HomogeneousEnzymeImmunoassayfor Theophyllinein Serum and Plasma1

introduction Today, theophylline is considered the primary drug of choice in the prevention and treatment of asthmatic symptoms in children and adults (1); theophylline is also used extensively to treat apnea in premature infants (2-6). Current data suggest that the safe and effective use of theophylline requires careful dosage adjustment basedon measurements of theophylline concentrations in serum (7-9). The first commonly used assay for measuring serum theophylline was an extraction/spectrophotometric method described by Schack and Waxler in 1949 (10). This procedure has been widely used in spite of the large sample volume required and the reported problems with specificity and variable background absorbance (11-13). To eliminate barbiturate interferences (e.g., by phenobarbital), this method was modified by Jatlow in 1975 (13). Gas-liquid chromatographic methods have also been developed for theophylline (14-19). Although these assays are more specific than spectrophotometric procedures, most require an extraction of the sample, followed by solvent evaporation (15-19). Recently, improved methods have been reported for measuring theophylline concentrations, involving “high-pressure” liquid chromatography and enzyme immunoassay (20-27). The liquid-chromatographic methods can be fast and accurate, but interferences from certain drugs (9,28,29) may occur with some of the procedures. The homogeneous enzyme immunoassay possesses good specificity and a short analysis time (11,27).