Evaluation of the Du Pont Theophylline assay adapted to a centrifugal analyzer

We evaluated the Du Pont Theophylline Assay, a particle-enhanced turbidimetric inhibition immunoassay (PETINIA) for the measurement of theophylline in human serum. The procedure was applied to the Cobas-Bio centrifugal analyzer, and was compared to an enzyme immunoassay (EMIT) method and a high performance liquid chromatographic (HPLC) procedure. Day-to-day precision was 3.7% (1 SD coefficient of variation) at 7.1 mg/L, 3.3% at 17.7 mg/L, and 3.9% at 27.8 mg/L. The assay was linear up to 40 mg/L, and the correlation between the PETINIA, EMIT, and HPLC methods was good [PETINIA/EMIT: y = 0.94x + 0.63, r = 0.98, Syx = 1.13; PETINIA/HPLC: y = 1.00x - 0.89, r = 0.99, Syx = 0.66, where Syx is the standard deviation of the residual error of regression], when evaluated using specimens from 135 patients receiving theophylline. No interference from hemolysis, lipemia, icterus, or other methylxanthines was observed. Of the major metabolites of theophylline, only 1,3-dimethyluric acid showed any significant cross-reactivity (2.1 mg/L apparent theophylline at 20 mg/L 1,3-dimethyluric acid). The method is reliable and cost-effective for the measurement of theophylline in serum.