the case of the endogenous substance potassium, urine measurements give the most accurate assessment of bioequivalence.
Now, this is in part because when potassium is absorbed, most of the absorbed dose is excreted through the urine, but also it's because, as Dr. Conner brought out earlier, serum potassium is a very insensitive measure. This is because body homeostatic mechanisms maintain serum potassium concentrations within a very narrow range. The normal range for serum potassium concentrations varies from 3.5 to 5 milliequivalents per liter.
We noted that in typical bioequivalence studies of potassium chloride oral dosage forms, serum concentrations increase by only about 5 percent after a single dose of 80 milligrams. What this means, recalling the schematic that Dr. Conner showed earlier, is that the baseline in serum is a very high amount relative to the increase that's observed following a dose. Therefore, measuring potassium in serum will not give an accurate measurement of bioequivalence of two formulations because the additional potassium in serum after dosing is a very