P < 0.0001), but not fvv. In patients with diabetes, a strong correlation was observed between renal clearance values, calculated from CT and corrected for renal volume, and GFR (r = 0.87; P < 0.0001). Dynamic CT can provide quantitative renal physiological information on a regional basis noninvasively.

Iohexol plasma clearance in determining glomerular filtration rate in diabetic patients

The plasma clearance of iohexol has recently been proposed as a new method for estimating GFR. The iohexol plasma clearance was compared with that of 51Cr-EDTA in 32 diabetic patients (12 IDDM, 20 NIDDM; age 23-70; diabetes duration 1-35 years) with normal to impaired renal function (serum creatinine: 0.8-6.4 mg/dL). Bolus i.v. injection of 51Cr-EDTA (1 muCi/kg) was followed by 5 mL slow i.v. injection of Omnipaque (Nycomed, Oslo, Norway). Samples for radioactivity and iohexol analysis were drawn at 0, 5, 10, 15, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300 min (+360 and 420 min if serum creatinine > 2.0; +1440 min if > 5.0 mg/dL). Iohexol was assayed in duplicate by HPLC throughout a Nova-Pak C18 column (Waters-Millipore, USA). Only the second peak obtained during elution of iohexol (about 4.5 min) was used for calculation. Dilution tests show highly linear regressions for concentrations between 3.25-650 micrograms/mL (r = 0.99). Imprecision of iohexol assay (the whole procedure from deproteinization to chromatography) was: intra-assay 1.4 +/- 1.5%, mlsd (95% CI: 1.0-1.8%); inter-assay 3.0 +/- 2.7% (1.4-4.6%). Iohexol plasma clearance ranged between 12.9 and 150.9 mL/min, while 51Cr-EDTA plasma clearance between 11.9 and 149.8 mL/min with excellent correlation (iohexol = 0.95 51Cr-EDTA + 2.49; r = O.995). Mean CV between the two methods was 1.7% (range 0-4.9%) with a significant negative correlation (r = 0.5 I, p = 0.007) with the GFR levels. Correlation between repeated measurements, performed in eight patients, was excellent (r = O.994, P = 0.0001). In diabetes, GFR measured by plasma clearance of iohexol shows an excellent agreement with plasma clearance of 51Cr-EDTA throughout a wide range of renal function. Iohexol provides an accurate alternative method for measuring GFR.

Measurement of serum iohexol by determination of iodine with inductively coupled plasma–atomic emission spectroscopy

We used inductively coupled plasma–atomic emission spectroscopy to measure serum iodine to determine plasma clearance of iohexol, an iodinated radiographic contrast agent. We determined I at 178.276 nm on the phosphorus 178.287 nm channel of the polychromator by utilization of spectrum shifter offset software, while correcting for P with the sequential P 214.914 nm emission line. Determination of I on the polychromator provided excellent precision in the measurement of serum I, even though the interelement correction of P was done with a sequential P line. Total imprecision (CV) (n = 13) was 16% (at 13.7 mg/L I), 8.6% (28.7 mg/L), 3.6% (59.0 mg/L), 2.6% (120.5 mg/L), 1.7% (237.8 mg/L), 1.2% (478.7 mg/L), and 1.8% (597 mg/L). The linear range was 15 to 600 mg/L. Iohexol added to serum (mg/L I) and recoveries (%) were 15 (91.3%), 30 (95.7%), 60 (98.3%), 120 (100.4%), 240 (99.1%), 480 (99.7%), and 600 (99.5%). Studies on dogs and cats administered a single intravenous injection of iohexol indicated that a dose of 300 mg I/kg body weight was sufficient for measurement of glomerular filtration rate by using a single compartment model for plasma clearance with three samples drawn 3 to 7 h after treatment. With this protocol, correlation coefficients were &gt;0.99 on the β phase of the plasma disappearance curve.

Clearance of iohexol, 51Cr-EDTA and endogenous creatinine for determination of glomerular filtration rate in pigs with reduced renal function: a comparison between different clearance techniques

In order to simplify and/or improve determination of glomerular filtration rate (GFR) the clearances of iohexol, 51Cr-EDTA and endogenous creatinine were simultaneously determined with different techniques in 21 anesthetized landrace pigs. Their GFR had been reduced to about 1/3 or less of normal GFR. After an intravenous bolus of the GFR markers, their plasma concentration curves were followed for 6 hours with 16 plasma samples. A bladder catheter collected urine during six 60-min periods. The plasma clearance was calculated by dividing "dose of marker" with "area under the plasma concentration curve" (AUC) from the time of injection to infinity using a one- (Clprovisional) and a three-compartment (ClAUC-3comp) model. The renal clearance of iohexol and 51Cr-EDTA was calculated by dividing the amount of marker excreted in the urine in a period by AUC in the same period. The AUC was for iohexol and 51Cr-EDTA determined by integrating the total area in the period (Clren adv)-our reference method representing the "true" GFR and for creatinine determined by using the arithmetic mean of the plasma concentration of the marker at the start and at the end of the urine collection period (Clren simple). Renal clearance of creatinine was significantly lower than renal clearance of iohexol (p = 0.0019) and 51Cr-EDTA (p = 0.0001). There were no significant differences between the renal clearances (Clren adv) of iohexol and 51Cr-EDTA or between their plasma clearances (ClAUC-3comp). For iohexol the median overestimation of the "true" GFR with Clprovisional was higher when "early" plasma samples (30-120 min) were used (4.5 ml min-1 10 kg-1) than when late samples (180-360 min) were used (1.9 ml min-1 10 kg-1). Subtraction of the median extrarenal clearance (known from a study of nephrectomized pigs) from the plasma clearances (ClAUC-3comp) of iohexol and 51Cr-EDTA in pigs with reduced renal function decreased the median overestimation of the "true" GFR from 1.9 to 1.0 ml min-1 10 kg-1 with iohexol and from 1.7 to 0.9 ml min-1 10 kg-1 with 51Cr-EDTA. The plasma clearance technique may be improved in pigs with reduced GFR by (i) including a "late" plasma sample in three- and one-compartment models, which tends to increase the AUC; (ii) introducing a correction formula by normalizing the GFR values of the one-compartment model to those of the three-compartment model, thereby compensating for the rapid early changes in plasma concentration of marker after the bolus injection of the marker; or (iii) subtracting a median (or mean) extrarenal clearance of the marker in pigs from the plasma clearance [according to (i) or (ii)]. The plasma clearance one-compartment technique may be improved in pigs with various levels of GFR values by normalizing the plasma clearance values to the renal clearance values, thereby compensating for both the early changes in plasma concentration of marker and the extrarenal clearance of marker.

The effect of endovascular aortic stents placed across the renal arteries

OBJECTIVES

To investigate renal artery patency and renal function after deployment of aortic stents covering the orifices of renal arteries.

DESIGN

Prospective open animal study.

SETTING

Department of Experimental Surgery at a university hospital.

MATERIALS

Twenty-three pigs were used.

METHODS

Ten pigs were observed for 1 h after graft-anchoring aortic stents, Gianturco (5) and Palmaz (5), were placed so that the stents covered the renal arterial orifices. In 13 pigs, Gianturco (6) and Palmaz (7) stents without grafts were placed over the renal arteries and left in situ for 7 days. Renal function and blood flow were measured by renograms, iohexol clearance and ultrasonic blood flow meter and patency was verified by angiograms. The kidneys were microscopically examined for signs of ischaemia and microemboli.

RESULTS

One renal artery covered by a graft-anchoring Gianturco stent occluded. The remaining renal arteries remained patent without any significant decrease in renal blood flow after stent deployment. Normal renal function and histology was maintained.

CONCLUSIONS

Aortic stents placed at the level of the renal arteries do not affect renal blood flow within 1 week in this experimental model. This may prove valuable in endovascular treatment of aortic aneurysms and in other procedures involving stents.

The use of a novel monitoring apparatus and modified Belzer hydroxyethyl starch perfusate for analysis of glomerular filtration during hypothermic perfusion preservation

The present study describes an experimental model for measurement of glomerular filtration during hypothermic perfusion preservation (HPP). To facilitate glomerular filtration during HPP, perfusate oncotic pressure was reduced by lowering the concentration of hydroxyethyl starch. Lewis rats underwent HPP at a mean perfusion pressure of 40-46 mmHg. An isograft model was used to demonstrate that retrieval and preparation for HPP did not impact adversely on renal function. Total cold ischemic time (CIT) consisted of the time from retrieval and preparation for perfusion (2 hr) added to the time of HPP. Tubular function studies demonstrated identical concentrations of Na+ and iohexol in ureteral effluent (UE) compared with circulating perfusate and, as such, established that UE flow represented a direct measure of glomerular filtration. Glomerular filtration rate (GFR) was then monitored during HPP by collecting UE in a beaker housed within a computerized Mettler balance system. GFR evolved in a characteristic, biphasic pattern during HPP, increasing from baseline values to reach a peak level at 4.8+/-0.3 hr of CIT and declining progressively thereafter. At 2.5 hr, time of peak values, 10 hr, 19.5 hr, and 24 hr of CIT, GFR values were 29+/-6 microl/min, 39+/-7 microl/min, 20+/-4 microl/min (n=15; P<0.01), 7+/-2 microl/min (n=14; P<0.001), and 14+/-6 microl/min (n=5), respectively. Intrarenal perfusate flows at the same time intervals were 4180+/-292 microl/min, 4083+/-290 microl/min, 3577+/-294 microl/min (P=NS), 1948+/-393 microl/min (P<0.001), and 2175+/-743 microl/min, respectively. Filtration fraction (FF) initially changed in parallel to glomerular filtration. Thereafter, FF either declined at a disproportionately slow rate compared with GFR (n=8) or increased rapidly (n=7). The data suggest that (1) primary change(s) in glomerular dynamics occur during HPP and (2) declining perfusate flow during the later stages of HPP reflects increasing renal vascular resistance localized at a postglomerular level. The data provide an experimental basis for investigating the clinical utility of monitoring glomerular filtration during HPP.

Clearance of iohexol, chromium-51-ethylenediaminetetraacetic acid, and creatinine for determining the glomerular filtration rate in pigs with normal renal function: comparison of different clearance techniques

RATIONALE AND OBJECTIVES

We wanted to improve determination of the glomerular filtration rate (GFR) with plasma clearance techniques because the alternative-renal clearance techniques-may involve inaccurate urine sampling or risk of urinary tract infection when bladder catheterization becomes necessary. Therefore, we compared the renal and plasma clearances of iohexol and chromium-51-ethylenediaminetetraacetic acid (51Cr-EDTA), as well as endogenous creatinine clearance, in 19 normal pigs using different techniques.

METHODS

After an intravenous bolus injection of the GFR markers, 16 plasma samples were used to plot the marker concentrations versus time for 4.5 hr. Urine was collected during nine 30-min periods. Plasma clearance was calculated by dividing the dose of marker with the area under the plasma concentration curve (AUC) from the time of injection to infinity using one-compartment (ClAUC-slope) and three-compartment (ClAUC-3comp) models. The renal clearance was calculated by dividing the amount of marker excreted in the urine in a period with the AUC in the same period. This AUC was determined by integrating the total area in the period (Clren adv)--our reference method representing the "true" GFR--or by using the arithmetic mean of the plasma concentrations of the marker at the beginning and end of the urine collection period (Clren simple). Creatinine clearance was determined according to Clren simple.

RESULTS

Renal clearances of iohexol and 51Cr-EDTA were significantly higher than creatinine clearance (P = .0002). There was no significant difference between the renal clearances of iohexol and 51Cr-EDTA or between their plasma clearances. The two mathematical methods of calculating the renal clearance of iohexol were highly correlated (rs = .99), as were the two methods of calculating its plasma clearance (rs = .95). Because of the extrarenal clearance of the markers, the plasma clearance methods for iohexol and 51Cr-EDTA always overestimated the true GFR. ClAUC-3comp was the method closest to the true GFR. For iohexol, the median overestimation of the GFR was higher with ClAUC-slope when early plasma samples (30-120 min) after injection of the marker were used (5.5 ml.min-1.10 kg-1) than when late samples (180-270 min) were used (4.0 ml.min-1.10 kg-1). After subtracting the median extrarenal clearances of iohexol and 51Cr-EDTA (previously determined in nephrectomized pigs) from their plasma clearances (ClAUC-3comp), the median overestimation of the true GFR was reduced from 2.0 to 1.1 ml.min-1.10 kg-1 with iohexol and from 2.1 to 1.3 ml.min-1.10 kg-1 with 51Cr-EDTA.

CONCLUSION

GFR determination with plasma clearance techniques can be improved in three- and one-compartment models by taking late plasma samples and by subtracting the extrarenal plasma clearance of the species. One-compartment models can be improved by determining a correction formula in the species for the early parts of the decay curve of the plasma concentration of the marker.

Evaluation of iohexol as a marker for the clinical measurement of glomerular filtration rate in dogs

The contrast medium iohexol can be used as a marker for the measurement of glomerular filtration rate. It has the advantages of sample stability and the availability of a simple, automated (though expensive) analyser. The clearance of iohexol was compared with the clearance of Tc-DTPA in 24 dogs with known or suspected renal impairment. The results were comparable but fractionally lower with iohexol. The correction factors developed for human beings were found to be satisfactory in the dogs, thus validating the software routines used in the automated analyser with the canine samples.

Renal computed tomography with 3-dimensional angiography and simultaneous measurement of plasma contrast clearance reduce the invasiveness and cost of evaluating living renal donor candidates

Renal computed tomography (CT), 3-dimensional CT angiography (3D-CTA), and simultaneous measurement of glomerular filtration rate (GFR) by x-ray fluorescence determination of plasma contrast clearance (PCC) are alternatives to intravenous urography (IVU), renal arteriography (RA), and 24-hr urine creatinine clearance (CrCl) for evaluation of renal structure and function in living renal donor (LRD) candidates. To determine if CT, 3D-CTA, and PCC provide data comparable to IVU, RA, and CrCl, both methods were used to evaluate 23 LRD candidates. Costs were also compared. Conventional RA identified 19 accessory arteries and one case of medial fibroplasia. Each of these anomalous vessels was recognized on 3D-CTA. Venous anatomy was more clearly delineated on 3D-CTA than the venous phase of conventional RA. CT demonstrated 3 benign cysts and a single, small intraparenchymal calcification in 3 renal units. GFRs measured by PCC and CrCl were 91 +/- 4 and 132 +/- 7 ml/min/1.73m2, respectively (r = 0.64, P < 0.05). Total cost for CT/3D-CTA/PCC was 46% less than that of IVU/RA/CrCl and 40% less than RA/CrCl. CT/3D-CTA/PCC provided reliable structural and functional data at substantially less cost, discomfort, and inconvenience to the living renal donor candidate. As such, CT/3D-CTA/PCC is superior to conventional methods for evaluation of the living renal donor candidate.

Determination of residual renal function with iohexol clearance in hemodialysis patients

Residual renal function (RRF) may contribute significantly to the total dialysis prescription. Conventional quantitation of RRF in hemodialysis (HD) patients is measured by urea clearance and requires a 24-hour urine collection which is often difficult to perform and inaccurate. The renal clearance of iohexol was evaluated as an alternative method for RRF assessment (iohexol-derived RRF) in hemodialysis patients. An intravenous bolus of iohexol (12 ml; 300 mg iodine/ml) was administered to 42 hemodialysis patients following routine HD. A single blood sample was obtained approximately 44 hours later (pre-HD) to determine the plasma clearance of iohexol using x-ray fluorescence methods. Total body clearance of iohexol (CTBio) and non-renal clearance of iohexol (CNRio) 2.87 +/- 0.3 ml/min (mean +/SEM) were used to calculate iohexol-derived RRF (CTBio-CNRio). Iohexol-derived RRF determinations were then compared to urea clearance-derived RRF measurements. The RRF contribution to the dialysis prescription was also calculated utilizing iohexol-derived RRF compared to urea-derived RRF. Iohexol-derived RRF did not differ from urea-derived RRF (2.48 +/- 0.3 vs. 2.64 +/- 0.4 ml/min, P = 0.21). The RRF contribution to the weekly dialysis prescription (Kt/V) did not differ when iohexol-derived RRF was compared to urea-derived RRF (0.94 +/- 0.1 vs. 0.93 +/- 0.1, P = 0.9). Additionally, the effect of iohexol on RRF was assessed in 17 HD patients. Urea-derived RRF determinations one week after iohexol exposure did not differ from those measured one week prior to iohexol exposure (3.17 +/- 0.6 vs. 2.91 +/- 0.5 ml/min, respectively). Thus, renal clearance of iohexol can be an accurate and safe measure of RRF in HD patients and potentially simplify delivery of the dialysis prescription.

Comparative evaluation of iohexol and inulin clearance for glomerular filtration rate determinations

We evaluated iohexol as a filtration marker in 150 children. The clearance of iohexol was compared with that of inulin or with a formula clearance. The single-sample clearance of iohexol showed a good correlation with the clearance of inulin (r = 0.834). The clearance of iohexol correlated well (r = 0.672) with the formula clearance. The optimal blood sampling time for iohexol clearance determinations appears to be between 120 and 180 min after injection, at least in patients with relatively normal filtration rates. We conclude that iohexol clearance is an accurate method of determining the glomerular filtration rate in clinical practice.