In recent years, saliva has been utilized for TDM. The advantage is that collection is noninvasive and painless and so it has been used as a specimen of choice in pediatric TDM. Due to the low protein content of saliva, it is considered to represent the unbound or free fraction of drug in plasma. Since this is the fraction considered available for transfer across membranes and therefore responsible for pharmacological activity, its usefulness is easy to understand. Saliva collection methods are known to influence drug concentrations but if these are compensated for and a standardized procedure utilized, correlation between plasma and saliva drug concentrations may be demonstrated for several drugs (e.g., phenytoin). Inconsistent results have been found for some drugs such as phenobarbital, so additional studies are needed to clearly define the limitations of testing saliva for TDM.

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