Lack of theophylline assay interference from pentoxifylline and its metabolites

Pentoxifylline, a hemorheologic agent, and its metabolites are structurally similar to theophylline and other xanthines; therefore, they have the potential to interfere with serum assays for theophylline. Any false elevation of theophylline serum concentrations could have a significant impact on drug therapy decisions. Serum was obtained from six elderly subjects who had been taking 400 mg of pentoxifylline three times daily for at least six months. A serum assay using the TDx fluorescence polarization immunoassay failed to detect any measurable amount of theophylline. Normal doses of pentoxifylline and the resulting metabolites do not appear to interfere with the TDx method of serum assay for theophylline.