In the creatinine clearance calculation, the urine volume term is expressed in which units?

• A. mL/min
• B. mL/day
• C. L/min
• D. μL/min

Answer: (A)

Creatinine clearance is a rate: it tells how much plasma is cleared of creatinine each minute. In the typical formula ClCr = (Ucr × V) / Pcr, the urine volume term V must be a volume per unit time to yield a rate. If Ucr and Pcr are in the same mass-per-volume units (for example mg/dL), those mass units cancel, leaving V as the remaining unit of volume per time. Using V in mL/min gives the clearance in mL/min, which is the standard way clinicians report it. If you used mL/day, you’d get mL/day instead, which isn’t the conventional clearance unit. So the urine volume term is expressed as mL/min.