To determine creatinine clearance, which samples are required?

• A. Timed urine collection plus serum creatinine
• B. Random urine sample and serum glucose
• C. 24-hour urine without blood tests
• D. Urine sample only

Answer: (A)

Creatinine clearance reflects how much plasma is cleared of creatinine per minute, which depends on the amount of creatinine excreted in urine over a defined period and the amount of creatinine in the blood. To determine it, you need a timed urine sample to know the total amount excreted (urine creatinine concentration times urine volume) and a serum creatinine measurement for the blood concentration. The calculation uses both: CrCl (mL/min) = (urine creatinine concentration × urine volume) / (serum creatinine concentration × collection duration in minutes). A random urine sample won’t give the total excretion, and urine alone doesn’t provide the blood creatinine level needed to relate excretion to filtration. Even a 24-hour urine collection isn’t enough by itself—you still need the corresponding serum creatinine value to complete the calculation. That’s why the required samples are a timed urine collection plus serum creatinine.