Theophylline Intoxication-Like Signs Despite Normal Serum Concentration in an Older Patient

Theophylline is a bronchodilator with a narrow therapeutic index. Theophylline toxicity can manifest as metabolic acidosis, hypokalemia, arrhythmia, and other life-threatening symptoms. A 90-year-old Asian woman with a 10-day history of hyporexia presented to the emergency room with shortness of breath and low SpO₂. Diagnostic tests revealed ketosis, metabolic acidosis, hypokalemia, and hypercalcemia. Diabetic ketoacidosis and alcoholic ketoacidosis were ruled out based on the patient's history and laboratory parameters. When it was discovered that the patient had been previously prescribed theophylline, theophylline toxicity was suspected, despite a serum concentration of 16.6 μg/mL, which was within the range typically considered safe. She received symptomatic infusion therapy and corrective treatment for electrolyte abnormalities and was discharged 15 days later. Theophylline intoxication can occur even when serum concentrations do not exceed the therapeutic range, and the severity may be higher among older patients.