Comparison of Plasma Clearance With Early-Compartment Correction Equations and Urinary Clearance in High GFR Ranges

Iohexol plasma clearance measurement protocol standardization for adults: a consensus paper of the European Kidney Function Consortium

International consensus supports the development of standardized protocols for measured glomerular filtration rate (mGFR) to facilitate the integration of mGFR testing in both clinical and research settings. To this end, the European Kidney Function Consortium convened an international group of experts with relevant experience in mGFR. The working group performed an extensive literature search to inform the development of recommendations for mGFR determination using 1-compartment plasma clearance models and iohexol as the exogenous filtration marker. Iohexol was selected as it is non-radio labeled, inexpensive, and safe, can be assayed at a central laboratory, and the other commonly used non-radio-labeled tracers have been (inulin) or are soon to be (iothalamate) discontinued. A plasma clearance model was selected over urine clearance as it requires no urine collection. A 1 compartment was preferred to 2 compartments as it requires fewer samples. The recommendations are based on published evidence complemented by expert opinion. The consensus paper covers practical advice for patients and health professionals, preparation, administration, and safety aspects of iohexol, laboratory analysis, blood sample collection and sampling times using both multiple and single-sample protocols, description of the mGFR mathematical calculations, as well as implementation strategies. Supplementary materials include patient and provider information sheets, standard operating procedures, a study protocol template, and support for mGFR calculation.

Accuracy of Shorter Iohexol GFR Measurement Protocols in Individuals with Preserved Kidney Function

Key Points

Shorter measured GFR protocols are accurate and precise compared with the reference standard measured GFR protocol in patients with preserved GFR. These shorter protocols can potentially improve the adoption of GFR measurement more widely by reducing procedural time and cost.

Background

Measured GFR (mGFR) using exogenous tracers is recommended in a number of settings. Plasma one-compartment multisample protocols (MSPs) are the most commonly used, with iohexol being the dominant tracer. The accuracy of MSPs has mostly been evaluated in the setting of reduced GFR where delayed initial and final samples are recommended. Much less is known about MSPs when GFR is not decreased, and the default protocol tends to include initial sampling at 120 minutes and final sampling at 240 minutes after iohexol injection. The recent Kidney Disease Improving Global Outcomes 2024 Clinical Practice Guideline for the Evaluation and Management of CKD includes research recommendations for the development of shorter more efficient mGFR protocols. The objective of this study was to assess the performance of shorter MSPs with earlier initial (60 and 90 minutes) and final (150, 180, and 210 minutes) sampling times in individuals with preserved GFR. Reference mGFR (R-mGFR) was calculated using five samples collected between 120 and 240 minutes.

Methods

Four different combinations of shorter sampling strategies were investigated. Performance was evaluated using measurements of bias, precision, and accuracy (P2, P5, and mean absolute error).

Results

The mean R-mGFR of the 43 participants was 102.3±13.7 ml/min per 1.73 m2. All shorter mGFRs had biases <1 ml/min per 1.73 m2 and mean absolute error <1.6 ml/min per 1.73 m2. All shorter mGFRs were within 5% of the R-mGFR, and the majority were within 2%.

Conclusions

These results demonstrate that shortening the mGFR procedure in individuals with preserved GFR provides similar results to the current standard while significantly decreasing procedure time.

A physiological model for iohexol plasma clearance supporting diagnostics of kidney function

BACKGROUND

There are several shortcomings in present methods for estimation of GFR from plasma clearance. The aim of the present study was therefore to develop a physiologically based method for calculation of plasma clearance of iohexol.

METHODS

A mechanistic model founded on classical biochemical engineering principles where in- and outgoing molecular flows of iohexol between plasma and surrounding tissues were balanced over time. After intravenous injections of iohexol, plasma samples were taken from the investigated subjects until complete elimination of iohexol. After tuning of the model parameters, the clearance value was calculated from the injected dose and the integral of the iohexol concentrations over the investigated period.

RESULTS

The mass balance model was able to predict the time course of iohexol distribution and elimination after parameterization of mass balance and kinetic equations. Four model structures were evaluated, all based on model parameters derived from published data and from internal tests, each complied at varying physiological conditions. Iohexol clearance was assessed through the model and compared with calculations from previously practiced methods. When testing the mass balance model on ten healthy subjects, clearance was estimated accurately.

CONCLUSIONS

The physiological and mechanistic character of the mass balance model may suggest that its derived clearance comes closer to actual in vivo conditions than data derived from previously practiced calculation methods. Although here, only verified with the clearance marker iohexol, the mass balance model should be applicable also to other renal clearance markers.

Performance of creatinine-based equations to estimate glomerular filtration rate in White and Black populations in Europe, Brazil and Africa

BACKGROUND

A new Chronic Kidney Disease Epidemiology Collaboration equation without the race variable has been recently proposed (CKD-EPIAS). This equation has neither been validated outside USA nor compared with the new European Kidney Function Consortium (EKFC) and Lund-Malmö Revised (LMREV) equations, developed in European cohorts.

METHODS

Standardized creatinine and measured glomerular filtration rate (GFR) from the European EKFC cohorts (n = 13 856 including 6031 individuals in the external validation cohort), from France (n = 4429, including 964 Black Europeans), from Brazil (n = 100) and from Africa (n = 508) were used to test the performances of the equations. A matched analysis between White Europeans and Black Africans or Black Europeans was performed.

RESULTS

In White Europeans (n = 9496), both the EKFC and LMREV equations outperformed CKD-EPIAS (bias of -0.6 and -3.2, respectively versus 5.0 mL/min/1.73 m², and accuracy within 30% of 86.9 and 87.4, respectively, versus 80.9%). In Black Europeans and Black Africans, the best performance was observed with the EKFC equation using a specific Q-value (= concentration of serum creatinine in healthy males and females). These results were confirmed in matched analyses, which showed that serum creatinine concentrations were different in White Europeans, Black Europeans and Black Africans for the same measured GFR, age, sex and body mass index. Creatinine differences were more relevant in males.

CONCLUSION

In a European and African cohort, the performances of CKD-EPIAS remain suboptimal. The EKFC equation, using usual or dedicated population-specific Q-values, presents the best performance in the whole age range in the European and African populations included in this study.

Standardization of serum creatinine is essential for accurate use of unbiased estimated GFR equations: evidence from three cohorts matched on renal function

BACKGROUND

Differences in the performance of estimated glomerular filtration rate (eGFR) equations have been attributed to the mathematical form of the equations and to differences between patient demographics and measurement methods. We evaluated differences in serum creatinine (SCr) and eGFR in cohorts matched for age, sex, body mass index (BMI) and measured GFR (mGFR).

METHODS

White North Americans from Minnesota (n = 1093) and the Chronic Renal Insufficiency Cohort (CRIC) (n = 1548) and White subjects from the European Kidney Function Consortium (EKFC) cohort (n = 7727) were matched for demographic patient characteristics (sex, age ± 3 years, BMI ± 2.5 kg/m2) and renal function (mGFR ± 3 ml/min/1.73 m2). SCr was measured with isotope dilution mass spectrometry (IDMS)-traceable assays in the Minnesota and EKFC cohorts and with non-standardized SCr assays recalculated to IDMS in the CRIC. The Minnesota cohort and CRIC shared a common method to measure GFR (renal clearance of iothalamate), while the EKFC cohort used a variety of exogenous markers and methods, all with recognized sufficient accuracy. We compared the SCr levels and eGFR predictions [for Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) and EKFC equations] of patients fulfilling these matching criteria.

RESULTS

For 305 matched individuals, mean SCr (mg/dL) was not different between the Minnesota and EKFC cohorts (females 0.83 ± 0.20 versus 0.86 ± 0.23, males 1.06 ± 0.23 versus 1.12 ± 0.37; P > .05) but significantly different from the CRIC [females 1.13 ± 0.23 (P < .0001), males 1.42 ± 0.31 (P < .0001)]. The CKD-EPI equations performed better than the EKFC equation in the CRIC, while the opposite was true in the Minnesota and EKFC cohorts.

CONCLUSION

Significant differences in SCr concentrations between the Minnesota and EKFC cohorts versus CRIC were observed in subjects with the same level of mGFR and equal demographic characteristics and can be explained by the difference in SCr calibration.

Endogenous markers of kidney function and renal drug clearance processes of filtration, secretion, and reabsorption

The kidneys are responsible for maintaining physiologic homeostasis. The kidneys clear a variety of drugs and other substances through passive (filtration) and active processes that utilize transport proteins. Renal clearance is comprised of the processes of glomerular filtration, tubular secretion, and tubular reabsorption. Endogenous biomarkers, such as creatinine and cystatin C, are routinely used to estimate renal clearance. Understanding the contributing components of renal function and clearance, through the use of biomarkers, is necessary in elucidating the renal pharmacology of drugs and other substances. While exogenous markers of kidney function have been known for decades, several complexities have limited their usage. Several endogenous markers are being evaluated and hold promise to elucidate the individual components of kidney function that represent filtration, secretion, and reabsorption.