Creatinine-based eGFR can misstate kidney function whenever creatinine production departs from the average, and cystatin C, which is largely independent of muscle mass and diet, is the practical cross-check.
Reach for a cystatin C-based estimate when creatinine is likely to mislead: at the extremes of muscle mass, in frailty, amputation or paralysis, in high muscle mass or creatine supplementation, in some ancestries where creatine synthesis differs, and whenever an eGFR does not fit the clinical picture or is about to drive a major decision such as drug dosing, contrast use or transplant referral. A combined creatinine-cystatin C equation is generally more accurate than either alone, and a measured GFR remains the arbiter when precision is essential.
The habit worth keeping is to confirm a borderline or surprising creatinine-based eGFR with cystatin C before acting on it, rather than treating a single creatinine estimate as ground truth.
- Cystatin C is largely independent of muscle mass and diet, unlike creatinine.
- Use it when muscle mass is unusually low or high, or with creatine supplementation.
- Use it when eGFR does not fit the clinical picture or drives a major decision.
- A combined creatinine-cystatin C equation is usually more accurate than either alone.
- Measured GFR remains the arbiter when precision is essential.
Why it matters
A single creatinine estimate can mislabel kidney function and misdose drugs; cystatin C is the available cross-check.
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