Increased Single-Nephron GFR in Normal Adults: Too Much of a Good Thing . . . or Maybe Not?

The Renal Pathology of Obesity: Structure-Function Correlations

The kidney is one of the target organs that may show health disorders as a result of obesity. Obesity-related glomerulopathy (ORG) is a kidney disease category based on a biopsy diagnosis that may occur secondary to obesity. Detailed clinicopathologic observations of ORG have provided significant knowledge regarding obesity-associated renal complications. Glomerulomegaly with focal segmental glomerulosclerosis of perihilar locations is a typical renal histopathologic finding in ORG, which has long been considered to represent a state of single-nephron glomerular hyperfiltration. This hypothesis was recently confirmed in ORG patients by estimating single-nephron glomerular filtration rate using a combined image analysis and biopsy-based stereology. Overshooting in glomerulotubular and tubuloglomerular interactions may lead to glomerular hyperfiltration/hypertension, podocyte failure, tubular protein-traffic overload, and tubulointerstitial scarring, constituting a vicious cycle of a common pathway to the further loss of functioning nephrons and the progression of kidney functional impairment.

Assessment of nephron number and single-nephron glomerular filtration rate in a clinical setting

Total nephron counts vary widely between individuals and may affect susceptibility to certain diseases, including hypertension and chronic kidney disease. Detailed analyses of whole kidneys collected from autopsy patients remain the only method for accurately counting nephrons in humans, with no equivalent option in living subjects. Current technological advances have enabled estimations of nephron numbers in vivo, particularly the use of total nephron number and whole-kidney glomerular filtration rate to estimate the mean single-nephron glomerular filtration rate. The use of this method would allow physicians to detect dynamic changes in filtration function at the single-nephron level rather than to simply count the number of nephrons that appear to be functioning. Currently available methods for estimating total nephron number in clinical practice have the potential to overcome limitations associated with autopsy analyses and may therefore pave the way for new therapeutic interventions and improved clinical outcomes.

Living kidney donor evaluation for all candidates with normal estimated GFR for age

Multiple days assessments are frequent for the evaluation of candidates to living kidney donation, combined with an early GFR estimation (eGFR). Living kidney donation is questionable when eGFR is <90 ml/min/1.73 m² (KDIGO guidelines) or 80 ml/min/1.73 m² (most US centres). However, age-related GFR decline results in a lower eGFR for older candidates. That may limit the number of older kidney donors. Yet, continuing the screening with a GFR measure increases the number of eligible donors. We hypothesized that in-depth screening should be proposed to all candidates with a normal eGFR for age. We compared the evolution of eGFR after donation between three groups of predonation eGFR: normal for age (S_age ) higher than 90 or 80 ml/min/1.73 m² (S₉₀ and S_80, respectively); across three age groups (<45, 45-55, >55 years) in a population of 1825 French living kidney donors with a median follow-up of 5.9 years. In donors younger than 45, postdonation eGFR, absolute- and relative-eGFR variation were not different between the three groups. For older donors, postdonation eGFR was higher in S₉₀ than in S₈₀ or S_age but other comparators were identical. Postdonation eGFR slope was comparable between all groups. Our results are in favour of in-depth screening for all candidates to donation with a normal eGFR for age.

A Systematic Review of Renal Functional Reserve in Adult Living Kidney Donors

Introduction

The kidney’s capacity to increase its glomerular filtration rate (GFR) in response to a higher functional demand is known as the renal functional reserve (RFR). Good short-term outcomes after living kidney donation have led to more acceptance of borderline donors (with hypertension, obesity, older age) due the ongoing shortage of donor organs. Given recent concerns about increased long-term risk in some donor subgroups, better donor stratification is needed. Measurement of RFR could inform assessment of donor risk.

Methods

A systematic literature review of studies that assessed RFR in donors pre- and/or post-donation was performed. Given study heterogeneity, descriptive analysis and narrative synthesis was conducted.

Results

Sixteen of 3250 identified studies published between 1956 and 2019 met inclusion criteria. Most studies were cross-sectional and conducted before (n = 8) and/or after (n = 16) kidney donation. Methods for measurement of GFR, effective renal plasma flow (ERPF) and RFR were not standardized. Changes in filtration fraction (FF) and ERPF relative to GFR observed after donation varied depending on stimulus used to induce RFR. Overall, RFR fell after donation; however, over the shorter term, RFR was largely preserved in young healthy donors. RFR was more significantly reduced in donors with hypertension, obesity, or older age.

Conclusion

Existing data suggest possible blunting of RFR post-donation in older, obese, and hypertensive donors, which may represent increased single-nephron GFR at baseline. The long-term implications of these changes deserve further study to determine utility in informing selection of borderline kidney donors.

Human nephron number, hypertension, and renal pathology

Recent studies have reported that total nephron number varies widely in human kidneys and some racial groups with low nephron number have a higher incidence of hypertension and kidney disease. Importantly, nephrogenesis normally reaches completion at about 34-36 weeks gestation, with no new nephrons formed for the lifetime in humans after this time. Although the loss of glomeruli varies among individuals due to aging, blood pressure, or additional inducers of kidney injury, much of the variation in nephron number is nowadays thought to be present at birth. According to the hyperfiltration hypothesis, this subsequent nephron loss results in compensatory hyperfiltration and/or hypertension of remaining glomeruli, thereby contributing to increased susceptibility to systemic hypertension. However, recent studies have suggested that the association between a low nephron number and systemic hypertension is not a universal finding. In most studies to date, nephron counts were performed on kidneys obtained at autopsy. Several recent studies have attempted to estimate nephron number in living human subjects, but further work is required to obtain accurate and precise estimates of nephron number using these noninvasive methods. Longitudinal studies in living humans have the potential to reveal associations between nephron number and hypertension/renal pathology.