Is 125I iothalamate an ideal marker for glomerular filtration?

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.

Novel insights in classic versus relative glomerular hyperfiltration and implications on pharmacotherapy

PURPOSE OF REVIEW

Glomerular filtration rate (GFR) assessment and its estimation (eGFR) is a long-lasting challenge in medicine and public health. Current eGFR formulae are indexed for standardized body surface area (BSA) of 1.73 m2, ignoring persons and populations wherein the ratio of BSA or metabolic rate to nephron number might be different, due to increased BSA, increased metabolic rate or reduced nephron number. These equations are based on creatinine, cystatin C or a combination of the two, which adds another confounder to eGFR assessment. Unusually high GFR values, also known as renal hyperfiltration, have not been well defined under these equations.

RECENT FINDINGS

Special conditions such as solitary kidney in kidney donors, high dietary protein intake, obesity and diabetes are often associated with renal hyperfiltration and amenable to errors in GFR estimation. In all hyperfiltration types, there is an increased intraglomerular pressure that can be physiologic, but its persistence over time is detrimental to glomerulus leading to progressive glomerular damage and renal fibrosis. Hyperfiltration might be underdiagnosed due to BSA standardization embedded in the formula. Hence, timely intervention is delayed. Reducing intraglomerular pressure in diabetes can be achieved by SGLT2 inhibitors or low protein diet to reverse the glomerulopathy process.

SUMMARY

Accurate identification of glomerular hyperfiltration as a pre-CKD condition needs accurate estimation of GFR in the above normal range should establish a threshold for timely intervention.

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.

Assessing Renal Function for Kidney Donation. How Low Is Too Low?

Kidney transplantation (KT) is the treatment of choice for patients with end-stage kidney disease (ESKD) with decreased morbi-mortality, improved life quality, and reduced cost. However, the shortage of organs from deceased donors led to an increase in KT from living donors. Some stipulate that living donors have a higher risk of ESKD after donation compared with healthy non-donors. The reason for this is not clear. It is possible that ESKD is due to the nephrectomy-related reduction in glomerular filtration rate (GFR), followed by an age-related decline that may be more rapid in related donors. It is essential to assess donors properly to avoid rejecting suitable ones and not accepting those with a higher risk of ESKD. GFR is a central aspect of the evaluation of potential donors since there is an association between low GFR and ESKD. The methods for assessing GFR are in continuous debate, and the kidney function thresholds for accepting a donor may vary according to the guidelines. While direct measurements of GFR (mGFR) provide the most accurate evaluation of kidney function, guidelines do not systematically use this measurement as a reference. Also, some studies have shown that the GFR decreases with age and may vary with gender and race, therefore, the lower limit of GFR in patients eligible to donate may vary based on these demographic factors. Finally, it is known that CrCl overestimates mGFR while eGFR underestimates it, therefore, another way to have a reliable GFR could be the combination of two measurement methods.

Reference values of glomerular filtration rate for healthy adults in southern China: a cross-sectional survey

Background:

Currently the global data on the glomerular filtration rate of healthy adults are insufficient, with relatively little data for other races and countries. Especially in China, there are no such figures.

Methods:

In this cross-sectional study, we included healthy Han adults in southern China. Participants completed a lifestyle and medical history questionnaire and had their blood pressure measured, and blood and urine samples collected. Serum creatinine was measured and used to estimated glomerular filtration rate (eGFR) by the Modification of Diet in Renal Disease (MDRD) and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formulae. The normal range of eGFR is described, and the influence of gender and age on eGFR is analyzed by the statistical method.

Results:

We provided the largest sample size of eGFR research in China at present. The mean age of the 20,930 healthy individuals was 40.9 ± 12.3 years, 58.8% were women. The eGFR_MDRD for women and men were 111.3 ± 17.4 mL/min per 1.73 m² and 103.3 ± 15.9 mL/min per 1.73 m², respectively. The eGFR_CKD-EPI for women and men were 110.3 ± 12.1 mL/min per 1.73 m² and 103.8 ± 13.3 mL/min per 1.73 m², respectively. The eGFR_MDRD of women and men in all age groups decreased continuously by 7.3 ml/min/1.73 m²/decade and 4.4 ml/min/1.73 m²/decade, respectively. The eGFR_CKD-EPI of women and men in all age groups decreased continuously by 8.4 ml/min/1.73 m²/decade and 6.9 ml/min/1.73 m²/decade.

Conclusions:

The eGFR of women is higher than men and with the increasing age, the eGFR of women declines faster than men.

Midazolam and hydroxymidazolam plasma concentrations can be monitored with selected biochemical and physiological parameters of palliative care patients

RATIONALE & OBJECTIVE

Midazolam is one of top three drugs used in palliative care. Its use increases in the last days of hospice patients' lives while safe dosage can be challenging. Equations currently used to estimate glomerular filtration rate, e.g: the Cockroft-Gault (eGFR_CR) and the Modification of Diet in Renal Disease (eGFR_MDRD) ones, do not generate precise calculations, especially in palliative patients exhibiting variations in body parameters. Our aim was to seek new relationships between mean midazolam (M_avg) and alfahydroxymidazolam (OH-M_avg) concentrations in plasma, and selected biochemical and physiological parameters of palliative patients, to enable optimal midazolam pharmacotherapy.

STUDY DESIGN, PARTICIPANTS AND INTERVENTIONS

The pilot study included 11 Caucasians, aged 42-95, with advanced cancer disease, receiving midazolam in a hospice in-patient unit. We tested correlations among M_avg, BMI, eGFR_MDRD, midazolam clearance (CL), OH-M_avg, bilirubin (Bil) and blood creatinine concentration (Cr). F test and leave-one out (LOO) validation was applied to verify the correlations' significance and predictive ability.

RESULTS

We found ten statistically significant (p < 0.05) correlations related to midazolam pharmacokinetics and physiological factors. We formulated two equations with high degree of predictive ability, based on the eGFR_MDRD→CL and the (Bil + BMI × Ln(Cr))→M_avg-(OH-M_avg) correlations. The limitations of the study mainly revolve around its pilot nature and the need to continue testing the results on a bigger population. No funding to disclose.

CONCLUSIONS

The significance of correlations corresponding to the arithmetic expressions confirms that Bil, BMI, Ln(Cr) analyzed simultaneously report a series of processes on which midazolam metabolism depends. Two of ten correlations proposed came close to meet all LOO validation criteria. Current findings can help optimize midazolam treatment in palliative therapy.

Shrunken pore syndrome - a common kidney disorder with high mortality. Diagnosis, prevalence, pathophysiology and treatment options

Invasive studies show that the glomerular sieving coefficients for 5-30 kDa plasma proteins in the human kidney may be selectively reduced compared to those for small molecules < 0.9 kDa, commonly used to measure glomerular filtration rate (GFR). Identification of this pathophysiological state, called shrunken pore syndrome (SPS), can easily and non-invasively be done by comparing estimations of GFR using cystatin C (13.3 kDa) and creatinine (0.113 kDa). SPS is present if the estimate of GFR using cystatin C is lower than 60 or 70% of the estimate using creatinine in the absence of non-renal influences on cystatin C or creatinine. All studies of SPS show that the 3- or 5-year mortality is strongly increased and high hazard ratios for mortality associated with SPS have been observed for many different patient cohorts, including cohorts with normal measured GFR, no albuminuria and no diagnosis. The prevalence of SPS in the cohorts so far investigated is between 0.2 and 36%. Proteome studies of SPS demonstrate that the high mortality associated with the syndrome might be caused by the accumulation of 10-30 kDa signalling proteins promoting development of atherosclerosis and thus suggesting use of monoclonal antibodies to reduce the levels of the most detrimental signalling proteins as a treatment option. The KDIGO recommendations for classification of chronic kidney disease (CKD) comprise determination, or estimation, of GFR and analysis of albuminuria and therefore cannot identify a large fraction of the patients with SPS. The high prevalence and mortality of SPS and the possible treatment options strongly suggest that the KDIGO recommendations should be expanded to include determination of cystatin C to be able to identify all patients with SPS.

Conventional Pig as Animal Model for Human Renal Drug Excretion Processes: Unravelling the Porcine Renal Function by Use of a Cocktail of Exogenous Markers

Over recent years, pigs have been promoted as potential animal model due to their anatomical and physiological similarities with humans. However, information about the contribution of distinct renal elimination processes [glomerular filtration rate (GFR), effective renal plasma flow (ERPF), tubular secretion, and reabsorption] in pigs is currently limited. Therefore, a cocktail of renal markers, consisting of iohexol (GFR), para-aminohippuric acid (ERPF and net tubular anion secretion), pindolol (net tubular cation secretion), and fluconazole (net tubular reabsorption) was administered intravenously to 7-week-old male conventional pigs. Plasma and urinary concentrations were determined using validated analytical methods. The clearance of iohexol (GFR) was 97.87 ± 16.05 ml/min/m² (mean ± SD). The ERPF, calculated as the renal clearance of PAH, was 226.77 ± 62.45 ml/min/m², whereas the net tubular secretion of PAH was 130.28 ± 52.62 ml/min/m². The net tubular secretion of R-pindolol and S-pindolol was 13.53 ± 12.97 and 18.01 ± 39.23 ml/min/m², respectively. The net tubular reabsorption of fluconazole was 78.32 ± 13.52 ml/min/m². Overall, this cocktail of renal markers was considered to be safe for use in pigs since no adverse effects were observed. Iohexol, PAH and fluconazole were considered suitable renal marker to assess the porcine renal function. Pindolol seems less appropriate due to the high degree of nonrenal clearance in pigs. The values of GFR, ERPF, and anion secretion are within the same range for both human and pig. Regarding the tubular reabsorption of fluconazole, slightly higher values were obtained for pigs. Nevertheless, these results indicate the conventional pig could be an appropriate animal model to study renal drug elimination processes in humans.

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.

Challenges in Measuring Glomerular Filtration Rate: A Clinical Laboratory Perspective

The assessment of kidney function is a cornerstone in the clinical management and health of the patient. Although the kidneys perform many physiologic functions and are essential for maintaining homeostasis, kidney function is typically evaluated, quantitated, and understood using the glomerular filtration rate (GFR). Although GFR can be directly measured using a variety of externally administered glomerular filtration markers, in general practice, the GFR is usually estimated (eGFR) using endogenous markers that are cleared primarily by kidney filtration. Common situations exist where the GFR needs to be measured (mGFR) in order to proceed with care. This manuscript will review laboratory challenges in the assessment of GFR. Key points to consider when implementing a mGFR testing protocol are the following: marker selection, clearance methodology (urinary vs solely plasma measurements of filtration marker), sample collection, number of samples to collect, staff required, and analytical measurement technology for the filtration marker selected. We suggest those wanting to implement mGFR testing examine site-specific institutional resources along with patient population and proceed with the approaches best suited for their clinical needs and laboratory resources available.

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.

Measurement and Estimation of Glomerular Filtration Rate in Children

Rapid, accurate, and precise measures of kidney function are essential for daily management of patients. While plasma and urinary clearances provide the greatest accuracy for assessing glomerular filtration rate (GFR), these are often impractical particularly for the care of children. Serum creatinine, the most commonly used endogenous marker, is simple, convenient, and practical but less accurate because of the influence of non-GFR determinants such as muscle mass, which increases with age in children. GFR estimating equations have been developed for adults and children to improve the accuracy of endogenous biomarkers, such as creatinine and cystatin C, by accounting for some of the non-GFR determinants, thus enhancing the practitioner's ability to assess GFR. In the steady state, when height is used as a surrogate for growth, there is a strong correlation between height/S_Cr and GFR. Current national guidelines recommend routine reporting of the estimated GFR alongside the serum creatinine value for adults using the Chronic Kidney Disease Epidemiology Collaboration creatinine-based formula and the updated Schwartz "bedside" formula (CKiD 2009) for children.

Relationships and Clinical Utility of Creatinine, Cystatin C, eGFRs, GFRs, and Clearances

BACKGROUND

This review addresses techniques for glomerular filtration rate (GFR), either measured by clearance tests such as with creatinine, iothalamate, inulin, or iohexol [measured GFR (mGFR)] or calculated by equations that determine the estimated GFR (eGFR) from serum measurements of creatinine and/or cystatin C. However, mGFR tests are slow and impractical for routine use. Therefore, calculations of eGFRs have been developed that have advantages over the mGFRs.

CONTENT

The eGFR is a serum creatinine and/or cystatin C adjusted for age, sex, and race, with mathematical manipulations to produce an average numerical agreement with the mGFR. However, all comparisons between eGFR and mGFR show wide scatter that appears to be related to the large variability of the mGFR. Procedures for mGFR often do not agree with each other and have both wide population variation (similar to plasma creatinine and cystatin C) and within-individual variation that is much larger than creatinine or cystatin C. Whether the measured GFR is even equivalent to serum creatinine and/or cystatin C for detecting early clinical changes in chronic kidney disease will be addressed.

SUMMARY

Procedures for measuring GFR are tedious and expensive, and have both wide population variation (similar to plasma creatinine and cystatin C), and within-individual variation that is much larger than either creatinine or cystatin C. Because the normal range for mGFR overlaps considerably with the stages 1 and 2 of chronic kidney disease, mGFR has significant clinical limitations. Instead of trying to mimic mGFRs, the focus should be on using eGFRs on their own clinical merits to detect impaired kidney function.

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.

Comparison between Two-sample Method with 99mTc-diethylenetriaminepentaacetic acid, Gates' Method and Estimated Glomerular Filtration Rate Values by Formula Based Methods in Healthy Kidney Donor Population

Purpose of the Study:

Glomerular filtration rate (GFR) is the most important parameter for the assessment of renal function. GFR by plasma sampling technique is considered accurate in the selection of donors for renal transplantation. Estimated GFR (eGFR) calculations using Gates’ method and Modification of Diet in Renal Disease (MDRD) and Cockcroft–Gault (CG) equations are simple methods but have not been validated in the Indian population. Hence, we aimed to assess the correlation between these three techniques.

Materials and Methods:

The plasma sampling technique was done using two samples at 60 and 180 min after injection of 1 mCi (37MBq) ^99mTc-diethylenetriaminepentaacetic acid (^99mTc-DTPA) in 66 healthy donors. Age, sex, height, weight, and plasma creatinine were recorded. Normalized GFR (nGFR) by two-sample method and eGFR (for Gates’, MDRD, and CG) values were calculated using formulae.

Results:

There were 14 male and 52 female donors. Mean age was 46.56 ± 12.88 years (24–69 years). Mean height was 153.74 ± 8.35 cm, whereas mean weight was 56.97 ± 11.88 kg. Mean nGFR value was 80.4 for two-sample method while mean eGFR value for Gates’, CG, and MDRD were 83.3, 89.36, and 97.47 ml/min/1.73 m² (eligibility value at our institution = 70), respectively. While the correlation between nGFR and eGFR CG and MDRD was weak moderate (correlation coefficient = 0.5), nGFR and eGFR Gates’ had a moderate correlation (0.686). Mean total bias for eGFR Gates’, CG, and MDRD were 2.87, 8.93, and 17.0, respectively. P₃₀ of eGFR Gates’, CG and MDRD were 60.6%, 57.6%, and 62.1%, respectively.

Conclusions:

Due to the large variability in eGFR Gates’, CG and MDRD, nGFR estimation using the plasma sampling technique with ^99mTc-DTPA appears necessary while screening healthy donors for renal transplantation.

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.

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.

Update on Estimation of Kidney Function in Diabetic Kidney Disease

The American Diabetes Association recommends annual assessment of glomerular filtration rate (GFR) to screen for diabetic nephropathy. GFR is measured indirectly using markers that, ideally, are eliminated only by glomerular filtration. Measured GFR, although the gold standard, remains cumbersome and expensive. GFR is therefore routinely estimated using creatinine and/or cystatin C and clinical variables. In pediatrics, the Schwartz creatinine-based equation is most frequently used even though combined creatinine and cystatin C-based equations demonstrate stronger agreement with measured GFR. In adults, the CKD Epidemiology Collaboration (CKD-EPI) equations with creatinine and/or cystatin C are the most accurate and precise estimating equations. Despite recent advances, current estimates of GFR lack precision and accuracy before chronic kidney disease stage 3 (GFR < 60 mL/min/1.73 m(2)). There is therefore an urgent need to improve the methods for estimating and measuring GFR. In this review, we examine the current literature and data addressing measurement and estimation of GFR in diabetes.

Methods of assessing renal function

Accurate assessment of renal function is critical for appropriate drug dosing of renally excreted compounds. Glomerular filtration rate (GFR) is considered the best marker of kidney function. Inulin clearance forms the gold standard for measuring GFR, both in adults and in children. The method is invasive, cumbersome, and smaller children require urinary catheterization for accurate timed urine collections. Nuclear medicine methods replaced inulin clearance in the 1970s after (51)Cr EDTA clearance was introduced. Inulin has no plasma protein binding, whereas all commonly used radioisotopes have a small amount of plasma protein binding that leads to lower values. Only iohexol does not have significant plasma protein binding. The underestimation due to plasma protein binding is partially offset by overestimation due to the use of non-compartmental pharmacokinetic modeling of the plasma disappearance of the radioisotope. The problem could be overcome with a urinary nuclear medicine clearance method, but these have not been validated in children. Endogenous markers of GFR include serum creatinine and low molecular weight proteins such as cystatin C and beta-trace protein. Of these, estimation of GFR using cystatin C appears to be the most promising, although its accuracy in pregnancy and in the neonatal period may be limited.

Improved equations estimating GFR in children with chronic kidney disease using an immunonephelometric determination of cystatin C

The Chronic Kidney Disease in Children study is a cohort of about 600 children with chronic kidney disease (CKD) in the United States and Canada. The independent variable for our observations was a measurement of glomerular filtration rate (GFR) by iohexol disappearance (iGFR) at the first two visits one year apart and during alternate years thereafter. In a previous report, we had developed GFR estimating equations utilizing serum creatinine, blood urea nitrogen, height, gender and cystatin C measured by an immunoturbidimetric method; however the correlation coefficient of cystatin C and GFR (-0.69) was less robust than expected. Therefore, 495 samples were re-assayed using immunonephelometry. The reciprocal of immunonephelometric cystatin C was as well correlated with iGFR as was height/serum creatinine (both 0.88). We developed a new GFR estimating equation using a random 2/3 of 965 person-visits and applied it to the remaining 1/3 as a validation data set. In the validation data set, the correlation of the estimated GFR with iGFR was 0.92 with high precision and no bias; 91% and 45% of eGFR values were within 30% and 10% of iGFR, respectively. This equation works well in children with CKD in a range of GFR from 15 to 75 ml/min per 1.73 m². Further studies are needed to establish the applicability to children of normal stature and muscle mass, and higher GFR.

Measurement of renal function in patients with chronic kidney disease

Chronic kidney disease affects millions of people worldwide and is associated with an increased morbidity and mortality as a result of kidney failure and cardiovascular disease. Accurate assessment of kidney function is important in the clinical setting as a screening tool and for monitoring disease progression and guiding prognosis. In clinical research, the development of new methods to measure kidney function accurately is important in the search for new therapeutic targets and the discovery of novel biomarkers to aid early identification of kidney injury. This review considers different methods for measuring kidney function and their contribution to the improvement of detection, monitoring and treatment of chronic kidney disease.

Comparison of serum cystatin C, serum creatinine, measured GFR, and estimated GFR to assess the risk of kidney failure in American Indians with diabetic nephropathy

BACKGROUND

We compared values of baseline serum cystatin C (SCysC), serum creatinine (SCr), and measured glomerular filtration rate (mGFR) for predicting end-stage renal disease (ESRD) in patients with type 2 diabetes and elevated albuminuria.

STUDY DESIGN

Observational longitudinal study.

SETTING & PARTICIPANTS

Pima Indians with type 2 diabetes and elevated albumin-creatinine ratio (ACR ≥30 mg/g).

PREDICTORS

Baseline SCysC, SCr, and mGFR.

OUTCOMES & MEASUREMENTS

Individuals were followed up from their first examination with diabetes and ACR ≥30 mg/g until December 2010, onset of ESRD, or death, whichever came first. Incidence rates adjusted for age and sex were computed by Mantel-Haenszel stratification. The abilities of SCysC, SCr, and mGFR values to predict ESRD were compared with receiver operating characteristic curves.

RESULTS

Of 234 Pima Indians with a mean age of 42.8 years who were followed up for a median of 10.7 (range, 0.6-21.3) years, 68 (29%) developed ESRD. The incidence of ESRD was significantly higher in patients in the lowest versus highest tertile of 1/SCysC (incidence rate ratio, 2.43; 95% CI, 1.31-4.50). By contrast, mGFR and 1/SCr had J-shaped associations with ESRD. In unadjusted analyses, 1/SCysC had the highest area under the receiver operating characteristic curve (AUROC; 0.719 ± 0.035) and mGFR had the lowest (0.585 ± 0.042; P < 0.001); the AUROC for 1/SCr was intermediate (0.672 ± 0.040; P = 0.1 and P = 0.03 vs 1/SCysC and mGFR, respectively). In analyses adjusted for age, sex, diabetes duration, height, weight, hemoglobin A1c level, and ACR, 1/SCysC had the highest AUROC (0.845 ± 0.026). Models with mGFR or 1/SCr alone had similar AUROCs (P = 0.9) and both were lower than the model with 1/SCysC alone (P = 0.02 and P = 0.03, respectively).

LIMITATIONS

The predictive values of the filtration markers are limited to the extent that their precision is based on a single measurement.

CONCLUSIONS

SCysC level was a better predictor of ESRD than mGFR or SCr level in Pima Indians with type 2 diabetes and elevated albuminuria.

Validation of the Lund-Malmö, Chronic Kidney Disease Epidemiology (CKD-EPI) and Modification of Diet in Renal Disease (MDRD) equations to estimate glomerular filtration rate in a large Swedish clinical population

OBJECTIVE

The aim of this study was to validate externally the Swedish Lund-Malmö revised creatinine-based glomerular filtration rate (GFR) equations (LM Revised) in a Swedish cohort in comparison with the North American Modification of Diet in Renal Disease (MDRD) and Chronic Kidney Disease Epidemiology (CKD-EPI) equations.

MATERIAL AND METHODS

The study included 1397 examinations [median age 61 years, median body mass index (BMI) 26 kg/m(2)] in 996 patients referred for iohexol clearance (median 44 ml/min/1.73 m(2)). Bias, precision [interquartile range (IQR)], accuracy expressed as percentage of estimates ± 10% (P(10)) and ± 30% (P(30)) of measured GFR, and classification ability for five GFR stages (<15, 15-29, 30-59, 60-89 and ≥90 ml/min/1.73 m(2)) were compared.

RESULTS

Overall, all three equations performed satisfactorily: LM Revised, MDRD, CKD-EPI showed, respectively, a median bias of -5.8%, -2.2% and 1.7%, IQR 11.9, 12.3 and 11.7 ml/min/1.73 m(2), P(10) 35%, 34% and 38%, P(30) 84%, 79% and 79% and correctly classified GFR stages 68%, 65% and 69%. LM Revised was at least as accurate in terms of P(30) as the other equations at GFR intervals <90, while CKD-EPI was the only unbiased and the most accurate equation at ≥90 ml/min/1.73 m(2). LM Revised was more stable in terms of bias and accuracy across age and BMI groups than MDRD and CKD-EPI. Both MDRD and CKD-EPI overestimated measured GFR among elderly patients and in the small group of underweight men.

CONCLUSION

The ideal all-purpose GFR prediction equation does not exist. LM Revised should be preferred in patients with suspected or known renal insufficiency, while CKD-EPI is most useful in settings where patients with no a priori suspicion of renal impairment are evaluated. Differences in creatinine measurements between laboratories may limit the generalizability of the present validation.

The impact of different GFR estimating equations on the prevalence of CKD and risk groups in a Southeast Asian cohort using the new KDIGO guidelines

Background

Recently, the Kidney Disease: Improving Global Outcomes (KDIGO) group recommended that patients with CKD should be assigned to stages and composite relative risk groups according to GFR (G) and proteinuria (A) criteria. Asians have among the highest rates of ESRD in the world, but establishing the prevalence and prognosis CKD is a problem for Asian populations since there is no consensus on the best GFR estimating (eGFR) equation. We studied the effects of the choice of new Asian and Caucasian eGFR equations on CKD prevalence, stage distribution, and risk categorization using the new KDIGO classification.

Methods

The prevalence of CKD and composite relative risk groups defined by eGFR from with Chronic Kidney Disease-Epidemiology Collaboration (CKD-EPI); standard (S) or Chinese(C) MDRD; Japanese CKD-EPI (J-EPI), Thai GFR (T-GFR) equations were compared in a Thai cohort (n = 5526)

Results

There was a 7 fold difference in CKD_3-5 prevalence between J-EPI and the other Asian eGFR formulae. CKD_3-5 prevalence with S-MDRD and CKD-EPI were 2 - 3 folds higher than T-GFR or C-MDRD. The concordance with CKD-EPI to diagnose CKD_3-5 was over 90% for T-GFR or C-MDRD, but they only assigned the same CKD stage in 50% of the time. The choice of equation also caused large variations in each composite risk groups especially those with mildly increased risks. Different equations can lead to a reversal of male: female ratios. The variability of different equations is most apparent in older subjects. Stage G3aA1 increased with age and accounted for a large proportion of the differences in CKD_3-5 between CKD-EPI, S-MDRD and C-MDRD.

Conclusions

CKD prevalence, sex ratios, and KDIGO composite risk groupings varied widely depending on the equation used. More studies are needed to define the best equation for Asian populations.

Validation of the CKD-EPI formula in patients after renal transplantation

BACKGROUND

Accurate calculation of glomerular filtration rate (GFR) is crucial in the management of patients after kidney transplantation (KTx). Recently, the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula was introduced to estimate GFR in chronic kidney disease patients. However, to date the diagnostic value of this equation remains to be determined in patients after KTx.

METHODS

We analysed the CKD-EPI formula in comparison to the re-expressed Modification of Diet in Renal Disease (MDRD) equation in 170 stable patients after renal transplantation. Correlation, bias, precision and accuracy within 30 and 50% of true GFR were determined. GFR was measured by technetium-diethylenetriamine pentaacetic acid clearance [39.6, 95% confidence interval (CI): 37.3-42.0 mL/min/1.73m(2)].

RESULTS

The results for the MDRD and CKD-EPI equations correlated well with GFR (0.82; 0.83, respectively). GFR calculated by MDRD (44.1, 95% CI: 41.6-46.8 mL/min/1.73m(2)) and CKD-EPI (47.7, 95% CI: 44.7-50.7 mL/min/1.73m(2)) overestimated true GFR significantly (P < 0.001). Precision was not significantly different between MDRD and CKD-EPI (10.9 versus 10.0 mL/min/1.73m(2), respectively). Accuracy within 30% of true GFR was significantly higher for MDRD (71.8%) than for CKD-EPI (64.1%, P = 0.0014). Accuracy within 50% of true GFR did not differ significantly (MDRD: 89.4% versus CKD-EPI: 84.7%, P = 0.06).

CONCLUSION

The new CKD-EPI formula did not improve the estimation of GFR in Caucasian patients after renal transplantation in this study.

Optimizing iohexol plasma disappearance curves to measure the glomerular filtration rate in children with chronic kidney disease

Measuring the glomerular filtration rate (GFR) by iohexol plasma disappearance in children requires optimization of duration and sampling times since shortening the study may overestimate GFR. To determine these parameters, we analyzed iohexol plasma disappearance curves using multiple sampling points in each of 27 children in the Chronic Kidney Disease in Children (CKiD) study. The GFR measured after 5 h was comparable to that after 6 h, but shortening to 4 h resulted in a significant 3% overestimation of GFR. We also sought to reduce the number of blood samples to determine GFR. This was done by adapting the Brochner-Mortensen equations to derive the relationship between a single-compartment two-sample (slow GFR model) disappearance curve and that from a double exponential analysis (two-compartment four-sample model) in the first clinic visit of 489 children. Using polynomial regression methods, we developed coefficients to accurately measure GFR from a single-compartment model. These coefficients were employed to recalculate the GFR and compare these to values measured with the two-compartment four-sample model in 361 of these children in their second clinic visit. There was excellent correlation (r=0.999) and no bias or change in between-individuals' dispersion. Hence, the GFR can be accurately measured in children with chronic kidney disease using the slow component of the iohexol plasma disappearance curve, provided the duration of study is at least 5 h.

Iothalamate quantification by tandem mass spectrometry to measure glomerular filtration rate

BACKGROUND

Glomerular filtration rate (GFR) can be determined by measuring renal clearance of the radiocontrast agent iothalamate. Current analytic methods for quantifying iothalamate concentrations in plasma and urine using liquid chromatography or capillary electrophoresis have limitations such as long analysis times and susceptibility to interferences. We developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to overcome these limitations.

METHODS

Urine and plasma samples were deproteinized using acetonitrile and centrifugation. The supernatant was diluted in water and analyzed by LC-MS/MS using a water:methanol gradient. We monitored 4 multiple reaction monitoring transitions: m/z 614.8-487.0, 614.8-456.0, 614.8-361.1, and 614.8-177.1. We compared the results to those obtained via our standard capillary electrophoresis (CE-UV) on samples from 53 patients undergoing clinical GFR testing.

RESULTS

Mean recovery was 90%-110% in both urine and plasma matrices. Imprecision was <or=15% for the m/z 614.8-487.0 and 614.8-456.0 transitions over a 10-day period at 1 mg/L. Method comparison for 159 patient samples (53 clearances) provided the following Passing-Bablok regressions: plasma iothalamate LC-MS/MS (y) vs CE-UV (x), y = 0.99x + 0.36; urine iothalamate LC-MS/MS vs CE-UV, y = 1.01x + 0.31; corrected GFR LC-MS/MS vs CE-UV, y = 1.00x + 0.00. Interfering substances prevented accurate iothalamate quantification by CE-UV in 2 patients, whereas these samples could be analyzed by LC-MS/MS.

CONCLUSIONS

Iothalamate can be quantified by LC-MS/MS for GFR measurement. This method circumvents potential problems with interfering substances that occasionally confound accurate GFR determinations.

Glomerular filtration rate measurement and prediction equations

Chronic kidney disease (CKD) is defined as the presence of kidney damage or a glomerular filtration rate (GFR) <60 mL/min/1.73 m(2) for three or more months. Measurement of serum creatinine is the most commonly used method to evaluated kidney function, but it must be included in formulas to estimate GFR, adjusting for age, gender and ethnicity, such as the Modification of Diet in Renal Disease (MDRD) study equation. The performance of this equation is acceptable for patients with CKD but appears to under-estimate GFR in populations with unknown kidney status. A new formula has been developed recently. The Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation appears to perform better than the MDRD equation. Cystatin C has been widely evaluated as a marker for GFR and seems to be more sensitive than creatinine. The aim of this review is to discuss the recommendations for detecting CKD, emphasizing the characteristics and limitations of GFR estimating equations and pitfalls in the evaluation of urinary albumin excretion.

Measurement and estimation of GFR in children and adolescents

GFR is the best indicator of renal function in children and adolescents and is critical for diagnosing acute and chronic kidney impairment, intervening early to prevent end-stage renal failure, prescribing nephrotoxic drugs and drugs cleared by a failing kidney, and monitoring for side effects of medications. Renal inulin clearance was the gold standard for GFR but is compromised by lack of availability, difficult assays, and problems of collecting timed urine samples, especially in children with vesicoureteral reflux or bladder dysfunction. Creatinine clearance-based estimates of GFR are often used in pediatrics. The addition of cimetidine to eliminate creatinine secretion permits accurate measurement of GFR in those who can completely empty their bladders to provide timed urine collections. Radioisotopes are used in plasma disappearance GFR determinations; however, these are not ideal for use in children, especially for repeated studies. The plasma disappearance of iohexol serves as a promising alternative GFR marker, because it is safe and not radioactive, easily measured, not metabolized or transported by the kidney, and excreted primarily by glomerular filtration. GFR estimating equations, based on serum concentrations of creatinine or cystatin C, are popular clinically and in research studies. Efforts are ongoing to improve these estimating equations for children and make the results readily available to clinicians obtaining standard chemistry profiles, as is being done for adults. However, at this time, there is no dependable substitute for an accurately determined GFR, and iohexol plasma disappearance offers the best combination of safety, accuracy, and reproducible precision.

Estimating glomerular filtration rate: Cockcroft-Gault and Modification of Diet in Renal Disease formulas compared to renal inulin clearance

BACKGROUND AND OBJECTIVES

Evaluation of renal function by estimation of the glomerular filtration rate (GFR) is very important for the diagnosis and treatment of patients with chronic kidney disease (CKD). The Cockcroft-Gault (CG) and Modification of Diet in Renal Disease (MDRD) formulas are the most commonly used estimations.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Estimated GFR values by each formula were compared with measured GFR (mGFR) by renal inulin clearance in 2208 European adults (46% women, 1.4% Caribbean blacks), with and without CKD, and mean mGFR 72.4 +/- 39.0 (range 2.2 to 177.2) ml/min/1.73 m(2).

RESULTS

Overall, the CG and MDRD formulas showed bias (mean difference) -3.5 ml/min/1.73 m(2) (5.3%), P < 0.001, and -9.8 ml/min/1.73 m(2) (-6.4%), P < 0.001; precision (SD of bias) 21.5 ml/min/1.73 m(2) (43.1%) and 20.0 ml/min/1.73 m(2) (33.0%); limits of agreement (2 SD by Bland-Altman method) 39.5 to -46.5 (range 86.0) ml/min/1.73 m(2) and 30.2 to -49.8 (range 80.0) ml/min/1.73 m(2); and accuracy within +/-30% of mGFR 70.8 and 69.0%, respectively. Both formulas showed a trend for decreasing accuracy with lower mGFR levels. According to the Kidney Disease Outcomes Quality Initiative (K/DOQI)-CKD classification's five GFR groups, the CG and MDRD formulas properly assigned 61.6 and 57.1% of the entire population and had a range of positive predictive values 42.6 to 81.8% and 39.6 to 85.2% and of negative predictive values 81.7 to 96.6% and 76.4 to 97.5%, respectively.

CONCLUSIONS

The CG and MDRD formulas had some limitations for proper GFR estimation and K/DOQI-CKD classification by GFR levels alone.

An easy to calculate equation to estimate GFR based on inulin clearance

BACKGROUND

For the estimation of renal function on the basis of serum creatinine, either the Cockcroft-Gault (CG) equation or the MDRD formula is commonly used. Compared to MDRD (using power functions), CG has the advantage of easy calculability at the bedside. MDRD, however, approaches glomerular filtration rate (GFR) more precisely than CG and gives values corrected for a body surface area (BSA) of 1.73 m(2). We wondered whether CG could be adapted to estimate GFR rather than creatinine clearance without losing the advantage of easy calculability. In this prospective study, inulin clearance under well-defined conditions was taken as the gold standard for GFR.

METHODS

In 182 living kidney donors, inulin clearance was measured under standardized conditions (protein, salt and water intake, overnight stay) before and after nephrectomy. Together with the serum creatinine level, and demographic and clinical data, 281 measurements of inulin clearance were used to compare the accuracy of different estimation equations. Using stepwise multiple regression, a new set of constants was defined for a CG-like equation in order to estimate GFR.

RESULTS

The MDRD equation underestimated GFR by 9%, and the quadratic equation suggested by Rule overestimated GFR by 12.4%. The new CG-like equation, even when calculated with 'mental arithmetic-friendly' rounded parameters, showed significantly less bias (1.2%). The adapted equation is GFR[mL/min] = ((155 - Age[years]) x weight [kg]/serum creatinine [micromol/L]) x 0.85 if female.

CONCLUSIONS

We propose the CG-like equation called IB-eGFR (Inulinclearance Based eGFR) to estimate GFR more reliably than MDRD, Rule's equation or the original Cockcroft-Gault equation. As our data represent a Caucasian population, the adapted equation is still to be validated for patients of other ethnicity.

Prediction of relative glomerular filtration rate in adults: new improved equations based on Swedish Caucasians and standardized plasma-creatinine assays

OBJECTIVE

To evaluate newly developed equations predicting relative glomerular filtration rate(GFR) in adult Swedish Caucasians and to compare with the Modification of Diet in Renal Disease(MDRD) and Mayo Clinic equations using enzymatic and zero-calibrated plasma creatinine assays.

MATERIAL AND METHODS

GFR was measured with iohexol clearance adjusted to 1.73 m(2). One population sample (n=436/Lund) was used to derive an equation based on plasma-creatinine/age/gender, and a second with the addition of lean body mass (LBM). Both equations were validated in a separate sample (n=414/Malmö). The coefficients of the equations were eventually fine-tuned using all 850 patients and yielding Lund-Malmö equations without (LM) and with LBM-term (LM(LBM)). Their performance was compared with the MDRD(CC) (conventional creatinine calibration), MDRD(IDMS) (isotope dilution mass spectroscopy traceable calibration) and Mayo Clinic equations.

RESULTS

The Lund equations performed similarly in both samples. In the combined set, the Mayo Clinic/MDRD(CC) resulted in +19.0/+10.2 % median bias, while bias for the other equations was < 10 %. LM(LBM) had the highest accuracy (86 % of estimates within 30 % of measured GFR), significantly (p < 0.001) better than for MDRD(IDMS) (80 %). In men with BMI < 20 kg/m(2), MDRD(IDMS)/LM had +46 %/+19 % median bias. MDRD(IDMS) also overestimated GFR by 22 %/14 % in men/women above 80 years of age. The LM(LBM) equation had < 10 % bias irrespective of BMI, age or GFR except for a 15 % negative bias at GFR > 90 mL/min/1.73 m(2).

CONCLUSION

The newly developed Lund-Malmö equations for GFR estimation performed better than the MDRD(IDMS) and Mayo Clinic equations in a Swedish Caucasian sample. Inclusion of an LBM term improved performance markedly in certain subgroups.