Stability of theophylline in human serum and whole blood

The effect of various storage conditions on serum theophylline results using a fluorescence polarization immunoassay (FPI) method was studied. Two human subjects received sufficient oral theophylline to produce low and high therapeutic serum theophylline concentrations. Aliquots of venous whole blood (without anti-coagulant) and serum from each subject were sealed in polypropylene plastic and glass containers and stored at 4 degrees C and 25 degrees C. Additional aliquots of serum from each subject were stored in glass and polypropylene containers at -20 degrees C. FPI assays were performed on days 0 and 3 for room temperature serum samples, days 3 and 7 for refrigerated serum samples, and days 3, 7, 14, 28, 56, 168, and 336 for frozen serum samples. All whole blood samples were assayed on day 3 after removal of the clot. Some samples were tested using FPI for 1,3-dimethyluric acid, a metabolite of theophylline, and results from spiked samples and samples tested with high-pressure liquid chromatography were compared with FPI results to determine cross-reactivity of the metabolite. No appreciable change in the assayed theophylline values occurred under any storage temperature, nor was there any difference in results from samples stored in glass versus polypropylene containers. At a concentration of 20 micrograms/ml, 1,3-dimethyluric acid produced a cross-reactivity of 3.5% with the FPI system. The FPI method is not affected by storage of samples under the specified conditions over the time period studied.