Which of the following is a commonly encountered xanthine that could potentially interfere with the determination of theophylline?

• A. Nicotine
• B. Caffeine
• C. Amphetamine
• D. Procainamide

Answer: (B)

Substances that resemble the analyte can interfere with its measurement, especially in immunoassays, when the analyte is a methylxanthine like theophylline. Theophylline and caffeine share a very similar chemical structure, both being methylxanthines, and caffeine is a common dietary exposure. Because of this similarity, caffeine can cross-react in some assays or co-elute in chromatography, causing interference and potentially falsely elevated or inaccurate theophylline readings. That makes caffeine the commonly encountered xanthine that could affect theophylline determination.

Nicotine, amphetamine, and procainamide are not xanthines; nicotine is a pyridine alkaloid, while amphetamine and procainamide are different classes of compounds. Their chemical nature makes them unlikely to cause the same type of interference in theophylline measurements as caffeine does. In practice, using more specific methods like LC-MS or adequately separated chromatographic assays helps minimize any caffeine-related interference.