The lithium ratio as a guide to patient compliance

Noncomplannce with medication regimens not only reduces the benefits of proper therapy to the patient, but also complicates the clinician's appraisal of response to therapy. The magnitude of this problem is evident from a review of 50 compliance studies. One-quarter to one-half of all patients failed to comply adequately with medication orders.¹ Noncompliance is not inherent in any particular personality, ethnic, or socioeconomic group.². Psychiatric patients may pose a particular problem because their illness itself may interfere with their capacity to cooperate with pharmacotherapy. This makes medication compliance a major variable in assessing the response to psychoactive drugs such as the lithium ion (Li⁺). With regard to Li⁺, a partial solution to this problem has been the ability to measure the plasma concentration of the cation. However, because absorption of Li⁺ from the gastrointestinal tract is rapid, a patient who takes only a few doses of lithium prior to seeing his physician may appear to have "therapeutic" plasma concentrations of Li⁺ while still being noncompliant. For this reason, measurement of plasma Li⁺ concentration may not always give an accurate assessment of whether or not a patient has adhered to the prescribed regimen. The present investigation was designed to determine if the measurement of Li⁺ in another body compartment might provide the physician with a parameter that would enable him to more accurately assess the compliance of his patients. Since the erythrocyte (RBC) behaves pharmacokinetically as a tissue compartment separate from plasma,³ we examined whether the erythrocyte concentration of Li⁺ might be used as an indicator of patient compliance.