Which test assesses kidney function by measuring creatinine clearance?

• A. Creatinine
• B. Creatinine Kinase
• C. Creatinine Clearance
• D. Blood Urea Nitrogen

Answer: (C)

Creatinine clearance is about how well the kidneys are filtering wastes from the blood. Creatinine is produced at a steady rate and is filtered by the kidneys with little being reabsorbed, so the amount of creatinine that the kidneys remove per minute closely reflects the glomerular filtration rate (GFR). To measure clearance, you typically collect all urine over a set period and measure creatinine in both the urine and the plasma. The clearance is calculated as (urine creatinine concentration × urine flow rate) divided by the plasma creatinine concentration. This gives the rate at which plasma is being cleared of creatinine, effectively estimating the kidney’s filtration capacity.

In practice, many labs estimate GFR from a serum creatinine value using formulas, which is more convenient than 24-hour urine collection, but the underlying principle remains the same: creatinine clearance assesses how effectively the kidneys filter waste.

Other tests measure different things. Creatinine alone is a concentration and doesn’t by itself quantify how much blood is cleared of creatinine per minute. Blood urea nitrogen can reflect kidney function but is influenced by hydration, protein intake, and liver function. Creatinine kinase is an enzyme related to muscle injury, not kidney filtration.