RESEARCH/PRACTICE REPORTS EFFECT OF ASSAY METHODOLOGY ON THE PREDICTION OF LITHIUM MAINTENANCE DOSAGE

ABSTRACT; Test dose methods have been used to predict the maintenance dosage of lithium carbonate tablets in bipolar patients. These methods require a lithium analytical method that is accurate to the second decimal place. Successful use of these test methods has employed the atomic absorption spectrophotometric method, yet many hospitals use a flame spectrophotometric method. We evaluated a two-point method to predict lithium maintenance dosages of slow-release lithium carbonate tablets in 20 bipolar patients using both assay methodologies. No clinically significant differences were found between the two sets of predictions, suggesting that either assay methodology has requisite sensitivity and accuracy for use in test dose predictions of lithium maintenance dosages. DIe? 1989;23:372-5. LITHIUM IS CONSIDERED the drug of choice in the treat-ment and prophylaxis of manic-depressive illness. 1.2 The drug has a narrow therapeutic serum concentration range, high toxicity potential, and a relatively long serum half-life.': " Steady-state serum concentrations vary widely among patients given standard dosages because of a large interpatient variability in systemic clearance.v " This necessitates the individualization of the dosage regimen. Various methods including single-point and two-point test dose methods have been tried to predict the maintenance dosage of lithium. 8-12 All the studies using these test dose methods successfully have used atomic absorption spectrophotometry. It has been emphasized that for the test dose methods to work, the laboratory must be able to accurately measure lithium to the sec-ond decimal place. 9 Many hospitals use flame photome