Evaluation of a single-point method for predicting theophylline dosage

Prediction of theophylline maintenance dosage based on a single serum theophylline determination following an intravenous aminophylline loading dose was studied. A previously reported nomogram was modified and used to predict retrospectively the maintenance dose needed to achieve a steady-state serum theophylline concentration of 15 mg/liter in 42 adult patients. The nomogram was based on the assumption that the patients had no theophylline in the body before the loading dose. The nomogram prediction was compared with maintenance doses calculated from a minimum three-point serum concentration-time curve (actual maintenance dose). The mean difference between the predicted and actual doses was 21.88% (range =-53.9 to 223.9%). Predictive error was clustered around the 0-20% range of overestimation of maintenance dose. Patients with total body clearances of greater than 40 ml/kg/hr had significantly less predictive error than other patients. No other significant differences were noted when patients were grouped according to duration of infusion time or amount of loading dose. The modified single-point nomogram predicts theophylline maintenance doses accurately in adult patients with total body clearances of greater than 40 ml/kg/hr. It should be used only with extreme caution in other patients.