Part of the book: Severe Sepsis and Septic Shock
Plasma drug concentration is not homogeneous within the intravascular space, being the arterial (PA) concentrations higher or lower than that in veins (PV) depending on whether the samples are taken during the drug absorption or the elimination phases, respectively. However, blood samples are currently withdrawn from peripheral veins and total (bound plus unbound) plasma drug concentration is assayed. Despite the fact that free plasma drug levels (PfV) are not determined in routine therapeutic drug monitoring (TDM), they could be assayed for research purposes. Salivary drug concentrations (S) approximate to their free plasma levels in the arteries of the great circulation. Saliva is recommended to be collected with stimulation to minimize the difference between the pHs of both fluids (saliva and blood), and thus, artery/vein-free drug concentration ratios (PfA/PfV) could be surrogated by the stimulated saliva/free-plasma in vein drug concentration ratios (SS/PfV). It is possible in this way not only to assess this S/P ratio but also to infer the brain (B)/PfV ratio, which is actually the most relevant for antiepileptic drugs (AEDs). Different cases of AEDs are considered in this review, taking into account their physiochemical properties and their ability to be transported by membrane carriers.
Part of the book: Epileptology