Evaluation of Drug Interferences in the Ultraviolet Spectrophotometric Analysis of Plasma Theophylline

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Abstract
Forty-five commonly used drugs were evaluated for potential Interference in the widely employed ultraviolet spectrophotometric analysis of plasma theophylline (7,8). Eighteen compounds interfered ≧ ± 10% when extracted from plasma at a concentration of 20 mg/liter and back-extracted into 0.1 N NaOH: group A—dicoumarol, gentamicin, methyclothiazide, nitrofurantoin, oxyphenbutazone, phenylbutazone, probenecid, and sulfisoxazole; group B—acetaminophen, 1,7-dimethylxanthine (caffeine metabolite), dyphylline, furosemide, sulfadiazine, sulfadimethoxine, sulfamethoxazole, theobromine, thiopental, and warfarin. Theophylline could be measured free from interference by the drugs in group A but not group B when back-extraction into 1 N HCl was performed. Of 1,750 theophylline analyses done in our laboratory by the Jatlow method (8) for 600 different patients during 1976 and 1977, 1.8% of analyses (3.7% of patients) showed interference from one or more of the drugs listed above: sulfamethoxazole (as Spectra® or Bactrim®) 50% of cases, furosemide (Lasix®), probenecid (Benemid®), acetaminophen (Tylenol®), gentamicin, and nitrofurantoin (Macrodantin®).