Intra-individual variability of caffeine elimination in healthy subjects.

Caffeine is a popular test substance for assessing the activity of hepatic drug metabolizing enzymes in vivo and in vitro. A correct estimation of the relative magnitudes of intra-individual and inter-individual variations in caffeine elimination is significant for the use of the elimination parameter of caffeine to characterize the biotransformation capacity of a specific form of cytochrome P-450 (1AII) in vivo. The purpose of this study is to demonstrate the magnitudes of fluctuation of caffeine-clearance and half-life as well as inter- and intra-individual comparison in 12 healthy male subjects. Compared to the high reproducibility of caffeine decay curves in each healthy male, caffeine elimination varied more extensively between subjects. The distribution of variance amounted to: intra-individual 21.4%, inter-individual 78.6%. The knowledge of variance provided precise evidence of the sample size, which is necessary to prove previously defined differences.