Competitive nephelometric immunoassay of theophylline in plasma.

We have developed an accurate, simple, and rapid method for the determination of theophylline in plasma. The principle of the method is based on inhibition of immunoprecipitation by hapten. A nephelometer is used to measure the scattered light from the immunoprecipitate. The macromolecule, which possesses numerous theophylline moieties and forms immunoprecipitate with anti-theophylline antibodies, can be easily prepared and used as the reagent for the assay. Theophylline in the assay mixture competitively inhibits the immunoprecipitation of the macromolecule. Therefore, theophylline can be determined by the measurement of the decrease of the scattered light. The assay is rapid (incubation time within 15 min), requires as little as 10 microliter of plasma, and requires neither troublesome pretreatment nor separation of antibody-bound from free antigen. Estimations of theophylline levels in patient plasma specimens correlate well to those obtained by the high performance liquid chromatography (correlation coefficient 0.971).