Which expression correctly represents creatinine clearance when normalization to body surface area is included?

Which expression correctly represents creatinine clearance when normalization to body surface area is included?

• 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 represents the volume of plasma cleared of creatinine per unit time. It is calculated from the rate of creatinine excretion in urine divided by the plasma concentration of creatinine. The rate of excretion is the urinary creatinine concentration multiplied by the urine flow rate, so using UCr in mg/L and urine flow in mL/min, the expression that gives clearance in mL/min is (UCr × V) / PCr. Normalizing to body surface area then adjusts this clearance to a standard body surface area (1.73 m^2). The correct form is urinary creatinine concentration times urine volume flow divided by plasma creatinine concentration, with the volume flow in mL/min, because it yields the proper clearance units (mL/min) and aligns with the concept of how much plasma is cleared per minute. Using volume in hours or arranging the terms in the denominator changes the units away from mL/min, making those options inappropriate unless you explicitly convert units first.

Creatinine clearance represents the volume of plasma cleared of creatinine per unit time. It is calculated from the rate of creatinine excretion in urine divided by the plasma concentration of creatinine. The rate of excretion is the urinary creatinine concentration multiplied by the urine flow rate, so using UCr in mg/L and urine flow in mL/min, the expression that gives clearance in mL/min is (UCr × V) / PCr. Normalizing to body surface area then adjusts this clearance to a standard body surface area (1.73 m^2).

The correct form is urinary creatinine concentration times urine volume flow divided by plasma creatinine concentration, with the volume flow in mL/min, because it yields the proper clearance units (mL/min) and aligns with the concept of how much plasma is cleared per minute. Using volume in hours or arranging the terms in the denominator changes the units away from mL/min, making those options inappropriate unless you explicitly convert units first.