Clinical predictors of theophylline blood levels in asthmatic patients.
To determine the usefulness of clinical information in predicting theophylline levels, 21 parameters were studied in 204 asthmatic patients. The best single parameter for predicting theophylline levels was the last outpatient level (r = 0.484), which was within +/- 5 micrograms/mL of the presenting theophylline level in 62.5% of cases. However, there was considerable variability in theophylline levels in the other 37.5% of cases. The best combination of predictors was the last outpatient level and time since the last dose in patients taking a short-acting preparation. Even with this combination, however, 20.8% of predicted levels fell outside a range of +/- 5 micrograms/mL of the presenting theophylline levels. The only patients in whom a theophylline level could be predicted reliably were those who reported taking a sustained-release or short-acting preparation more than 15 or 8 hours prior to evaluation, respectively. In these patients, all presenting theophylline levels were in the subtherapeutic range (ie, less than 10 micrograms/mL), with 92% of the levels less than 5 micrograms/mL. Except for these patients, readily available theophylline determinations are necessary in order to optimize theophylline therapy with minimal risk of toxicity.