Why does my creatinine change when my kidneys haven't?

Creatinine is a waste product your muscles release at a fairly steady rate, and your kidneys clear it. Labs use it to estimate GFR — but anything that changes how much creatinine you produce, or how concentrated your blood is, moves the number without your kidneys changing at all.

The common culprits: a big cooked-meat meal the day before the draw, dehydration, a new exercise program or creatine supplement, and certain medications (like trimethoprim or cimetidine) that block creatinine's exit route in the kidney tubule without harming the kidney itself.

This is why nephrologists read creatinine as a trend over several draws, and why a surprising single result is usually repeated — sometimes with a cystatin C test, which isn't affected by muscle mass — before anyone changes your stage or your plan.