For optimal theophylline therapy of asthma it is now strongly recommended to aim at plasma or serum levels of 10-20μg/ml. However, significant intrapatient variability in theophylline kinetics is often observed during the course of theophylline treatment. In the present study, 19 asthmatic children were repeatedly checked for theophylline clearance and were also clinically scored based on physical examina tions. Theophylline clearance values ranged from 0.034 to 0 .146 1/kg/hr among these patients. The highest percentage change in clearancewithin the same individual was found to be 48% . There was no significant relationship between changes in the clearance value and the clinical score (N=30, r=0.291). Dose-dependent eli mination kinetics of theophylline was indicated for some patients even within the desirable therapeutic serum concentration range. However, intrapatient variations in clear ance caused difficulties in assessing the dosedependent kinetics.
Since large intrapatient variability can occur, a single determination of theophylline clearance cannot be used safely to predict future dosage requirements . It is necessary to monitor theophylline levels regulary even without changes in the dose, because significant variability in theophylline clearance with time may lead to disproportionate changes in plasma theophylline concentration with serious clinical consequences.
