Creatinine comes from muscle. In cirrhosis, sarcopenia and malnutrition, production falls, so serum creatinine stays low and estimated GFR or creatinine clearance overstates true kidney function. Renally cleared drugs dosed on these estimates — vancomycin, aminoglycosides, some antivirals — can accumulate. Cystatin C-based estimates and measured drug levels are less affected by muscle mass.
- A 'normal' creatinine in a wasted patient may hide significant renal impairment.
- Consider cystatin C where muscle mass is low and dosing is critical.
- Measure levels early for renally cleared drugs with narrow therapeutic windows.
- Low albumin and ascites also alter drug distribution.
Why it matters
Dosing errors in liver disease often start with a creatinine that looked reassuring.
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