Determination of plasma hydrochlorothiazide levels in humans

A method for determining plasma hydrochlorothiazide levels was developed with a sensitivity of 5 ng/ml. Accuracy and precision were demonstrated over the 5–648-ng/ml range by an overall recovery of 95 ± 8%. The detector response was linear for the 5–250-ng/ml range. The method was sufficiently sensitive for hydrochlorothiazide bioavailability studies and also was applicable for the determination of whole blood drug levels. Plasma levels in two subjects reached peak levels of 428 and 450 ng/ml at 2.5 and 2 hr, respectively, after a 50-mg dose. Whole blood levels at 3 hr after the same dose were 547 and 851 ng/ml and were approximately 2.5 times the 3-hr plasma levels.