Clinical evaluation of a dry chemistry strip theophylline assay

In this dry chemistry strip immunoassay, flavin adenine dinucleotide (FAD) conjugated theophylline competes with theophylline in the sample for binding to a specific monoclonal antibody. Unbound conjugate can then activate apoglucose oxidase, the activity of which is coupled to a peroxidase chromogenic reaction that allows kinetic monitoring by reflectance photometry. Thirty microliters of diluted sample is placed onto the reagent pad. The strip is inserted into the analyzer, and a digital readout appears 80 s later. Within-run precision was 4.5% based on patient duplicates. Between-day precision was 7.6% at 10 mg/L (n = 29) and 4.5% at 20 mg/L (n = 27). The assay was linear to at least 35 mg/L. There was good agreement between the test method and fluorescence polarization immunoassay: y (test) = -0.2865 + 1.085x (reference), n = 79, r = 0.980, Syx = 1.64. The assay showed good specificity except in patients receiving dimenhydrinate or suffering from renal failure. This system of assay provides reliable measurement of theophylline and will allow easy accessibility to this measurement for physicians working in emergency departments or private offices.