CreatinineeGFR

Creatinine is a waste product made steadily as muscles use energy. The kidneys filter it out of the blood into the urine, so the amount left in the blood reflects how well the kidneys are clearing it.

It matters because creatinine — and the eGFR (estimated glomerular filtration rate) worked out from it — is the standard everyday check of kidney function, and kidney health is one component of overall biological ageing. Because it comes from muscle, very muscular people can read a little higher without any kidney problem.

In practice, creatinine is interpreted as an eGFR adjusted for age and sex and read alongside other markers; a single value tells you less than the trend over time.