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Cystatin C vs Creatinine eGFR Muscle Mass — ESENeph MCQ

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ModerateChronic Kidney DiseaseCystatin C vs Creatinine eGFR Muscle MassESENeph

A 48-year-old woman with CKD G3b (eGFR 38 mL/min/1.73m2) has a urine ACR of 85 mg/mmol. Her creatinine-based eGFR and cystatin C-based eGFR differ significantly (creatinine eGFR 38, cystatin C eGFR 28 mL/min/1.73m2). She is a competitive bodybuilder. What does this discrepancy most likely reflect?

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Correct answer: A — Creatinine-based eGFR overestimates true GFR in this patient due to high muscle mass

Creatinine is produced from creatine metabolism in skeletal muscle. In patients with high muscle mass (bodybuilders, athletes), serum creatinine is disproportionately elevated relative to GFR, causing creatinine-based eGFR to overestimate true kidney function (the formula 'expects' normal muscle mass). Cystatin C is produced at a constant rate by all nucleated cells, independent of muscle mass, making cystatin C-based eGFR more accurate in this scenario. KDIGO 2024 recommends confirming eGFR with cystatin C when creatinine-based estimates may be unreliable (extreme muscle mass, amputees, malnutrition, vegetarians). The true GFR is closer to 28 mL/min/1.73m2, significantly worse than suggested by creatinine alone.

Reference: KDIGO 2024 – CKD Guideline; Inker et al 2012 – CKD-EPI Cystatin C Equation