Interference of allopurinol and alloxanthine in ultraviolet theophylline assay

A modification of several spectrophotometric methods for assaying theophylline in biological fluids is presented; the maximum absorbances of theophylline, allopurinol and alloxanthine solutions were measured. Plots of absorbance vs wavelength from 320 to 190 nm wer made for four samples in 0.1 N sodium hydroxide. The maximum absorbances for the theophylline sample (20 microgram/ml) were at 274-275 and 217 nm; for allopurinol (1 microgram/ml), at 274, 275 and 216 nm; for alloxanthine (10 microgram/ml), a plot similar to that for allopurinol; and for an equal mixture of the three solutions, at 274 and 218 nm. Especially with allopurinol and alloxanthine, and possibly in the presence of drugs with similar basic structures, one might wich to use a method other than spectrophotometry for the assay of theophylline or discontinue administration of the interfering medication.