Relationship between intraindividual variation of the saliva/plasma- and of the arteriovenous concentration ratio as demonstrated by the administration of caffeine.

The use of saliva instead of venous blood for monitoring drugs has many advantages. Its usefulness has, however, been questioned, because of the variability of the intraindividual saliva/plasma ratio. Among other reasons, this variability can result from the fluctuation of the arteriovenous blood concentration ratio. The present work represents a detailed investigation of this phenomenon, using caffeine as an example. This phenomenon is observed when the concentration of free diffusible substances differs in the central and peripheral compartment, depending on the pharmacokinetic phase. In such cases, the salivary concentration more closely reflects the cellular concentration in organs of the central compartment than that of peripheral venous blood.