Creatinine
What it measures, and what changes the number.
Creatinine is a waste product created when muscles break down creatine during normal activity. The kidneys filter it out of the blood, so the blood level is widely used as an indicator of how well the kidneys are filtering. Spanish language reports may list it as 'Creatinina'.
It is usually read with eGFR, which is calculated from creatinine along with age and sex, and with urea or BUN. Cystatin C is sometimes added because it is less influenced by muscle mass.
Muscle mass, a large meal of cooked meat shortly before the draw, intense exercise, hydration and some medications can all change the value. Labs use either the Jaffe method or an enzymatic method, and these can differ slightly, so comparing results across labs needs care.
Often read alongside: eGFR (CKD-EPI), Urea, BUN (urea nitrogen), Cystatin C, Creatinine clearance