1000 mg of paracetamol was given by IV bolus and urine and plasma samples taken over 12 hours. The plasma concentration at 6 hours following dosing was found to be 350 µg/ml. The total amount of drug eliminated unchanged in the urine over 12 hours was 420 mg. What is the renal clearance?[Fill in the blank]

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BlackTom AI · Accepted Answer

Question restatement: A patient received 1000 mg paracetamol IV. Over 12 hours, total unchanged drug excreted in urine = 420 mg. Plasma concentration at 6 hours after dosing = 350 µg/mL. What is the renal clearance?

Option analysis:

- a. 2.34 L/h: To reach this value, you would need a higher ratio of urine excretion rate to plasma concentration. First compute the urine excretion rate: 420 mg/12 h = 35 mg/h. Next, convert plasma concentration to consistent units: 350 µg/mL equals 350 mg/L. Clearance = excretion rate (mg/h) ÷ plasma conc (mg/L) = 35 mg/h ÷ 350 mg/L = 0.1 L/h. 0.1 L/h is far from 2.34 L/h, so this option is incorrect.

- b. 0.42 ml/h: This would imply a clearance on the order of a few tenths of a milliliter per hour, which is far too small for a typical renal clearance calculation at these values. Using the same formula (35 mg/h ÷ 350 mg/L) gives 0.1 L/h, i.e., 100 mL/h, not 0.42 mL/h. Therefore, this is incorrect.

- c. 35 mg/h: This represents the hourly amount of drug eliminated in urine, not the clearance. Renal clearance uses the ratio of urine rate to plasma concentration, yielding a volume per time (L/h). 35 mg/h is the excretion rate, not the clearance, so this option is incorrect.

- d. 0.1 L/h: This matches the calculation precisely. Excretion rate = 420 mg/12 h = 35 mg/h. Plasma conc = 350 µg/mL = 350 mg/L. Clearance = 35 mg/h ÷ 350 mg/L = 0.1 L/h. This is the correct unit and value for renal clearance in this scenario.

Summary: The correct approach is to convert the urinary excretion rate to mg/h, convert plasma concentration to mg/L, and divide the former by the latter to obtain clearance in L/h. Among the options, 0.1 L/h is the value that results from 35 mg/h divided by 350 mg/L. The other options either misinterpret clearance as an excretion rate, misconvert units, or are orders of magnitude off.

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