Estimation of theophylline absorption rate by means of the Wagner-Nelson equation.

The elimination rate constant of theophylline to be used in calculating the cumulative amount absorbed per milliliter of the volume of distribution is estimated from terminal theophylline concentrations of the same data set as that analyzed for absorption kinetics. Erroneous results are obtained if the elimination rate constant is obtained from a different treatment such as an oral solution or elixir or from intravenous data, and/or if the wrong asymptotic value is used. So-called "trickle absorption" is shown to be an artifact. Published data indicate appreciable intrasubject variation in the elimination rate constant of theophylline.