Simultaneous glomerular filtration rate determination using inulin, iohexol, and 99mTc-DTPA demonstrates the need for customized measurement protocols

Nonindexed versus Body Surface Area-Indexed Measured GFR Determinations as a Criterion of Living Donor Acceptance

Key Points

Living kidney donors with discordant values of measured GFR were at increased risk of developing eGFR <45 ml/min per 1.73 m2. Careful consideration should be given when assessing potential LKDs with large differences in their nonindexed and body surface area–indexed measured GFR.

Background

When GFR is measured (measured GFR [mGFR]) using iohexol plasma clearance, results are reported both as nonindexed (ml/min) and body surface area (BSA) indexed to 1.73 m2. When these two values differ, there is no consensus as to which is preferable to use to determine suitability for living kidney donor. We sought to compare the difference between nonindexed and BSA-indexed mGFR in LKDs and the association with postdonation eGFR.

Methods

Between January 1, 2007, and January 1, 2023, 627 adult living kidney donors (LKD) at the University of Minnesota had predonation mGFR by iohexol plasma clearance and a minimum 6-month follow-up. LKD acceptance was based on a nonindexed mGFR ≥80 ml/min (age younger than 60 years) or ≥75 ml/min (age 60 years and older). Primary outcomes included eGFR at 1 year postdonation and sustained eGFR <45 ml/min per 1.73 m2.

Results

Among 627 LKDs, 561 (90%) had both a nonindexed and BSA-indexed mGFR above the age-based threshold (concordant), while 66 (11%) had nonindexed measurements above and BSA indexed below (discordant). Compared with concordant LKDs, discordant LKDs were older (median: 54.1 versus 42.8 years, P < 0.001) and had higher body mass indices (28.0 versus 26.1, P < 0.001). At 1 year postdonation, mean eGFR was higher among concordant LKDs, although the difference in relative change from predonation eGFR measurements was similar. During a median follow-up of 2.3 years, six of 66 discordant LKDs (9%) experienced sustained eGFR <45 ml/min per 1.73 m2 compared with five of 561 concordant LKDs (0.9%) (hazard ratio, 10.7; 95% confidence interval, 3.21 to 35.6).

Conclusions

Discordant LKDs had lower eGFR measurements postdonation and experienced a higher risk of eGFR <45 ml/min per 1.73 m2.

Cisplatin eligibility in the neoadjuvant setting of patients with muscle-invasive bladder cancer undergoing radical cystectomy

BACKGROUND

To examine the agreement of different calculated estimated glomerular filtration rate (eGFR) formulas and measured creatinine clearance (CrCI) at the primary diagnosis of muscle-invasive bladder cancer (MIBC).

MATERIALS AND METHODS

We performed a multicenter analysis of patients with MIBC, treated with cisplatin-based neoadjuvant chemotherapy (NAC) and radical cystectomy (RC), or with RC alone, between 2011 and 2021. Baseline eGFR was computed using 4 calculated serum equations including Cockcroft-Gault (CG), MDRD, CKD-EPI 2009, and race-free CKD-EPI 2021. To examine the association between calculated eGFR and measured CrCI, subgroup analyses were performed among patients in whom measured 24-hour urine CrCl was determined. Cisplatin-ineligibility was defined as CrCI and/or eGFR < 60 mL/minute per 1.73 m2.

RESULTS

Of 956 patients, 30.0%, 33.3%, 31.9%, and 27.7% were found to be cisplatin-ineligible by the CG, MDRD, CKD-EPI, and race-free CKD-EPI equations (P = .052). The concordance between calculated eGFR formulas was rated substantial (Cohen's kappa (k): 0.66-0.95). Among the subgroup (n = 245) with measured CrCl, 37 (15.1%) patients had a CrCI less than 60 mL/minute. Concordance between measured CrCl and calculated eGFR was poor (ĸ: 0.29-0.40). All calculated eGFR formulas markedly underestimated the measured CrCI. Specifically, 78%-87.5% of patients with a calculated eGFR between 40 and 59 mL/minute exhibited a measured CrCI ≥ 60 mL/minute.

CONCLUSIONS

Comparing calculated eGFR formulas, similar percentages of patients with MIBC were deemed cisplatin-ineligible. However, a significant number of patients could be upgraded by being cisplatin-fit based on measured CrCI, particularly when the calculated eGFR was falling within the gray range of 40-59 mL/minute.

Comparison of eGFR Equations to Guide Dosing of Medications for Kidney Transplant Recipients

BACKGROUND

Clinicians caring for kidney transplant recipients (KTRs) most commonly use estimated glomerular filtration rate (eGFR) to guide medication dosing as it is the most readily available measure of kidney function. Which eGFR equations provide the most accurate medication dosing guidance for KTRs remains uncertain.

METHODS

We studied 415 stable KTRs in Canada and New Zealand. Participants completed same-day measurements of creatinine and cystatin C and measured GFR (diethylenetriaminepentaacetic acid). Chronic Kidney Disease Epidemiology Collaboration, European Kidney Function Consortium, and transplant-specific eGFR equations were compared with both Cockcroft-Gault creatinine clearance (CrCl) and measured GFR. eGFR equations were assessed both indexed to a standardized body surface area (BSA) of 1.73 m 2 (milliliter per minute per 1.73 m 2 , as is conventional reporting from most clinical laboratories) and nonindexed (milliliter per minute) accounting for actual BSA. The primary outcome was the proportion of medication dosing discordance relative to Cockcroft-Gault CrCl or measured GFR for 8 commonly prescribed medications. Stratified analyses were performed on the basis of obesity status.

RESULTS

Nonindexed eGFR equations (milliliter per minute) resulted in substantially lower medication dosing discordance compared with indexed eGFR equations (milliliter per minute per 1.73 m 2 ). These findings were most pronounced among KTRs with obesity, in whom underdosing was frequent. When compared with Cockcroft-Gault CrCl, the lowest proportion of discordance was found with the nonindexed 2023 transplant-specific equation. When compared with measured GFR, the lowest proportion of discordance was found with the nonindexed 2021 Chronic Kidney Disease Epidemiology Collaboration Cr/CysC equation.

CONCLUSIONS

Nonindexed eGFR values accounting for actual BSA should be used by clinicians for medication dosing in KTRs. These findings may inform KT providers about which eGFR equations provide the safest, most accurate medication dosing guidance for KTRs.

Serum total testosterone is associated with phosphate and calcium excretion in response to oral phosphate loading in healthy middle-aged males

Androgen receptors are expressed in the kidney and serum testosterone is negatively associated with serum phosphate in males, suggesting a role of testosterone in renal phosphate handling. In this cross-sectional study, we examined the association of serum total and free testosterone with acute phosphate and calcium excretion in males in response to an oral phosphate challenge. Thirty-five healthy adult males with normal baseline testosterone levels consumed a 500 mg phosphorus drink and the urinary excretion of minerals, as well as levels of relevant circulating parameters, were assessed at baseline and hourly for 4 h. Serum total testosterone was positively associated with overall phosphate excretion (r = 0.35, p = 0.04) and calcium excretion (r = 0.44, p = 0.00) in response to the challenge. Serum free testosterone was positively associated with post-challenge calcium excretion (r = 0.34, p = 0.048), but significance was not reached for phosphate excretion (r = 0.31, p = 0.07). Serum total and free testosterone were not associated with parathyroid hormone, fibroblast growth factor-23, or vitamin D-key factors implicated in phosphate and calcium regulation. Overall, higher serum total testosterone levels in healthy middle-aged males are associated with a greater capacity to acutely excrete phosphate and calcium after a single oral phosphate challenge, suggesting potential ramifications of testosterone deficiency related to mineral homeostasis.

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.

Clinical validation of the novel fluorescent glomerular filtration rate tracer agent relmapirazin (MB-102)

The fluorescent compound relmapirazin has been rationally designed for use in point-of-care measurement of glomerular filtration rate (GFR), with attributes including negligible protein binding, negligible metabolites in vivo, negligible tubular secretion, and excellent chemical and photo stability. Twenty-four nonclinical assays were performed in accordance with FDA requirements yielding negligible toxicology concerns. Here, a clinical study was performed to validate relmapirazin as a GFR tracer in patients by comparison to iohexol. This was evaluated in 120 adults at three clinical sites with eGFR values ranging from normal to Stage 4 chronic kidney disease. Relmapirazin and iohexol were administered intravenously in consecutive boluses to each subject and serial blood samples obtained over the subsequent 12 hours. Plasma concentrations were measured and the corresponding plasma GFR for each agent was determined using a standard two-compartment pharmacokinetic assessment. Urine from each subject was collected for the entire 12-hour study period to measure the amount of administered dose appearing in the urine. A near perfect linear regression correlation was observed between the GFRs measured by these two tracers (r2=0.99). Bland-Altman analysis confirmed agreement between these two measures of GFR (limits of agreement -7.0 to +5.6 mL/min; mean of -0.7 mL/min). The GFR determined by relmapirazin was independent of GFR stratification by chronic kidney disease stage, and importantly by race. The percent of the administered relmapirazin dose recovered in the urine was greater than or equal to that of iohexol with no reported severe adverse events. Thus, relmapirazin may be used as a GFR tracer agent in humans.

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.

Residual Kidney Function in Hemodialysis: Its Importance and Contribution to Improved Patient Outcomes

Individuals afflicted with advanced kidney dysfunction who require dialysis for medical management exhibit different degrees of native kidney function, called residual kidney function (RKF), ranging from nil to appreciable levels. The primary focus of this manuscript is to delve into the concept of RKF, a pivotal yet under-represented topic in nephrology. To begin, we unpack the definition and intrinsic nature of RKF. We then juxtapose the efficiency of RKF against that of hemodialysis in preserving homeostatic equilibrium and facilitating physiological functions. Given the complex interplay of RKF and overall patient health, we shed light on the extent of its influence on patient outcomes, particularly in those living with advanced kidney dysfunction and on dialysis. This manuscript subsequently presents methodologies and measures to assess RKF, concluding with the potential benefits of targeted interventions aimed at preserving RKF.

Discrepancies between transcutaneous and estimated glomerular filtration rates in rats with chronic kidney disease

Accurate assessment of the glomerular filtration rate (GFR) is crucial for researching kidney disease in rats. Although validation of methods that assess GFR is crucial, large-scale comparisons between different methods are lacking. Both transcutaneous GFR (tGFR) and a newly developed estimated GFR (eGFR) equation by our group provide a low-invasive approach enabling repeated measurements. The tGFR is a single bolus method using FITC-labeled sinistrin to measure GFR based on half-life of the transcutaneous signal, whilst the eGFR is based on urinary sinistrin clearance. Here, we retrospectively compared tGFR, using both 1- and 3- compartment models (tGFR_1c and tGFR_3c, respectively) to the eGFR in a historic cohort of 43 healthy male rats and 84 male rats with various models of chronic kidney disease. The eGFR was on average considerably lower than tGFR-1c and tGFR-3c (mean differences 855 and 216 μL/min, respectively) and only 20 and 47% of measurements were within 30% of each other, respectively. The relative difference between eGFR and tGFR was highest in rats with the lowest GFR. Possible explanations for the divergence are problems inherent to tGFR, such as technical issues with signal measurement, description of the signal kinetics, and translation of half-life to tGFR, which depends on distribution volume. The unknown impact of isoflurane anesthesia used in determining mGFR remains a limiting factor. Thus, our study shows that there is a severe disagreement between GFR measured by tGFR and eGFR, stressing the need for more rigorous validation of the tGFR and possible adjustments to the underlying technique.

Serum Cystatin C as a Risk Factor for Supratherapeutic Digoxin Concentration in Elderly Patients with Heart Failure and Chronic Kidney Disease

BACKGROUND

Digoxin is primarily metabolized by the kidney, and its toxicity is strongly associated with high concentrations, particularly in elderly patients. The purpose of this study was to evaluate the predictive performance of renal function biomarkers for supratherapeutic digoxin concentrations in elderly patients with heart failure (HF) and chronic kidney disease (CKD).

METHODS

Data were retrospectively obtained from elderly patient with HF and CKD who received digoxin treatment from January 2022 and December 2022. Logistic regression was used to assess independent risk factors for supratherapeutic concentrations. The predictive performance of serum creatinine, serum cystatin C, and blood urea nitrogen on supratherapeutic concentrations was compared by receiver operating characteristic analysis.

RESULTS

A total of 115 elderly patients with HF and CKD were enrolled in our study. Supratherapeutic concentrations were detected in 49 patients. Logistic regression analysis showed that estimated glomerular filtration rate calculated by serum cystatin C [eGFRCysC, odds ratio (OR): 0.962, P = 0.006], heart rate (OR: 1.024, P = 0.040), and NYHA class (OR: 3.099, P = 0.010) were independent risk factors for supratherapeutic concentration. Cutoff value for eGFRCysC between the two groups was 41 ml/min/1.73m2. Predictive performance of serum cystatin C was further improved in patients with obesity, CKD stage 4-5, and older than 75 years compared with normal weight, CKD stage 3, and aged 60-75-year-old patients.

CONCLUSIONS

Serum cystatin C is a sensitive renal function biomarker to predict supratherapeutic digoxin concentration in elderly patients with HF and CKD.

External Validation of a Novel Multimarker GFR Estimating Equation

Key Points

Using multiple markers may improve GFR estimation especially in settings where creatinine and cystatin C are known to be limited. Panel eGFR is a novel multimarker eGFR equation consisting of age, sex, cystatin C, and nuclear magnetic resonance–measured creatinine, valine, and myo-inositol. eGFR-Cr and eGFR-Cr-CysC may underestimate measured GFR, while panel eGFR was unbiased among younger Black male individuals.

Background

Using multiple markers may improve accuracy in GFR estimation. We sought to externally validate and compare the performance of a novel multimarker eGFR (panel eGFR) equation among Black and White persons using the Genetic Epidemiology Network of Arteriopathy cohort.

Methods

We included 224 sex, race/ethnicity, and measured GFR (mGFR) category–matched persons, with GFR measured using urinary clearance of iothalamate. We calculated panel eGFR using serum creatinine, valine, myo-inositol, cystatin C, age, and sex. We compared its reliability with current eGFR equations (2021 CKD Epidemiology Collaboration creatinine [eGFR-Cr] and creatinine with cystatin C [eGFR-Cr-CysC]) using median bias, precision, and accuracy metrics. We evaluated each equation's performance in age, sex, and race subgroups.

Results

In the overall cohort, 49% were Black individuals, and mean mGFR was 79 ml/min per 1.73 m2. Panel eGFR overestimated mGFR (bias: −2.4 ml/min per 1.73 m2; 95% confidence interval [CI], −4.4 to −0.7), eGFR-Cr-CysC underestimated mGFR (bias: 4.8 ml/min per 1.73 m2; 95% CI, 2.1 to 6.7), while eGFR-Cr was unbiased (bias: 2.0 ml/min per 1.73 m2; 95% CI, −1.1 to 4.6). All equations had comparable accuracy. Among Black male individuals younger than 65 years, both eGFR-Cr (bias: 17.0 ml/min per 1.73 m2; 95% CI, 8.6 to 23.5) and eGFR-Cr-CysC (bias: 14.5 ml/min per 1.73 m2; 95% CI, 6.0 to 19.7) underestimated mGFR, whereas panel eGFR was unbiased (bias: 1.7 ml/min per 1.73 m2; 95% CI, −3.4 to 10.0). Metrics of accuracy for all eGFRs were acceptable in all subgroups except for panel eGFR in Black female individuals younger than 65 years (P30: 73.3%).

Conclusions

Panel eGFR can be used to estimate mGFR and may have utility among Black male individuals younger than 65 years where current CKD Epidemiology Collaboration equations are biased.

Longitudinal Estimated Glomerular Filtration Rate Trajectories in Children with Type 1 Diabetes

Although children with type 1 diabetes (T1D) are at risk for developing diabetic kidney disease (DKD), clinical practice guidelines do not uniformly recommend routine serum creatinine (SCr) monitoring, and data describing changes in renal function from diagnosis are lacking. As part of a quality improvement initiative, the Diabetes Clinic at British Columbia Children's Hospital in Vancouver, Canada, implemented routine serum SCr monitoring. This study describes estimated glomerular filtration rate (eGFR) trajectories and prevalence of decreased eGFR, hypertension, and albuminuria and their relationship to patterns of nephrology referral in a cohort of children aged ≤18 years (n = 307) with T1D recruited between December 2016 and February 2019. Annualized eGFR (ml/min/1.73 m2 per year) was calculated using the CKiD U25 formula and categorized as declining (<-3), stable (-3 to +3), and inclining (>+3). eGFR was categorized as normal (≥90), mildly decreased (60 to <90), and chronic kidney disease (CKD, <60). In this cohort, 54% were male; the median age at diagnosis and duration of T1D was 6.2 years and 6.9 years, respectively. Over a median follow-up of 2.3 years, declining, stable, and inclining trajectories were observed in 33%, 32%, and 35%, respectively. During their follow-up, 32% had mildly decreased eGFR, elevated blood pressures (≥90th percentile), and/or abnormal urine albumin-creatinine ratios (≥2 mg/mmol), with <10% referred for nephrology assessment. Twenty-three percent of subjects had an eGFR <90; this subgroup was more highly represented in the declining trajectory group (vs. stable and inclining). Logistic regression analysis found female sex and higher baseline eGFR to be associated with a declining eGFR trajectory. In conclusion, these data challenge the commonly held paradigm that renal function remains stable in childhood T1D and supports systematic monitoring of renal function in children with T1D, as well as collaboration across disciplines, particularly endocrinology and nephrology, to provide evidence-based individualized care.

A back propagation neural network approach to estimate the glomerular filtration rate in an older population

BACKGROUND

The use of creatinine-based glomerular filtration rate (GFR)-estimating equations to evaluate kidney function in elderly individuals does not appear to offer any performance advantages. We therefore aimed to develop an accurate GFR-estimating tool for this age group.

METHODS

Adults aged ≥ 65 years who underwent GFR measurement by technetium-99 m-diethylene triamine pentaacetic acid (^99mTc-DTPA) renal dynamic imaging were included. Data were randomly split into a training set containing 80% of the participants and a test set containing the remaining 20% of the subjects. The Back propagation neural network (BPNN) approach was used to derive a novel GFR estimation tool; then we compared the performance of the BPNN tool with six creatinine-based equations (Chronic Kidney Disease-Epidemiology Collaboration [CKD-EPI], European Kidney Function Consortium [EKFC], Berlin Initiative Study-1 [BIS1], Lund-Malmö Revised [LMR], Asian modified CKD-EPI, and Modification of Diet in Renal Disease [MDRD]) in the test cohort. Three equation performance criteria were considered: bias (difference between measured GFR and estimated GFR), precision (interquartile range [IQR] of the median difference), and accuracy P30 (percentage of GFR estimates that are within 30% of measured GFR).

RESULTS

The study included 1,222 older adults. The mean age of both the training cohort (n = 978) and the test cohort (n = 244) was 72 ± 6 years, with 544 (55.6%) and 129 (52.9%) males, respectively. The median bias of BPNN was 2.06 ml/min/1.73 m², which was smaller than that of LMR (4.59 ml/min/1.73 m²; p = 0.03), and higher than that of the Asian modified CKD-EPI (-1.43 ml/min/1.73 m²; p = 0.02). The median bias between BPNN and each of CKD-EPI (2.19 ml/min/1.73 m²; p = 0.31), EKFC (-1.41 ml/min/1.73 m²; p = 0.26), BIS1 (0.64 ml/min/1.73 m²; p = 0.99), and MDRD (1.11 ml/min/1.73 m²; p = 0.45) was not significant. However, the BPNN had the highest precision IQR (14.31 ml/min/1.73 m²) and the greatest accuracy P30 among all equations (78.28%). At measured GFR < 45 ml/min/1.73 m², the BPNN has highest accuracy P30 (70.69%), and highest precision IQR (12.46 ml/min/1.73 m²). The biases of BPNN and BIS1 equations were similar (0.74 [-1.55-2.78] and 0.24 [-2.58-1.61], respectively), smaller than any other equation.

CONCLUSIONS

The novel BPNN tool is more accurate than the currently available creatinine-based GFR estimation equations in an older population and could be recommended for routine clinical use.

Sex Differences in Phosphate Homeostasis: Females Excrete More Phosphate and Calcium After an Oral Phosphate Challenge

CONTEXT

Dietary consumption of phosphate is increasing, and elevated serum phosphate is associated with increased cardiovascular disease (CVD) risk. Sex differences in phosphate homeostasis and response to changes in dietary phosphate intake, which are not captured by clinically measured analytes, may contribute to differences in CVD presentation and bone disease.

OBJECTIVE

To assess sex differences in acute phosphate homeostasis in response to a single oral phosphate challenge.

DESIGN

Cross-sectional.

SETTING

General community.

PARTICIPANTS

78 participants (40-76 years) with measured glomerular filtration rate >60 mL/min/1.73 m2 and no clinically diagnosed CVD and 14 young healthy adults.

MAIN OUTCOME MEASURES

To elucidate subtle alterations in phosphate homeostasis, we employ an acute challenge whereby the hormonal response, circulating mineral levels, and urinary excretion are assessed following an oral challenge of phosphate.

RESULTS

Although both males and females had similar changes in circulating phosphate, calcium, and parathyroid hormone in response to the challenge, females excreted ∼1.9x more phosphate and ∼2.7x more calcium than males, despite not consuming calcium. These sex differences were recapitulated in healthy young adults. This excretion response did not correlate to age, serum phosphate, or estradiol levels. The females with greater excretion of phosphate had higher levels of bone resorption markers compared to formation markers.

CONCLUSIONS

Taken together, these data identify sex differences in acute phosphate homeostasis, specifically that females may mobilize and excrete endogenous sources of calcium and phosphate in response to oral phosphate compared to males. While high levels of dietary phosphate negatively impact bone, our results suggest that females may incur more risk from these diets.

Hepatic and renal improvements with FXR agonist vonafexor in individuals with suspected fibrotic NASH

BACKGROUND & AIMS

The LIVIFY trial investigated the safety, tolerability, and efficacy of vonafexor, a second-generation, non-bile acid farnesoid X receptor agonist in patients with suspected fibrotic non-alcoholic steatohepatitis (NASH).

METHODS

This double-blind phase IIa study was conducted in two parts. Patients were randomised (1:1:1:1) to receive placebo, vonafexor 100 mg twice daily (VONA-100BID), vonafexor 200 mg once daily (VONA-200QD), or 400 mg vonafexor QD (VONA-400QD) in Part A (safety run-in, pharmacokinetics/pharmacodynamics) or placebo, vonafexor 100 mg QD (VONA-100QD), or VONA-200QD (1:1:1) in Part B. The primary efficacy endpoint was a reduction in liver fat content (LFC) by MRI-proton density fat fraction, while secondary endpoints included reduced corrected T1 values and liver enzymes, from baseline to Week 12.

RESULTS

One hundred and twenty patients were randomised (Part A, n = 24; Part B, n = 96). In Part B, there was a significant reduction in least-square mean (SE) absolute change in LFC from baseline to Week 12 for VONA-100QD (-6.3% [0.9]) and VONA-200QD (-5.4% [0.9]), vs. placebo (-2.3% [0.9], p = 0.002 and 0.012, respectively). A >30% relative LFC reduction was achieved by 50.0% and 39.3% of patients in the VONA-100QD and VONA-200QD arms, respectively, but only in 12.5% in the placebo arm. Reductions in body weight, liver enzymes, and corrected T1 were also observed with vonafexor. Creatinine-based glomerular filtration rate improved in the active arms but not the placebo arm. Mild to moderate generalised pruritus was reported in 6.3%, 9.7%, and 18.2% of participants in the placebo, VONA-100QD, and VONA-200QD arms, respectively.

CONCLUSIONS

In patients with suspected fibrotic NASH, vonafexor was safe and induced potent liver fat reduction, improvement in liver enzymes, weight loss, and a possible renal benefit.

CLINICAL TRIAL NUMBER (EUDRACT)

2018-003119-22.

CLINICALTRIALS

GOV IDENTIFIER

NCT03812029.

IMPACT AND IMPLICATIONS

Non-alcoholic steatohepatitis (NASH) has become a leading cause of chronic liver disease worldwide. Affected patients are also at higher risk of developing chronic kidney disease. There are no approved therapies and only few options to treat this population. The phase IIa LIVIFY trial results show that single daily administration of oral vonafexor, an FXR agonist, leads in the short term to a reduction in liver fat, liver enzymes, fibrosis biomarkers, body weight and abdominal circumference, and a possible improvement in kidney function, while possible mild moderate pruritus (a peripheral FXR class effect) and an LDL-cholesterol increase are manageable with lower doses and statins. These results support exploration in longer and larger trials, with the aim of addressing the unmet medical need in NASH.

Optimal Glomerular Filtration Rate Equations for Various Age Groups, Disease Conditions and Ethnicities in Asia: A Systematic Review

(1) Background: The performance of estimated glomerular filtration rate (eGFR) equations in the Asian population has been widely questioned. The primary objective of this study was to gather evidence regarding optimal GFR equations in Asia for various age groups, disease conditions, and ethnicities. The secondary objective was to see whether the equations based on the combination of creatinine and cystatin C biomarkers if employed are satisfactory across different age groups and disease conditions in various ethnicities in Asia compared to those based on either of the single biomarkers. (2) Methods: Validation studies that had both creatinine and cystatin C-based equations either alone or in combination, validated in specific disease conditions, and those which compared the performance of these equations with exogenous markers were eligible only. The bias, precision, and 30% accuracy (P30) of each equation were recorded accordingly. (3) Results: Twenty-one studies consisting of 11,371 participants were included and 54 equations were extracted. The bias, precision, and P30 accuracies of the equations ranged from -14.54 to 9.96 mL/min/1.73 m2, 1.61 to 59.85 mL/min/1.73 m2, and 4.7% to 96.10%. The highest values of P30 accuracies were found for the JSN-CKDI equation (96.10%) in Chinese adult renal transplant recipients, for the BIS-2 equation (94.5%) in Chinese elderly CKD patients, and Filler equation (93.70%) also in Chinese adult renal transplant recipients. (4) Conclusions: Optimal equations were identified accordingly and it was proven that combination biomarker equations are more precise and accurate in most of the age groups and disease conditions. These can be considered equations of choice for the specific age groups, disease conditions, and ethnicities within Asia.

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.

Performance of the 2021 Race-Free CKD-EPI Creatinine- and Cystatin C-Based Estimated GFR Equations Among Kidney Transplant Recipients

RATIONALE & OBJECTIVE

Race-free estimated glomerular filtration rate (eGFR) equations incorporating creatinine with and without cystatin C were recently developed and recommended for routine use. However, the performance of these equations among kidney transplant recipients (KTRs) remains unknown.

STUDY DESIGN

Cross-sectional study to validate the 2021 race-free Chronic Kidney Disease (CKD) Epidemiology Collaboration (CKD-EPI) eGFR equation based on creatinine alone (eGFR_cr) or based on creatinine and cystatin C (eGFR_cr-cys) among KTRs.

SETTING & PARTICIPANTS

KTRs in stable condition (N = 415) from Canada and New Zealand with same-day measurements of creatinine, cystatin C, and glomerular filtration rate (GFR) using radiolabeled diethylenetriaminepentaacetic acid.

TESTS COMPARED

The 2009 CKD-EPI eGFR_cr, 2021 CKD-EPI eGFR_cr, 2012 CKD-EPI eGFR_cr-cys, 2021 CKD-EPI eGFR_cr-cys, 2012 CKD-EPI eGFR_cys, and Modification of Diet in Renal Disease (MDRD) Study eGFR equations were compared with measured GFR.

OUTCOMES

Bias, precision, accuracy, and correct classification by CKD stage. Bias was defined as the difference between estimated and measured GFR. Precision was represented by the interquartile range. Accuracy was defined as the percentages of participants with eGFRs within 10%/20%/30% (P₁₀/P₂₀/P₃₀) of measured GFR, root mean square error, and mean absolute error.

RESULTS

87% of patients studied were White, 3% Black, and 10% other races. Mean measured GFR was 53 ± 19 (SD) mL/min/1.73 m². The 2009 and 2021 CKD-EPI eGFR_cr equations demonstrated similar median bias (-2.3 vs -0.2 mL/min/1.73 m², respectively), precision (14.5 vs 14.9 mL/min/1.73 m²), and accuracy (P₁₀/P₂₀/P₃₀, 32%/65%/84% vs 33%/63%/84%). The 2012 and 2021 CKD-EPI eGFR_cr-cys equations also demonstrated similar median bias (-3.6 vs 0.3 mL/min/1.73 m², respectively), precision (13.3 vs 14.3 mL/min/1.73 m²), and accuracy (P₁₀/P₂₀/P₃₀, 32%/63%/80% vs 32%/67%/83%). No clear difference in performance was detected between the 2021 CKD-EPI eGFR_cr and eGFR_cr-cys equations among KTRs. The proportion of correct classification by CKD stage was similar across all eGFR equations.

LIMITATIONS

Moderate sample size, few patients had a GFR <30 mL/min/1.73 m², and the large majority of patients were White.

CONCLUSIONS

Among KTRs, the 2021 race-free CKD-EPI eGFR equations perform similarly to the previous CKD-EPI equations that included race correction terms. No significant difference in performance was observed between the 2021 CKD-EPI eGFR_cr and eGFR_cr-cys equations in the kidney transplant population.

Quantifying Individual-Level Inaccuracy in Glomerular Filtration Rate Estimation : A Cross-Sectional Study

BACKGROUND

Although the population-level differences between estimated glomerular filtration rate (eGFR) and measured glomerular filtration rate (mGFR) are well recognized, the magnitude and potential clinical implications of individual-level differences are unknown.

OBJECTIVE

To quantify the magnitude and consequences of the individual-level differences between mGFRs and eGFRs.

DESIGN

Cross-sectional study.

SETTING

Four U.S. community-based epidemiologic cohort studies with mGFR.

PATIENTS

3223 participants in 4 studies.

MEASUREMENTS

The GFRs were measured using urinary iothalamate and plasma iohexol clearance; the eGFR was calculated from serum creatinine concentration alone (eGFR_CR) and with cystatin C. All GFR results are presented as mL/min/1.73 m².

RESULTS

The participants' mean age was 59 years; 32% were Black, 55% were women, and the mean mGFR was 68. The population-level differences between mGFR and eGFR_CR were small; the median difference (mGFR - eGFR) was -0.6 (95% CI, -1.2 to -0.2); however, the individual-level differences were large. At an eGFR_CR of 60, 50% of mGFRs ranged from 52 to 67, 80% from 45 to 76, and 95% from 36 to 87. At an eGFR_CR of 30, 50% of mGFRs ranged from 27 to 38, 80% from 23 to 44, and 95% from 17 to 54. Substantial disagreement in chronic kidney disease staging by mGFR and eGFR_CR was present. Among those with eGFR_CR of 45 to 59, 36% had mGFR greater than 60 whereas 20% had mGFR less than 45; among those with eGFR_CR of 15 to 29, 30% had mGFR greater than 30 and 5% had mGFR less than 15. The eGFR based on cystatin C did not provide substantial improvement.

LIMITATION

Single measurement of mGFR and serum markers without short-term replicates.

CONCLUSION

A substantial individual-level discrepancy exists between the mGFR and the eGFR. Laboratories reporting eGFR should consider including the extent of this uncertainty to avoid misinterpretation of eGFR as an mGFR replacement.

PRIMARY FUNDING SOURCE

National Institutes of Health.

Measurement of kidney function in Malawi, South Africa, and Uganda: a multicentre cohort study

BACKGROUND

The burden of kidney disease in many African countries is unknown. Equations used to estimate kidney function from serum creatinine have limited regional validation. We sought to determine the most accurate way to measure kidney function and thus estimate the prevalence of impaired kidney function in African populations.

METHODS

We measured serum creatinine, cystatin C, and glomerular filtration rate (GFR) using the slope-intercept method for iohexol plasma clearance (mGFR) in population cohorts from Malawi, Uganda, and South Africa. We compared performance of creatinine and cystatin C-based estimating equations to mGFR, modelled and validated a new creatinine-based equation, and developed a multiple imputation model trained on the mGFR sample using age, sex, and creatinine as the variables to predict the population prevalence of impaired kidney function in west, east, and southern Africa.

FINDINGS

Of 3025 people who underwent measured GFR testing (Malawi n=1020, South Africa n=986, and Uganda n=1019), we analysed data for 2578 participants who had complete data and adequate quality measurements. Among 2578 included participants, creatinine-based equations overestimated kidney function compared with mGFR, worsened by use of ethnicity coefficients. The greatest bias occurred at low kidney function, such that the proportion with GFR of less than 60 mL/min per 1·73 m² either directly measured or estimated by cystatin C was more than double that estimated from creatinine. A new creatinine-based equation did not outperform existing equations, and no equation, including the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) 2021 race-neutral equation, estimated GFR within plus or minus 30% of mGFR for 75% or more of the participants. Using a model to impute kidney function based on mGFR, the estimated prevalence of impaired kidney function was more than two-times higher than creatinine-based estimates in populations across six countries in Africa.

INTERPRETATION

Estimating GFR using serum creatinine substantially underestimates the individual and population-level burden of impaired kidney function in Africa with implications for understanding disease progression and complications, clinical care, and service provision. Scalable and affordable ways to accurately identify impaired kidney function in Africa are urgently needed.

FUNDING

The GSK Africa Non-Communicable Disease Open Lab.

TRANSLATIONS

For the Luganda, Chichewa and Xitsonga translations of the abstract see Supplementary Materials section.

The metabolism of 1,25(OH)2D3 in clinical and experimental kidney disease

Chronic kidney disease (CKD) results in calcitriol deficiency and altered vitamin D metabolism. The objective of this study was to assess the 24-hydroxylation-mediated metabolism of 25(OH)D₃ and 1,25(OH)₂D₃ in a cross-sectional analysis of participants with a range of kidney function assessed by precise measured GFR (mGFR) (N = 143) and in rats with the induction and progression of experimental kidney disease. Vitamin D metabolites were assessed with LC–MS/MS. Circulating measures of 24-hydroxylation of 25(OH)D₃ (24,25(OH)₂D₃:25(OH)D₃) precisely decreased according to mGFR in humans and progressively in rats with developing CKD. In contrast, the 1,24,25(OH)3D3: 1,25(OH)₂D₃ vitamin D metabolite ratio increased in humans as the mGFR decreased and in rats with the induction and progression of CKD. Human participants taking cholecalciferol had higher circulating 1,24,25(OH)₃D₃, despite no increase of 1,25(OH)₂D₃. This first report of circulating 1,24,25(OH)₃D₃ in the setting of CKD provides novel insight into the uniquely altered vitamin D metabolism in this setting. A better understanding of the uniquely dysfunctional catabolic vitamin D profile in CKD may guide more effective treatment strategies. The potential that 24-hydroxylated products have biological activity of is an important area of future research.

Sportomics suggests that albuminuria is a sensitive biomarker of hydration in cross combat

We have been using sportomics to understand hypermetabolic stress. Cross Combat (CCombat) has recently been initiated as a high-intensity functional training method inspired by CrossFit. We used a CCombat session to induce metabolic stress and evaluated its effects on hydration and kidney function. Blood samples were collected from 16 elite-level professional male athletes engaged in training sessions over a 96-h protocol. Blood myoglobin increased by ~ 3.5-fold (119 ± 21 to 369 ± 62 nmol/L; p = .001) in response to the protocol, returning to the pre-exercise level within 48 h. Furthermore, d-dimer levels increased from 6.5 ± 0.6 to 79.4 ± 21.3 μmol/L (p < .001) in response to exercise decreasing during recovery with high variability among the studied athletes. Albuminemia and creatininemia increased ~ 10% and cystatin C increased ~ 240% (1.7 ± 0.1 to 5.7 ± 0.5 mg/L; p < .001; effect size = 2.4) in response to the protocol. We measured albuminuria (HuA) to assess kidney permeability to albumin caused by exercise. HuA increased ~ 16-fold (0.16 ± 0.03 to 2.47 ± 0.41 μmol/L; p < .001; effect size = 1.4) in response to exercise, dropping and reaching basal levels during 48 h. Here, we suggest that microalbuminuria can be used as an early, sensitive, easy, and inexpensive biomarker to evaluate hydration status changes during intensive exercise, decreasing chronic impairment in renal function.

Evaluation of novel glomerular filtration rate estimation equations in adolescents and young adults with type 1 diabetes

AIMS

Individuals with type 1 diabetes (T1D) are at an increased risk of chronic kidney disease making estimation of glomerular filtration rate (eGFR) an important component of diabetes care. Which eGFR equation is most appropriate to use in patients with T1D during the transition to adult care is unclear. We, therefore, sought to evaluate the performance of five eGFR equations in adolescents and young adults with T1D.

METHODS

Measured iohexol-based glomerular filtration rate was compared to the Chronic Kidney Disease and Epidemiology Collaboration (CKD-EPI) eGFR, Chronic Kidney Disease in Children (CKiD) eGFR, and three recently developed age-adjusted versions of these in 53 patients with T1D and preserved GFR using bias, precision, and accuracy.

RESULTS

The best performance was found in the sex-dependent CKiD equation (bias: -0.8, accuracy: 11.8 ml/min/1.73 m²). Bias and accuracy (26.4 and 26.8 ml/min/1.73 m²) were worst in the CKD-EPI equation. Age-dependent adjustment improved performance for this equation (bias: 5.3, accuracy: 13.4 ml/min/1.73 m²), but not for the CKiD equation (bias: 15.5, accuracy: 18.8 ml/min/1.73 m²).

CONCLUSION

Age-adjustment improved performance for the CKD-EPI equation, but not for the CKiD equation. The sex-adjusted CKiD equation performed best out of all equations.

Measurement of glomerular filtration rate using endogenous d-serine clearance in living kidney transplant donors and recipients

BACKGROUND

Endogenous molecules that provide an unbiased and a precise evaluation of kidney function are still necessary. We explored the potential of clearance of d-serine, a rare enantiomer of serine and a biomarker of kidney function, as a measure of glomerular filtration rate (GFR).

METHODS

This was a cross-sectional observational study of 200 living kidney transplant donors and recipients enrolled between July 2019 and December 2020 in a single Japanese center, for whom GFR was measured by clearance of inulin (C-in). Clearance of d-serine (C-dSer) was calculated based on blood and urine levels of d-serine, as measured by two-dimensional high-performance liquid chromatography. Analytical performance was assessed by calculating biases. Utilizing data from 129 participants, we developed equations for C-in based on C-dSer and C-cre using a linear regression model, and the performance was validated in 68 participants.

FINDINGS

The means of C-in and C-dSer were 66.7 and 55.7 mL/min/1.73 m2 of body surface area, respectively, in the entire cohort. C-dSer underestimated C-in with a proportional bias of 22.0% (95% confidence interval, 14.2-29.8%) and a constant bias of -1.24 (-5.78-3.31), whereas the proportional bias was minor to that of C-cre (34.6% [31.1-38.2%] and 2.47 (-1.18-6.13) for proportional and constant bias, respectively). Combination of C-dSer and C-cre measured C-in with an equation of 0.391 × C-dSer + 0.418 × C-cre + 3.852, which reduced the proportional bias (6.5% [-0.2-13.1%] and -4.30 [-8.87-0.28] for proportional and constant bias, respectively). In the validation dataset, this equation performed well with median absolute residual of 3.5 [2.3-4.8], and high ratio of agreement (ratios of 30% and 15% different from C-in [P30 and P15] of 98.5 [91.4-100] and 89.7 [80.0-95.2], respectively).

INTERPRETATION

The smaller proportional bias compared to that of C-cre is an advantage of C-dSer as a measure of C-in. Combinational measurement of d-serine and creatinine, two endogenous molecules, has the potential to serve as a measure of GFR with precision and minor biases and can support important clinical decisions.

FUNDING

Japan Society for the Promotion of Science (JSPS, grant number 17H04188), Japan Agency of Medical Research and Development (AMED, JP20gm5010001), Osaka Kidney Bank (OKF19-0010), Shiseido Co., Ltd and KAGAMI Inc.

The « race » correction in estimating glomerular filtration rate: an European point of view

PURPOSE OF REVIEW

There is currently a heated debate ongoing whether or not to use the race coefficient for black people in the Modification of Diet in Renal Diseases and Chronic Kidney Disease Epidemiology-equation. The use of the race coefficient is thought by several American authors as a source of discrimination.

RECENT FINDINGS

It has recently been shown that the race coefficient is inaccurate in European and African black people. Therefore, it seems that the race correction is more a correction for black Americans, rather than for black in general. This 'correction' at the glomerular filtration rate (GFR)-level has been criticized, as it is misleading, and should be abandoned, as it has not been shown that GFR is different between black and white people. However, as differences in creatinine generation between black and white people might exist, a correction or adjustment, different for black and white people, at the creatinine level might be required, very similar to the different scaling of creatinine for males and females.

SUMMARY

The current debate on the race coefficient is particularly difficult because of the absence of good scientific data in black subjects and there lies the real discrimination in our opinion. We therefore call for future dedicated studies, both in Europe and USA.

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

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

Targeted design of a recombinant tracer for SPECT renal imaging

Rationale: A robust radiopharmaceutical has high uptake in the target and low retention in non-target tissues. However, traditional tracers for renal imaging that chemically chelate ^99mTc are excreted through the renal route with transient resident time in the kidney. Following a rational design approach, we constructed a protein-based radiotracer, designated PBT-Fc, to sequentially bind tubular neonatal Fc-receptor and subsequently proximal tubular basement membrane for its targeted sequestration in kidney parenchyma. In this process, the tracer participates in physiologic glomerular filtration and tubular reabsorption while escaping lysosomal catabolism and urinary clearance. Methods: To specifically target renal receptors in navigating the urinary passage in the kidney, we produced a recombinant fusion protein with two separate functional parts: a polybasic PBT segment derived from human Vascular Endothelial Growth Factor and Fc segment of IgG1. The chimeric fusion of PBT-Fc was labeled with radionuclide ^99mTc and tested in rodent models of kidney diseases. Planar scintigraphy and single-photon emission computerized tomography (SPECT) were performed to evaluate renal-specificity of the tracer. Results: When injected in mouse and rat, following a brief 10 - 15 min dynamic redistribution phase in circulation, ~ 95% of the [^99mTc]-PBT-Fc signal was concentrated in the kidney and lasted for hours without urinary loss or surrounding tissue activities. Long-lasting tracer signals in the kidney cortex in conjunction with SPECT greatly augmented the image quality in detecting pathological lesions in a variety of disease models, including ischemic acute kidney injury, drug-induced renal toxicity, and chronic kidney disease from renin-angiotensin system (RAS) overactivation. Conclusion: Exclusive renal retention of the recombinant radiotracer greatly facilitated static-phase signal acquisition by SPECT and achieved submillimeter spatial resolution of kidney alternations in glomerular and tubular disease models.

Assessment of kidney function: clinical indications for measured GFR

In the vast majority of cases, glomerular filtration rate (GFR) is estimated using serum creatinine, which is highly influenced by age, sex, muscle mass, body composition, severe chronic illness and many other factors. This often leads to misclassification of patients or potentially puts patients at risk for inappropriate clinical decisions. Possible solutions are the use of cystatin C as an alternative endogenous marker or performing direct measurement of GFR using an exogenous marker such as iohexol. The purpose of this review is to highlight clinical scenarios and conditions such as extreme body composition, Black race, disagreement between creatinine- and cystatin C-based estimated GFR (eGFR), drug dosing, liver cirrhosis, advanced chronic kidney disease and the transition to kidney replacement therapy, non-kidney solid organ transplant recipients and living kidney donors where creatinine-based GFR estimation may be invalid. In contrast to the majority of literature on measured GFR (mGFR), this review does not include aspects of mGFR for research or public health settings but aims to reach practicing clinicians and raise their understanding of the substantial limitations of creatinine. While including cystatin C as a renal biomarker in GFR estimating equations has been shown to increase the accuracy of the GFR estimate, there are also limitations to eGFR based on cystatin C alone or the combination of creatinine and cystatin C in the clinical scenarios described above that can be overcome by measuring GFR with an exogenous marker. We acknowledge that mGFR is not readily available in many centres but hope that this review will highlight and promote the expansion of kidney function diagnostics using standardized mGFR procedures as an important milestone towards more accurate and personalized medicine.

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

Introduction

Glomerular filtration rate (GFR) is measured from the late plasma disappearance curve of an exogenous tracer, after correction for the early decay—corresponding to the distribution of the tracer—using various equations. These equations display the highest discrepancies in the GFR range above 90 ml/min per 1.73 m², and their respective performances against a reference, urinary GFR measurement are unclear.

Methods

In patients with mGFR >90 ml/min per 1.73 m² from 6 different cohorts, we compared GFR obtained from the plasma clearance of iohexol or ⁵¹Cr-ethylenediamine tetraacetic acid (EDTA), after correction using Chantler (C), Bröchner-Mortensen (BM), Fleming (F), Jodal-Bröchner-Mortensen (JBM), and Ng (N) equations, with urinary clearance of the same tracers or inulin.

Results

In 438 participants (median age 41 [39–42] years, 43% women), the median urinary clearance was 100.8 (94.7–112.6) ml/min per 1.73 m². Plasma clearances using the correction equations were 105.7 (96.8–119.2), 102.4 (95.2–112.9), 100.7 (93.6–111.1), 102.6 (95.2–113.4), and 106.0 (98.2–117.6) ml/min per 1.73 m² for C, BM, F, JBM, and N, respectively. Concordance correlation coefficients between plasma and urinary clearances were poor for all equations. Compared with urinary clearances, BM, F, and JBM displayed the best accuracy within 10% (73%, 72%, and 71%, respectively, vs. 63% and 66% for C and N), whereas BM and JBM had the lowest median biases. Accuracy of all equations was especially low in the hyperfiltration range (urinary clearance >130 ml/min per 1.73 m²).

Conclusion

The BM and JBM equations displayed the best overall performances to correct for the early disappearance curve. Results of these equations should be interpreted with caution, especially in the highest GFR range.

A plasma creatinine- and urea-based equation to estimate glomerular filtration rate in rats

Monitoring renal function is a vital part of kidney research involving rats. The laborious measurement of glomerular filtration rate (GFR) with administration of exogenous filtration markers does not easily allow serial measurements. Using an in-house database of inulin clearances, we developed and validated a plasma creatinine- and plasma urea-based equation to estimate GFR in a large cohort of male rats [development cohort n = 325, R² = 0.816, percentage of predictions that fell within 30% of the true value (P30) = 76%] that had high accuracy in the validation cohort (n = 116 rats, R² = 0.935, P30 = 79%). The equation was less accurate in rats with nonsteady-state creatinine, in which the equation should therefore not be used. In conclusion, applying this equation facilitates easy and repeatable estimates of GFR in rats.NEW & NOTEWORTHY This is the first equation, that we know of, which estimates glomerular filtration rate in rats based on a single measurement of body weight, plasma creatinine, and plasma urea.

GFR Estimation After Cystatin C Reference Material Change

Introduction

Glomerular filtration rate (GFR) is routinely estimated with cystatin C. In June 2010, the International Federation of Clinical Chemistry (IFCC) released a certified cystatin C reference material (ERM-DA471/IFCC), and new cystatin C glomerular filtration rate estimation (eGFR) equations were developed with the IFCC standard. Early in 2018, Siemens discontinued their nonstandardized cystatin C reagent kits and replaced them with IFCC-calibrated kits in the US market. The aim of the current study was to assess the effect of IFCC calibration on cystatin C values and corresponding GFR estimations.

Methods

Cystatin C concentration was measured in 81 pediatric patients using a plasma sample from their nuclear GFR measurement with 99mTc-diethylenetriaminepentaaccetic acid. Calibration curves were generated using Siemens nonstandardized and IFCC-standardized kits to measure paired cystatin C concentrations in each sample. GFR-estimating equations using pre-IFCC and IFCC cystatin C values were compared using Bland-Altman analyses.

Results

The IFCC-standardized assay resulted in a mean increase in the measured cystatin C value of 24%. Estimating equations consistently overestimated GFR prior to IFCC standardization. Following incorporation of the IFCC standard, the Full Age Spectrum equation demonstrated the best overall performance, whereas the Chronic Kidney Disease in Children (CKiD) equation was more accurate in children with decreased GFR.

Conclusion

Incorporation of the IFCC standard significantly increased cystatin C values and affected the performance of GFR estimating equations. Clinical laboratories and providers may need to update the equation used for cystatin C-based estimation of GFR following adoption of the IFCC reference standard.