What do creatinine and eGFR measure?

Creatinine is a waste product produced by normal muscle activity and removed from the blood by the kidneys. Measuring how much is in your blood gives an indirect picture of how well that removal is working.

eGFR, or estimated glomerular filtration rate, takes that creatinine result and combines it with factors such as age and sex to estimate how much blood your kidneys are filtering. It is the number more often used to describe kidney function, because raw creatinine alone is heavily influenced by muscle mass.

That influence is worth knowing. A very muscular person and a frail person with identical kidney function will not produce identical creatinine results, which is part of why the estimate exists and why interpretation belongs with a clinician who knows your build and your history.