Pitfalls in the interpretation of serum theophylline levels

In two groups of patients, noncompliance with drug regimens resulted in misinterpretation of serum theophylline levels. All six chronic asthmatics in the first group, found in a retrospective review of 43 outpatient charts, had outpatient serum theophylline levels in the therapeutic range (10 to 20 microgram/mL) and at least a 7-microgram/mL greater inpatient theophylline level while receiving the same dosage. When hospitalized, four of these patients had serum theophylline levels in the toxic range (greater than 20 microgram/mL). In the second group four hospitalized patients had persistently low serum theophylline levels despite an adequate theophylline dose. When compliance was enforced, serum theophylline levels rose significantly. Compliance cannot be assumed in the outpatient with a serum theophylline level in the therapeutic range or in the hospitalized patient. Determination of serum theophylline level after supervised drug administration is recommended in inpatients requiring unusually high doses of theophylline or in those whose condition is poorly controlled despite having serum theophylline levels in the therapeutic range (10 to 20 microgram/mL).