A patient on your ward suddenly develops renal failure. In order to rapidly estimate the renal clearance of captopril in the patient you collect urine from 0-2 hours following dosing of 50 mg of drug and the urine is found to contain 5 mg of drug. You also take a midpoint plasma sample (at 1 hour post-dosing) and the plasma drug concentration is 10 mg/L. What is the estimated renal clearance for this drug in this patient?

Question

BlackTom AI · Accepted Answer

To estimate renal clearance (CLR) of captopril, we compare the rate at which the drug is removed by the kidneys to the concentration of drug in plasma.
First, determine the amount of drug excreted in urine over the collection interval: 5 mg collected from 0 to 2 hours means an excretion rate of 5 mg / 2 h = 2.5 mg/h.
Next, note the plasma concentration used for comparison: at 1 hour post-dosing the plasma concentration is 10 mg/L. This value serves as the average driving concentration for the clearance calculation over the interval.
Now compute CLR using the relation CLR = (urine excretion rate) / (plasma concentration) = (2.5 mg/h) / (10 mg/L) = 0.25 L/h.
So, the renal clearance is 0.25 L/h. The other options can be checked by similar dimensional reasoning:
- If CLR were 50 L/h, the required excretion rate would be 50 L/h × 10 mg/L = 500 mg/h, far exceeding the observed 2.5 mg/h, which is implausible here.
- If CLR were 5 L/h, the excretion rate would be 5 L/h × 10 mg/L = 50 mg/h, still higher than the measured 2.5 mg/h, so that would overshoot.
- If CLR were 0.05 L/h, the excretion rate would be 0.05 L/h × 10 mg/L = 0.5 mg/h, which is lower than the actual 2.5 mg/h, so this underestimates clearance.
Thus, the correct estimation aligns with 0.25 L/h, given the provided data.

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