creatinine clearance

Doctors need a quick way to judge how well a patient's kidneys are filtering, but measuring filtration directly is awkward. They use a clever stand-in: creatinine, a waste product of muscle that the body produces at a steady rate and the kidney handles in a convenient way. How fast the kidney clears creatinine becomes a practical proxy for overall filtering capacity.

Creatinine clearance estimates the glomerular filtration rate because creatinine is freely filtered and only slightly secreted. It can be measured from a timed urine collection, but is more often estimated from a blood creatinine level using formulas such as Cockcroft–Gault, which adjust for age, weight and sex because muscle mass affects how much creatinine is produced.

In pharmacology this number is the workhorse for dose adjustment. Drugs that are cleared mainly by the kidney are dosed in proportion to creatinine clearance: a patient with half the normal value generally needs a reduced dose or a longer interval. The estimate has limits, though, since it slightly overstates true filtration and is unreliable in people with very low or very high muscle mass.