Use of Saliva as an Alternative Matrix to Serum/Plasma for Therapeutic Drug Monitoring Using Reverse-Phase HPLC

This study was conducted to examine and verify the use of saliva as an alternative matrix for monitoring phenytoin drug levels in patients with epilepsy. Drug concentrations are measured to evaluate whether a suitable drug level has been achieved to minimize the risk for toxicity, inadequate efficacy, or therapy resistance and compliance issues.

Quantitative analysis was performed by using reverse-phase HPLC after sample pretreatment with acetonitrile. Seventy-eight patients who met the inclusion/exclusion criteria were examined in this study. Trough concentrations of both saliva and serum were taken at steady state.

Of the 78 patients enrolled, only 11 (14.1%) had normal levels. Twenty-eight patients (35.9%) had subtherapeutic levels, and 39 (50%) had toxic levels. Simultaneously, salivary phenytoin levels were analyzed; only 13 patients (17.3%) had therapeutic levels, 25 patients (33.3%) had subtherapeutic levels, and 37 (49.3%) had toxic levels. Among the study population, most of the patients were aged 31 to 40 years (25.6%) followed by the age group 21 to 30 years (19.2%). The lowest percentage of patients were in the age groups 71 to 80 years and >80 years (1.3%) each. This study found a statistically significant relationship between free serum and salivary phenytoin levels (P < 0.001). A very weak and insignificant correlation was observed between serum/salivary phenytoin levels and sex/age of the study population. The results of the present study support the use of saliva as an alternative to serum/plasma for monitoring phenytoin therapy.

The free concentration of a drug represents the freely diffusible drug fraction, which is the therapeutically active form. Accordingly, the free drug concentration correlates to clinical efficacy and drug toxicity better than total concentration.