We can calculate the renal clearance of any substance if we know which pair of values?
a. Urine flow rate and urine concentration
b. Plasma concentration and urine concentration
c. GFR and urinary excretion rate
d. Plasma concentration and urinary excretion rate
(d) The excretion rate of a substance divided by its plasma concentration yields the clearance.
A drug X has a short plasma half-life and must be administered frequently to maintain therapeutic levels. The urinary concentration of X is much higher than the plasma concentration. A substantial amount of X also appears in the feces. What can we say about the renal clearance of X compared with the metabolic clearance rate of X?
a. The metabolic clearance rate is higher than the renal clearance.
b. The renal clearance is higher than the metabolic clearance rate.
c. The 2 clearances are the same.
d. There is insufficient information to answer the question.
(a) The metabolic clearance rate represents the sum of all clearance routes. Since there are 2 major routes of clearance (kidneys and feces) the metabolic clearance must be higher than either one alone. The fact that the drug has a higher urinary concentration than plasma concentration mainly reflects the reabsorption of water.
Inulin clearance is measured twice: the first time at a low inulin infusion rate, and the second time at a higher infusion rate that results in a higher plasma inulin concentration during the test. Assuming the kidneys behave the same in both cases, which measurement will yield a higher inulin clearance?
c. Both measurements are the same
d. There is insufficient information to answer the question
(c) In the second test both the plasma concentration and filtered load (and hence rate of excretion) are increased, yielding offsetting effects on the calculation.
Which of the following indicates correct relative renal clearances?
a. Sodium clearance is greater than urea clearance.
b. PAH clearance is greater than inulin clearance.
c. Urea clearance is greater than PAH clearance.