Homogeneous fluorescent immunoassay with dry reagents

We described an assay for theophylline in serum in which the reagents for the substrate-labeled fluorescent immunoassay procedure are dry reagents in a paper matrix. This assay format is both rapid and convenient. In one such paper strip assay, the competing drug conjugate is present in the diluting buffer (dilution-into-conjugate strip assay); in another, all necessary assay reagents are contained on the strip (integral strip assay). The latter strip can be prepared by using appropriate solvents in a "two-dip" procedure, which prevents premature reaction of the assay constituents. The dilution-into-conjugates strip assays yield results that compare well with those by the corresponding solution assay (r = 0.975, standard error of estimate = 1.67 mg/L) and by "high-performance" liquid chromatography (r = 0.980, standard error of estimate = 1.49 mg/L). The integral strip results were compared with those by an enzyme immunoassay for theophylline (r = 0.990, standard error of estimate = 2.07 mg/L). Both strip assays require only a single dilution step before analysis.