Cystatin C

A small protein produced at a steady rate by every cell in the body and filtered out by the kidneys.

Unlike creatinine it does not depend on muscle mass, so it gives a fairer estimate of kidney function in people who are very muscular, very frail, or elderly.

It is the better number where creatinine is misleading, and reading the two together is more informative than either alone. Steroids and thyroid disease shift it independently of the kidneys.

Worth discussing a rising value, particularly when creatinine looks normal.