Creatinine clearance (uncorrected for body mass) is most accurately represented by which expression?

• A. urinary creatinine (mg/L) / [volume of urine (mL/min) × plasma creatinine (mg/L)]
• B. [urinary creatinine (mg/L) × volume (mL/min) / plasma creatinine (mg/L)]
• C. urinary creatinine (mg/L) / [volume of urine (mL/hour) × plasma creatinine (mg/L)]
• D. [urinary creatinine (mg/L) × volume (mL/hour)] / plasma creatinine (mg/L)

Answer: (B)

Creatinine clearance estimates how quickly the kidneys remove creatinine from the blood, and the standard expression is the urinary creatinine concentration times the urine flow rate, divided by the plasma creatinine concentration. When you use urine creatinine in mg/L, urine flow in mL/min, and plasma creatinine in mg/L, the units cancel properly to give mL/min, which is the typical clearance unit. That’s why the form with Ucr × V over Pcr is the correct representation: it directly reflects Ccr = (Ucr × V) / Pcr, yielding a clearance value per minute. The other forms misplace the volume term or use a different time basis (hour instead of minute), which would give units that are not the standard clearance unit or would require an extra conversion.