Bulk OEM Reagents (Reagent 1/ Reagent 2) for automated chemistry analyzers. Hydrochlorothiazide (HCT) belongs to the thiazide class of diuretics. It reduces blood volume by acting on the kidneys to reduce sodium (Na) reabsorption in the distal convoluted tubule. The onset of the diuretic effect and peak plasma levels of the drug occur within two hours after administration. Peak drug action takes place at four hours however the diuretic activity may persist for up to twelve hours after which HCT is excreted rapidly into the urine. An accurate and reliable means for the measurement of Hydrochlorothiazide concentration in patient samples is necessary for therapeutic dose management. A high throughput detection method with high sensitivity and specificity is presented herein. Hydrochlorothiazide Assay provides a convenient and efficient method to monitor the drug concentration in human urine.