Cystatin C- and Creatinine-based Estimated GFR Differences: Prevalence and Predictors in the UK Biobank

Glomerular filtration rate and sexual dimorphism: lessons from animal and human studies

PURPOSE OF REVIEW

Integrating sex-based analyses is becoming a key point in the new recommendations, particularly in nephrology.

RECENT FINDINGS

Whereas single nephron glomerular filtration rate (GFR) is not different between men and women, male sex is associated, after multiple adjustments, with a higher number of nephrons. However, after indexation to body surface area, measured GFR (mGFR) in healthy potential kidney donors is not different between men and women between 20 and 50 years of age. After 50 years, mGFR decline seems faster in women than in men, which is concordant with the protective role of estrogens on renal function, as demonstrated in animal and some human studies. Conversely, testosterone has a detrimental effect on renal function. Of note, although testosterone has been shown to increase the kidney volume of the remnant kidney after a unilateral nephrectomy in animal models, this may generate deleterious hyperfiltration in the longer term.

SUMMARY

Taken together, these data highlight the impact of sex on GFR, notably through sexual hormones whose receptors are expressed in glomerular cells.

Discordant Results Between Creatinine- and Cystatin C-based Equations for Estimating GFR

Introduction

Discordant results between cystatin C and creatinine in estimating glomerular filtration rate (GFR) are an important medical issue. However, the equation that should be used when GFR estimates are discordant remains unclear.

Methods

This cross-sectional analysis included 15,485 participants with GFR measured by the clearance of an exogenous marker, serum creatinine, and cystatin C. We studied the proportion of discordant results defined as an absolute (> 15 ml/min per 1.73 m2) or relative (> 20%) difference between creatinine-based estimated GFR (eGFR, eGFRcrea) and cystatin C-based eGFR (eGFRcys) using different equations (Chronic Kidney Disease Epidemiology Collaboration [CKD-EPI], European Kidney Function Consortium [EKFC], and reexpressed Lund-Malmö [r-LMR]). We also researched for the best estimating equations to be used in subjects with concordant or discordant results to estimate measured GFR (mGFR).

Results

In the total cohort, the proportion of subjects with discordant results (absolute or relative) was larger for CKD-EPI (35.1 and 40.6%) than for EKFC (21.9 and 31.7%) or r-LMR (22.8 and 32.8%) equations. Among discrepant results, the proportion of eGFRcys < eGFRcrea was significantly higher than the proportion of eGFRcrea < eGFRcys for the CKD-EPI equations, whereas the occurrence of discrepancy was similar in the 2 discrepant groups for EKFC or r-LMR. For the EKFC and r-LMR equations, but not for the CKD-EPI, the equation combining creatinine and cystatin C was consistently the closest to the mGFR in the discrepant groups.

Conclusion

Based on the lower discrepancy proportion, better balance between eGFRcrea and eGFRcys, and better concordance with mGFR, the EKFC, and r-LMR equations should be preferred over the CKD-EPI to estimate GFR.

Brief Report: Cystatin C Provides Substantially Higher Glomerular Filtration Rate Estimates Than Creatinine in a Subset of Black People With HIV on Current Antiretroviral Regimens

BACKGROUND

In African populations, estimated glomerular filtration rate by cystatin C (eGFRcys) is better aligned with gold-standard GFR measurements than eGFR by creatinine (eGFRcr). Moreover, eGFRcys is unaffected by the effects of antiretroviral therapy (ART) on tubular secretion and may thus provide better estimates of GFR in people with HIV on ART.

SETTING

Observational cohort study of people of African ancestry living with suppressed HIV RNA on ART in London, United Kingdom.

METHODS

Cross-sectional analysis of 360 paired serum creatinine and cystatin C measurements. Participants whose eGFRcys substantially (>10%) exceeded eGFRcr were identified, and factors associated with this outcome were identified in logistic regression analysis.

RESULTS

The median age of participants was 52 years, 56% were women, and 82% born in Africa or the Caribbean. The eGFRcys substantially exceeded eGFRcr in 42% of participants in the overall cohort, and in 68% of those with eGFRcr 45-75 mL/min/1.73 m2. In multivariable analysis, a higher eGFRcr was associated with lower odds (0.59 [0.50, 0.68] per 10 mL/min/1.73 m2 increase) of eGFRcys substantially exceeding eGFRcr; a higher BMI was also associated with this outcome, while ART regimens inhibiting tubular secretion of creatinine were not predictive. Of the 22 participants with eGFRcr 45-60 mL/min/1.73 m2, 16 (73%) had eGFRcys >60 mL/min/1.73 m2.

CONCLUSIONS

We report substantially higher eGFRcys than eGFRcr in a subset of people of African ancestry with suppressed HIV, particularly among those with eGFRcr 45-75 mL/min/1.73 m2. In this population, eGFRcys provides clinically useful information irrespective of ART regimen.

The Metabolism of Creatinine and Its Usefulness to Evaluate Kidney Function and Body Composition in Clinical Practice

Serum creatinine levels are the most used clinical marker to estimate renal function as the glomerular function rate because it is simple, fast, and inexpensive. However, creatinine has limitations, as its levels can be influenced by factors such as advanced age, physical activity, protein-rich diets, male gender, medications, and ethnicity. Serum cystatin C and its combination with serum creatinine may serve as an alternative since these factors do not affect it. Most creatinine synthesis occurs in the muscles, making it a valuable marker for assessing lean body mass within body composition. This measurement is crucial for evaluating and monitoring nutritional status in patients with chronic kidney disease. This review aimed to discuss the literature on creatinine metabolism, its advantages and disadvantages in assessing renal function, and its utility in measuring lean body mass. The variability in the creatinine generation rate among individuals should be considered when assessing the glomerular function rate.

Chronic Kidney Disease-Part 1: Evaluation & Risk Assessment in CKD, Methods to Delay CKD Progression

Chronic kidney disease (CKD) is common and leads to significant morbidity and mortality. Early detection and deployment of effective therapies could prevent CKD progression and related cardiovascular diseases. This review provides a case-based overview of key issues surrounding the diagnosis, staging, and management of CKD.

Testosterone deficiency and chronic kidney disease

Testosterone's biological functions are extensive, influencing reproductive and systemic health. It plays a vital role in sexual functions, muscle protein synthesis, bone metabolism, fat distribution, and cardiovascular health. The hormone also affects mood, cognitive function, and erythropoiesis, underscoring its importance in both physical and mental health. Testosterone deficiency, or male male hypogonadism, is increasingly recognized as a significant health issue affecting various bodily systems, also in the context of chronic kidney disease (CKD). Recent research indicates a complex interplay between testosterone levels and renal health, suggesting that male male hypogonadism may both impact and be impacted by CKD. The latter is characterized by a gradual loss of kidney function, affects millions globally and is often associated with diabetes mellitus, arterial hypertension, and autoimmune diseases. Men with CKD frequently experience lower testosterone levels, which can exacerbate muscle wasting, reduce quality of life, and increase cardiovascular risk. Overall, low testosterone levels in CKD patients are associated with increased morbidity and mortality. Several mechanisms explain the relationship between CKD and testosterone deficiency. The uremic environment in CKD disrupts the hypothalamic-pituitary-gonadal axis, impairing hormone production. Nutritional deficiencies and chronic inflammation common in CKD patients further suppress gonadal function. The consequences of low testosterone in CKD are profound, with studies suggesting that testosterone replacement therapy (TRT) might improve clinical outcomes, though the long-term effects and causal relationships remain under investigation. The potential benefits of TRT in CKD patients might be significant. TRT can enhance muscle mass and strength, address anemia by stimulating erythropoiesis, improve bone density, and possibly offer cardiovascular benefits by improving body composition and insulin sensitivity. General symptoms of male hypogonadism, such as deteriorated psychological, sexual and physical wellbeing, can be improved by TRT. However, these benefits must be weighed against potential risks. TRT may exacerbate fluid retention, arterial hypertension, or exacerbate existing heart failure, particularly in CKD patients with pre-existing cardiovascular comorbidities. Additionally, concerns about the progression of renal disease via several testosterone affected pathways involving renal tubular integrity exist, highlighting the need for careful patient selection and monitoring. Understanding this relationship is crucial for developing comprehensive treatment strategies that address both renal and endocrine dysfunctions, highlighting the need for integrated patient care, which means good collaboration between subspecialists like nephrologists, endocrinologists, urologists and primary care providers, aiming to improve outcomes and quality of life while mitigating adverse effects.