Comparative performance of cystatin C and creatinine eGFR equations in boys with posterior urethral valves: An...

Serum cystatin C is increasingly recognized as a valuable biomarker for estimating glomerular filtration rate (eGFR) in adults and children, due to its lower susceptibility to extrarenal influences compared to serum creatinine. Pediatric patients, particularly those with posterior urethral valves (PUV), pose challenges in accurately assessing kidney function due to variability in age, muscle mass, and nutritional status. Accurate eGFR measurement is crucial for managing chronic kidney disease (CKD) progression in these patients.

The objective of this study was to evaluate and compare eGFR estimates using serum cystatin C and serum creatinine in children diagnosed with PUV, utilizing established pediatric formulae.

We conducted a retrospective analysis of pediatric patients (<18 years) diagnosed with PUV and treated with valve ablation or vesicostomy within the first year of life between 2000 and 2020. Patients included had paired serum cystatin C and creatinine measurements (not standardized to timing, fasting, or hydration status) within a three-month interval. eGFR was calculated using the Chronic Kidney Disease in Children (CKiD) bedside creatinine (Schwartz) formula and the CKiD Under 25 (U25) equations (creatinine-based, cystatin C-based, and combined creatinine-cystatin C). Differences between formulae were evaluated using statistical tests for paired measurements.

Twenty-four patients met inclusion criteria, yielding 93 measurement pairs. Median age at cystatin C measurement was 11.5 years. The CKiD bedside creatinine (Schwartz) formula consistently yielded slightly higher eGFR values (median differences ranging from 1.5 to 2.6 mL/min/1.73 m²) compared to the CKiD U25 formulas. Cystatin C-based eGFR resulted in higher CKD stage classification (upstaging) for 11-27 % of the children. However, longitudinal analyses showed consistent trends in eGFR across all formulae.

Our findings support previous literature demonstrating slightly higher eGFR estimates with creatinine-based formulas compared to cystatin C-based formulas, potentially reflecting creatinine's susceptibility to extrarenal factors, notably muscle mass. While differences were statistically significant, clinical implications were limited due to small absolute differences. Limitations include the study's retrospective nature, small sample size, absence of direct GFR measurement, and lack of long-term clinical outcomes, potentially affecting generalizability and prognostic evaluation.

This study confirms that creatinine-based formulas slightly overestimate eGFR compared to cystatin C-based methods in children with PUV. Despite these discrepancies, consistent trends across methods emphasize the importance of maintaining a consistent biomarker for patient monitoring. Prospective research is needed to fully elucidate cystatin C's role in clinical practice and prognostication.