A 24-hour urine has a total volume of 1,200 mL. A 1:200 dilution yields a creatinine result of 0.8 mg/dL. The serum value is 1.2 mg/dL. What is the creatinine excretion in grams per 24 hours?

A 24-hour urine has a total volume of 1,200 mL. A 1:200 dilution yields a creatinine result of 0.8 mg/dL. The serum value is 1.2 mg/dL. What is the creatinine excretion in grams per 24 hours?

• A. 1.92
• B. 0.08
• C. 80
• D. 19

Answer: (A)

Understanding how much creatinine is excreted in a day comes from the urine creatinine concentration in the original urine sample multiplied by the total urine volume. Because the measurement was done on a 1:200 dilution, you first back-calculate the true urine concentration: 0.8 mg/dL × 200 = 160 mg/dL in the original urine. Convert to mg/mL: 160 mg/dL is the same as 1.6 mg/mL (since 1 dL = 100 mL). Then multiply by the total urine volume: 1.6 mg/mL × 1200 mL = 1920 mg, which is 1.92 g per 24 hours. The serum creatinine value isn’t needed for this calculation; it would be relevant for estimating clearance, not the excretion amount.

Understanding how much creatinine is excreted in a day comes from the urine creatinine concentration in the original urine sample multiplied by the total urine volume. Because the measurement was done on a 1:200 dilution, you first back-calculate the true urine concentration: 0.8 mg/dL × 200 = 160 mg/dL in the original urine. Convert to mg/mL: 160 mg/dL is the same as 1.6 mg/mL (since 1 dL = 100 mL). Then multiply by the total urine volume: 1.6 mg/mL × 1200 mL = 1920 mg, which is 1.92 g per 24 hours. The serum creatinine value isn’t needed for this calculation; it would be relevant for estimating clearance, not the excretion amount.