Evaluation of theophylline therapeutic drug monitoring service

Background Therapeutic monitoring of theophylline serum levels is required due to its narrow therapeutic range and marked interindividual pharmacokinetic variability. We evaluated therapeutic drug monitoring service for theophylline in Slovenian clinical setting, which currently includes no pharmacokinetic evaluation of measured theophylline serum concentrations. Methods We  retrospectively evaluated 127 randomly selected theophylline serum level determinations performed in 2010 in a tertiary clinical setting in Slovenia. Demographic data, information on theophylline dosing and blood sampling was collected from patients’ data files. Authors evaluated the appropriateness of the following procedures: indications for theophylline serum concentration measurement, timing of blood sampling and dosage adjustments made after theophylline levels had been reported. On the basis of collected data, population pharmacokinetic model for theophylline was built and further used for the evaluation of dosage adjustments. Results Out of 127 cases, 107 (84.3%) had clinically justified indication for theophylline serum level measurement. Near half of measurements (44.9%) were performed before the steady state of theophylline concentrations was established. 65% of measured concentrations were subtherapeutic and the average measured concentration was below therapeutic range (53.1 μmol/L). Despite subtherapeutic concentrations the dose of theophylline was mainly not increased. Pharmacokinetic model enabled the calculation of average optimal daily dose which was significantly higher than the average actual daily dose used (876 mg vs. 572 mg, p < 0.001). Conclusions Theophylline TDM service should be optimized and pharmacokinetic interpretation of theophylline serum levels should be integrated into clinical practice.