Bulk OEM Reagents (Reagent 1/ Reagent 2) for automated chemistry analyzers. Theophylline is a drug primarily used to treat bronchial asthma, chronic bronchitis and cardiac asthma, acute and chronic obstructive pulmonary disease. Therapeutic drug monitoring by determining Theophylline concentration in serum or plasma if often required. A narrow therapeutic index necessitates an accurate and reliable means for the measurement of theophylline concentration in human body fluid such as serum and plasma. Due to the low therapeutic index, the increasing number of theophylline intoxications and the large number of potential patients worldwide, a high throughput detection method with high sensitivity and specificity is presented herein. Theophylline Assay provides a convenient and efficient method to monitor the drug concentration in serum or plasma.