A study on the bioequivalence of lithium and valproate salivary and blood levels in the treatment of bipolar disorder

Lithium (Li) and valproate (VPA) are used in the treatment of bipolar disorder (BD), with narrow therapeutic window requiring periodic control of serum levels. This prevents intoxication, lack of efficacy due to low serum concentrations, and allows monitoring adherence. We aimed at evaluating the bioequivalence of salivary and blood levels of LI or VPA in a sample of adult BD patients. Secondarily, lithium bioequivalence was evaluated across different patients' lifespans. BD patients treated with either Li or VPA underwent contemporary standard serum and salivary measurements. Blood levels of both drugs were taken according to standard procedures. Li salivary levels were performed by an adapted potentiometric method on the AVL9180 electrolyte analyzer. VPA salivary levels were taken with an immune-assay method with turbidimetric inhibition. A total of 50 patients (38 on Li, 12 on VPA) were enrolled. Blood-saliva bioequivalence for VPA was not found due to a high variability in salivary measures. Li measures resulted in a high correlation (r=0.767, p<0.001), showing no partial correlation with age (r=0.147, p=0.380). Li salivary test is a reliable method of measuring Li availability and is equivalent to serum levels. Potential advantages of Li salivary testing are its non-invasive nature and the possibility of doing the test during the usual appointment with the psychiatrist.