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eGFR Discrepancy Muscle Mass — ESENeph MCQ

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ModerateChronic Kidney DiseaseeGFR Discrepancy Muscle MassESENeph

A 55-year-old man with CKD G3b has been referred for a second opinion on his eGFR. His creatinine-based eGFR is 38 mL/min/1.73m2. He is a bodybuilder with significant muscle mass (BMI 32, low body fat). Cystatin C-based eGFR is 52 mL/min/1.73m2. What is the best interpretation?

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Correct answer: A — The eGFRcr is likely underestimating true GFR due to high creatinine generation from muscle mass; eGFRcr-cys (combined equation) should be used per KDIGO 2024

Creatinine production is directly proportional to muscle mass. In bodybuilders or very muscular individuals, creatinine is disproportionately high, causing eGFRcr to underestimate true GFR. Cystatin C is not affected by muscle mass (it is produced by all nucleated cells at a relatively constant rate). KDIGO 2024 recommends eGFRcr-cys (combined equation) when there is discrepancy between eGFRcr and clinical assessment, particularly in patients with extremes of body composition. Measured GFR (iohexol/inulin clearance) is the gold standard if the combined estimate is uncertain.

Reference: KDIGO 2024 – CKD Guideline