Mechanism of renal excretion of various X-ray contrast materials in rabbits

Measurement of Glomerular Filtration Rate in Patients Undergoing Renal Surgery for Cancer: Estimated Glomerular Filtration Rate versus Measured Glomerular Filtration Rate in the Era of Precision Medicine

BACKGROUND

In the era of precision medicine, determining reliable renal function assessment remains a critical and debatable issue, especially in nephrology and oncology.

SUMMARY

This paper delves into the significance of accurately measured glomerular filtration rate (mGFR) in clinical practice, highlighting its essential role in guiding medical decisions and managing kidney health, particularly in the context of renal cancer (RC) patients undergoing nephrotoxic anti-cancer drugs. The limitations and advantages of traditional glomerular filtration rate (GFR) estimation methods, primarily using serum biomarkers like creatinine and cystatin C, are discussed, emphasizing their possible inadequacy in cancer patients. Specifically, newer formulae designed for GFR estimation in cancer patients may not perform at best in RC patients. The paper explores various methods for direct GFR measurement, including the gold standard inulin clearance and alternatives like iohexol plasma clearance.

KEY MESSAGE

Despite the logistical challenges of these methods, their implementation is crucial for accurate renal function assessment. The paper concludes by emphasizing the need for continued research and innovation in GFR measurement methodologies to improve patient outcomes, particularly in populations with complex medical needs.

The complexity of kidney disease and diagnosing it - cystatin C, selective glomerular hypofiltration syndromes and proteome regulation

Estimation of kidney function is often part of daily clinical practice, mostly done by using the endogenous glomerular filtration rate (GFR)-markers creatinine or cystatin C. A recommendation to use both markers in parallel in 2010 has resulted in new knowledge concerning the pathophysiology of kidney disorders by the identification of a new set of kidney disorders, selective glomerular hypofiltration syndromes. These syndromes, connected to strong increases in mortality and morbidity, are characterized by a selective reduction in the glomerular filtration of 5-30 kDa molecules, such as cystatin C, compared to the filtration of small molecules <1 kDa dominating the glomerular filtrate, for example water, urea and creatinine. At least two types of such disorders, shrunken or elongated pore syndrome, are possible according to the pore model for glomerular filtration. Selective glomerular hypofiltration syndromes are prevalent in investigated populations, and patients with these syndromes often display normal measured GFR or creatinine-based GFR-estimates. The syndromes are characterized by proteomic changes promoting the development of atherosclerosis, indicating antibodies and specific receptor-blocking substances as possible new treatment modalities. Presently, the KDIGO guidelines for diagnosing kidney disorders do not recommend cystatin C as a general marker of kidney function and will therefore not allow the identification of a considerable number of patients with selective glomerular hypofiltration syndromes. Furthermore, as cystatin C is uninfluenced by muscle mass, diet or variations in tubular secretion and cystatin C-based GFR-estimation equations do not require controversial race or sex terms, it is obvious that cystatin C should be a part of future KDIGO guidelines.

Cystatin C-based equations for estimating glomerular filtration rate do not require race or sex coefficients

Estimation or measurement of glomerular filtration rate (GFR) is generally required for optimal treatment of patients. Plasma creatinine has been used for estimation of GFR since 1926 and plasma cystatin C since 1979. The creatinine level is strongly dependent upon muscle mass and as the average muscle mass of different populations may vary, creatinine-based GFR-estimating equations have since 1999 used more than 10 different race coefficients to improve the diagnostic performance of such equations. But 'race' cannot be determined by biological measurements and is thus an ill-defined biological entity and controversial as it involves self-reporting and social considerations. In contrast, cystatin C-levels are virtually independent of muscular mass and cystatin C-based GFR-estimating equations do not require race coefficients for reliable estimation of GFR. The use of cystatin C-based GFR-estimating equations, alone or in conjunction with creatinine-based GFR-estimating equations, is therefore highly recommended to eliminate the use of race coefficients in estimating GFR. Although sex is a more biology-oriented parameter than race, sex terms may in some cases be controversial, involving self-reporting and social considerations. However, sex terms are not required for adequate estimation of GFR using cystatin C-based equations.

Creatinine clearance after cimetidine administration in a new short procedure: comparison with plasma and renal clearances of iohexol

Background

Creatinine clearance after cimetidine administration (Cim-CreatClr) was once proposed as a method of glomerular filtration rate (GFR) measurement, but has been largely abandoned. We investigated whether a new short procedure for Cim-CreatClr determination could be considered an appropriate method for GFR measurement.

Methods

A 150-min protocol involving oral cimetidine administration was developed to determine Cim-CreatClr. In total, 168 patients underwent simultaneous assessments of creatinine clearance before and after cimetidine administration [basal creatinine clearance (Basal-CreatClr) and Cim-CreatClr, respectively], renal iohexol clearance and plasma iohexol clearance (R-iohexClr and P-iohexClr, respectively). We compared the agreement between the various methods of GFR measurement, using Bland–Altman plots to determine biases, precisions (standard deviation of the biases) and accuracy (proportions of GFR values falling within 10, 15 and 30% of the mean: P10, P15 and P30, respectively).

Results

After cimetidine administration, Basal-CreatClr decreased by 19.8% [95% reference limits of agreement (95% LoA): −2.2 to 41.7%]. The bias between Cim-CreatClr and P-iohexClr was −0.6% (95% LoA −26.8 to 28%); the precision was 14.0%; P10, P15 and P30 were 57.1% [95% confidence interval (95% CI) 49.3 to 64.7%], 73.2% (95% CI 65.8 to 79.7%) and 97.0% (95% CI 93.2 to 99.0%), respectively. Due to the positive bias (16.7%; 95% LoA −3.6 to 36.9%) of Cim-CreatClr relative to R-iohexClr, accuracy of Cim-CreatClr relative to R-iohexClr was poor despite a good precision (10.3%).

Conclusions

Our study shows a high level of agreement between Cim-CreatClr and P-iohexClr. These results suggest that this short Cim-CreatClr procedure is a valid method for GFR measurement, which might be useful, in particular, in situations in which P-iohexClr is not suitable or not available.

GFR estimation based on standardized creatinine and cystatin C: a European multicenter analysis in older adults

BACKGROUND

Although recommended by the Kidney Disease Improving Global Outcomes, the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPICR) creatinine equation was not targeted to estimate glomerular filtration rate (eGFR) among older adults. The Berlin Initiative Study (BIS1CR) equation was specifically developed in older adults, and the Lund-Malmö revised (LMRCR) and the Full Age Spectrum (FASCR) equations have shown promising results in older adults. Our aim was to validate these four creatinine equations, including addition of cystatin C in a large multicenter cohort of Europeans ≥70 years.

METHODS

A total of 3226 individuals (2638 with cystatin C) underwent GFR measurement (mGFR; median, 44 mL/min/1.73 m2) using plasma iohexol clearance. Bias, precision (interquartile range [IQR]), accuracy (percent of estimates ±30% of mGFR, P30), eGFR accuracy diagrams and probability diagrams to classify mGFR<45 mL/min/1.73 m2 were compared.

RESULTS

The overall results of BIS1CR/CKD-EPICR/FASCR/LMRCR were as follows: median bias, 1.7/3.6/0.6/-0.7 mL/min/1.73 m2; IQR, 11.6/12.3/11.1/10.5 mL/min/1.73 m2; and P30, 77.5%/76.4%/80.9%/83.5% (significantly higher for LMR, p<0.001). Substandard P30 (<75%) was noted for all equations at mGFR<30 mL/min/1.73 m2, and at body mass index values <20 and ≥35 kg/m2. LMRCR had the most stable performance across mGFR subgroups. Only LMRCR and FASCR had a relatively constant small bias across eGFR levels. Probability diagrams exhibited wide eGFR intervals for all equations where mGFR<45 could not be confidently ruled in or out. Adding cystatin C improved P30 accuracy to 85.7/86.8/85.7/88.7 for BIS2CR+CYS/CKD-EPICR+CYS/FASCR+CYS/MEANLMR+CAPA.

CONCLUSIONS

LMRCR and FASCR seem to be attractive alternatives to CKD-EPICR in estimating GFR by creatinine-based equations in older Europeans. Addition of cystatin C leads to important improvement in estimation performance.

A Simple Method to Measure Renal Function in Swine by the Plasma Clearance of Iohexol

There is no simple method to measure glomerular filtration rate (GFR) in swine, an established model for studying renal disease. We developed a protocol to measure GFR in conscious swine by using the plasma clearance of iohexol. We used two groups, test and validation, with eight animals each. Ten milliliters of iohexol (6.47 g) was injected into the marginal auricular vein and blood samples (3 mL) were collected from the orbital sinus at different points after injection. GFR was determined using two models: two-compartment (CL2: all samples) and one-compartment (CL1: the last six samples). In the test group, CL1 overestimated CL2 by ~30%: CL2 = 245 ± 93 and CL1 = 308 ± 123 mL/min. This error was corrected by a first-order polynomial quadratic equation to CL1, which was considered the simplified method: SM = −47.909 + (1.176xCL1) − (0.00063968xCL1²). The SM showed narrow limits of agreement with CL2, a concordance correlation of 0.97, and a total deviation index of 14.73%. Similar results were obtained for the validation group. This protocol is reliable, reproducible, can be performed in conscious animals, uses a single dose of the marker, and requires a reduced number of samples, and avoids urine collection. Finally, it presents a significant improvement in animal welfare conditions and handling necessities in experimental trials.

Measurement and Estimation of Residual Kidney Function in Patients on Dialysis

Residual kidney function (RKF) in patients on dialysis is strongly associated with survival and better quality of life. Assessment of kidney function underlies the management of patients with chronic kidney disease before dialysis initiation. However, methods to assess RKF after dialysis initiation are just now being refined. In this review, we discuss the definition of RKF and methods for measurement and estimation of RKF, highlighting the unique aspects of dialysis that impact these assessments.

Accuracy of GFR estimating equations in a large Swedish cohort: implications for radiologists in daily routine and research

Background Guidelines recommend estimation of glomerular filtration rate (eGFR) prior to iodine contrast media (CM) examinations. It is also recommended that absolute eGFR in mL/min, not commonly used relative GFR (adjusted to body surface area; mL/min/1.73 m²), should be preferred when dosing and evaluating toxicity of renally excreted drugs. Purpose To validate the absolute Lund-Malmö equation (LM-ABS) in comparison with the absolute Cockcroft-Gault (CG) equation and the relative equations, revised Lund-Malmö (LM-REV), MDRD, and CKD-EPI, after converting relative estimates to absolute values, and to analyze change in eGFR classification when absolute instead of relative eGFR was used. Material and Methods A total of 3495 plasma clearance of iohexol to measure GFR (mGFR) served as reference test. Bias, precision, and accuracy (percentage of estimates ±30% of mGFR; P₃₀) were compared overall and after stratification for various mGFR, eGFR, age, and BMI subgroups. Results The overall P₃₀ results of CG/LM-ABS/LM-REV/MDRD/CKD-EPI were 62.8%/84.9%/83.7%/75.3%/75.6%, respectively. LM-ABS was the most stable equations across subgroups and the only equation that did not exhibit marked overestimation in underweight patients. For patients with relative eGFR 30-44 and 45-59 mL/min/1.73 m², 36% and 58% of men, respectively, and 24% and 32% of women, respectively, will have absolute eGFR values outside these relative eGFR intervals. Conclusion Choosing one equation to estimate GFR prior to contrast medium examinations, LM-ABS may be preferable. Unless absolute instead of relative eGFR are used, systematic inaccuracies in assessment of renal function may occur in daily routine and research on CM nephrotoxicity may be flawed.

Contrast Media-Induced Renal Morphologic Lesions in Diabetic Rats

Diabetes mellitus was induced in rats with streptozotocin and after 3 months the animals (n = 48) received an i.v. injection of 1 or 3 g I/kg in the form of high-osmolar diatrizoate, low-osmolar iopromide or iohexol, or of 0.6 g I/kg of high-osmolar Gd-DTPA. The controls were given an i.v. injection of physiologic saline. After 2 hours the kidneys were fixed by perfusion and the renal morphologic changes were semiquantitatively analyzed by two independent observers unaware of the agent administered. The contrast media (CM) induced pronounced cytoplasmic vacuolization in the proximal convoluted tubular cells. Such a lysosomal alteration may indicate CM uptake into the cell, and the ultrastructural evaluation revealed intracellular injuries related to the process. The alterations were most marked following administration of iohexol, but diatrizoate also induced a statistically highly significant vacuolization (p < 0.001). The lysosomal alterations following iopromide administration were not as striking, and Gd-DTPA induced only minor changes.

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.

Pharmacokinetics of lipophilically different 3-substituted 2,2,5,5-tetramethylpyrrolidine-N-oxyl radicals frequently used as redox probes in in vivo magnetic resonance studies

3-Carboxy-, 3-carbamoyl-, 3-hydroxymethyl, and 3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl radicals (CxP, CmP, HMP, and MCP, respectively) have been widely used as redox probes in in vivo magnetic resonance studies. Knowledge of the pharmacokinetics of these probes is essential for redox analyses. The apparent partition coefficient (Kp) of these probes at neutral pH was in the order of MCP>HMP>CmP>CxP. After these probes had been injected intravenously, their blood levels decayed in a bi-phasic manner, namely, fast decay followed by slow decay. The order of the area under the curve (AUC) was CxP»HMP>MCP≥CmP, which roughly coincided with that of Kp in the opposite direction, except for CmP. Decay in the slow phase largely affected the AUC of these probes. The reduction of these probes contributed to their decay in the slow phase. A two-compartment model analysis of blood levels, cyclic voltammetry, and magnetic resonance imaging provided the following pharmacokinetic information. The distribution of the probes between the central and peripheral compartments rapidly reached an equilibrium. In addition to lipophilicity, reduction potential may also be involved in the rate of in vivo reduction of the probes. Hydrophilic probes, such as CxP and CmP, were predominantly excreted in the urine. MCP was distributed to the peripheral tissues and then rapidly reduced. HMP was unique due to its moderate lipophilicity and slower reduction. Among the probes examined, the liver and kidney appear to be included in the central compartment in the two-compartment model analysis. MCP and HMP were rapidly distributed to the brain.

Discordance Between Iothalamate and Iohexol Urinary Clearances

BACKGROUND

Iothalamate and iohexol are contrast agents that have supplanted inulin for the measurement of glomerular filtration rate (GFR) in clinical practice. Previous studies have noted possible differences in renal handling of these 2 agents, but clarity about the differences has been lacking.

STUDY DESIGN

Study of diagnostic test accuracy.

SETTING & PARTICIPANTS

150 participants with a wide range of GFRs were studied in an outpatient clinical laboratory facility.

INDEX TESTS

Simultaneous urinary clearances of iothalamate, iohexol, and creatinine.

REFERENCE TEST

None.

OUTCOME

Relative differences between the urinary clearances. Iohexol and iothalamate in plasma and urine were assayed concurrently by a novel liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay.

RESULTS

Mean iohexol, iothalamate, and creatinine clearances were 52±28 (SD), 60±34, and 74±40 mL/min/1.73 m(2), respectively. The proportional bias of iohexol to iothalamate urinary clearance was 0.85 (95% CI, 0.83-0.88) and was proportional across the GFR range. The mean proportional bias of iohexol clearance compared with creatinine clearance is 1.27 (95% CI, 1.20-1.34), whereas that of iothalamate clearance compared with creatinine clearance is 1.09 (95% CI, 1.03-1.15).

LIMITATIONS

Lack of reference standard.

CONCLUSIONS

This study reveals a significant and consistent difference between urinary clearances of iothalamate and iohexol. Comparison of studies reporting renal clearance measurements using iohexol versus iothalamate must account for this observed bias.

Effect of radiologic contrast material on cell volume regulation in proximal renal tubules from trout (Salmo trutta)

RATIONALE AND OBJECTIVES

Most radiographic contrast media (CM) are hyperosmotic and pose an osmotic threat to cells they are in contact with. To study these effects at the cellular level, cell volume regulatory mechanisms were observed in proximal renal tubules following exposure to the CM iohexol, ioxaglate, and iodixanol.

MATERIALS AND METHODS

Isolated renal tubules from trout (Salmo trutta) were exposed to 5% vol/vol iohexol (326 mOsm), ioxaglate (314 mOsm), or iodixanol (300 mOsm) or mannitol (to achieve the same osmolalities), and cell volume changes were observed videometrically.

RESULTS

Iohexol and ioxaglate solutions induced a rapid shrinkage (12%-13%) not followed by cell volume regulation. Without CM (same osmolality), the cells shrank 11% but then showed a 77%-88% volume recovery. This reswelling was inhibited by 55% with the Na+, K+, Cl- symporter inhibitor bumetanide (50 micromol/L). Iodixanol did not significantly affect cell volume. Tubules preincubated with CM or mannitol were then stimulated with a hypoosmotic Ringer solution (160 mOsm) resulting in a 26%-36% cellular volume increase. Compared with results of experiments without mannitol and CM, preexposure to iohexol or ioxaglate almost completely inhibited the expected regulatory shrinkage phase, while previous exposure to hyperosmotic solutions with mannitol reduced the shrinkage response by 40%-53%.

CONCLUSION

In this system, the hyperosmotic iohexol and ioxaglate cause cell shrinkage followed by an impaired cell volume regulatory response. Exposure to these two CM also inhibits cell volume regulation on hypoosmotic stimulation. The isosmotic iodixanol has no such effects. These changes appear to some extent to be a result of the CM's degree of hyperosmolality, but this property alone does not explain these findings.

Indicator measurement in tissues: CT with iohexol versus storage-phosphor autoradiography with carbon-14-labeled inulin

RATIONALE AND OBJECTIVES

Computed tomography (CT) provides accurate measurement of blood iodine concentration in vivo, as well as in phantoms simulating tissue; however, its ability to measure radiopaque agents in biologic tissues in comparison with a standard technique does not seem to have been demonstrated. To validate the performance of CT imaging for quantification of contrast media in a variety of biologic tissues in vivo, a comparison between CT imaging with an iodinated contrast agent (iohexol) and the reference tracer quantification technique (storage-phosphor autoradiography with carbon-14-labeled inulin) was performed.

MATERIALS AND METHODS

Six New Zealand White rabbits were injected intravenously with a cocktail of iohexol and C-14-labeled inulin at different dose ratios and sacrificed shortly after injection to arrest blood flow at different stages of tissue tracer distribution. One rabbit received no iohexol-inulin mixture and provided baseline data. Liver, spleen, kidneys, testis, and heart were excised and rapidly frozen. Each organ was scanned with CT (1-mm contiguous sections) to determine tissue iodine distribution. Twenty-micrometer tissue slices were made in the same planes in which the CT images had been acquired, and storage-phosphor screen autoradiography was performed to quantify C-14-labeled inulin distribution.

RESULTS

Digital image analysis of CT images and autoradiograms was performed on spatially matched regions, and resultant tracer concentrations were compared. Tracer concentrations were highly correlated, with resultant R2 values exceeding 0.9 in all tissues.

CONCLUSION

The highly correlated results for iodinated tracer quantification in tissues for CT versus those obtained with the reference technique validate the performance of CT as an accurate means of measuring concentration of radiopaque agent in tissue, independent of tracer dose.

Measurement of renal perfusion and blood flow with fast computed tomography

Fast computed tomography (CT) is one of the few methods available to measure cortical and medullary renal blood flow (RBF) directly. Because these measurements are complicated by passage of the contrast medium into extravascular compartments, we used the residual opacity following the vascular blush as an index to account for extravascular iohexol. Kidneys of anesthetized dogs were examined in situ by fast CT following intra-aortic injections of iohexol. Perfusion was analyzed during a control period and three subsequent periods in which RBF was reduced by 10%, 30%, and 50%. Cortical microvascular distribution volume changed from 19.7 +/- 2.8% to 19.8 +/- 1.7%, 15.3 +/- 1.2%, and 9.9 +/- 1.7%, respectively, without significant alterations in cortical mean transit time. Microvascular distribution volume was divided by mean transit time to determine tissue perfusion. Cortical perfusion changed from 3.8 +/- 0.7 to 3.9 +/- 0.6, 3.1 +/- 0.5, and 2.2 +/- 0.5 mL.min-1.mL tissue-1. Total cortical blood flow (cortical perfusion multiplied by cortical volume) decreased from 164 +/- 32 to 159 +/- 31, 117 +/- 20, and 86 +/- 22 mL/min, respectively. Medullary microvascular distribution volume, mean transit time, perfusion, and total blood flow remained unchanged. Fast CT-determined total RBFs (cortex plus medulla) were similar to simultaneous electromagnetic flow measurements. These results indicate that renal regional perfusion is more dependent on the microvascular distribution volume than mean transit time and that variations in renal tissue perfusion with reduction of RBF are more apparent in the cortex than in the medulla.

Contrast media for angiography: physicochemical properties, pharmacokinetics and biocompatibility

Contrast agents are used as diagnostic molecules for the visualization of the vascular system. Despite their rapid pharmacokinetic distribution, and their excretion within a few minutes, their injection is associated with clinical symptoms of relative bioincompatibility. Allergoid reactions and disturbances of the hemostatic system represent the main fields of biological investigations. Due to the extent of clinical and experimental works the ubiquitous interactions between these molecules and cellular and/or protein systems have emerged. The development of a new family of low osmolality ionic or non-ionic contrast molecules had decreased the incidence of minor reactions, but did not modify the frequency of severe accidents and even led to the emergence of new iatrogenic syndromes. Despite extensive laboratory investigations there are still no predictive criteria nor any specific therapeutic prevention of these allergoid reactions. The suggested future line of investigation concerns the physicochemical interaction of CM and targeted biological systems which may allow the analysis and predictivity of these interactions at the molecular level.

Three-dimensional canine renovascular structure and circulation visualized in situ with the dynamic spatial reconstructor

The dynamic spatial reconstructor--a unique, high speed, volume-scanning, X-ray computed tomographic imaging system--was utilized to examine canine renovascular anatomy and renal circulation in situ. In each of the four kidneys examined in this study initial scans were done during bolus injections of angiographic contrast material into the renal artery. A subsequent scan was then performed following an injection of methyl-methacrylate-based casting compound that had been contrast enhanced with ethiodol. After the scans, each kidney was removed, and its parenchyma was digested in potassium hydroxide to expose the vascular cast. Comparison of casts with their reconstructed images and with images obtained during injection of contrast material showed that interlobar arteries and occasionally arcuate arteries could be clearly detected. Although discrete vessels less than 1 mm in diameter could not be resolved, dynamic changes in parenchymal distribution of density during passage of contrast material allowed interpretation of flow through the multiple capillary beds of the kidney. Such analysis indicated that maximal density was in the outer-middle zone of the cortex throughout the duration of the scan. Analysis of artery-to-vein transit time showed arrival of contrast material in the renal vein as soon as 3 sec, and continuation for longer than 8 sec, after the renal artery bolus. In conclusion, renal circulation in the dog can be discretely visualized with the dynamic spatial reconstructor up to the level of the arcuate arteries; however, capillary flow as a whole can be followed through the cortex, and the results suggest the presence of both rapid and slow components of peritubular circulation.