Particle-enhanced turbidimetric inhibition immunoassay for theophylline evaluated with the Du Pont aca

We evaluated the Du Pont Particle-Enhanced Turbidimetric Inhibition Immunoassay (PETINIA) for theophylline. The imprecision (CV) of the assay was less than 4.7% between-run and less than 3.6% within-run for theophylline concentrations between 5 and 30 mg/L. Standard curves for the assay were linear for theophylline concentrations from 0 to 46 mg/L and were stable throughout the study (i.e., for at least three months). The monoclonal antibody against theophylline used in this assay increases specificity; of the possibly interfering drugs, metabolites, and anticoagulants tested, only 1,3-dimethyluric acid and EDTA showed measurable effects. Bilirubin (less than 300 mg/L), hemoglobin (less than 6 g/L), or lipemia (triglycerides less than 6 g/L) does affect the quality of the assay. Analytical recovery of theophylline added to serum (5 to 40 mg/L) averaged 98% (range 93% to 112%). Comparison of results for patients' sera by the PETINIA method with those by enzyme immunoassay (EMIT) and by "high-performance" liquid chromatography yielded slopes and intercepts not significantly different from 1.0 and 0.0, respectively, and correlation coefficients ranging from 0.986 to 0.995.