A simpler and more accurate equation to predict daily lithium dose

Recently, the authors suggested that the lithium dose prediction equation c reated by Zetin and associates cannot always accurately predict a required lithium dose and that the inclusion of renal function data may improve the accuracy of the equation. The charts of 70 patients were reviewed to obtain data regarding factors thought to affect serum lithium concentrations, inc luding renal function, and an equation to estimate the dose intended to ach ieve an expected concentration was derived by stepwise multiple linear regr ession. The equation was also applied to 30 other patients to evaluate its accuracy. The authors obtained the following equation: daily lithium carbon ate dose (in milligrams) = 100.5 + 752.7 x (expected lithium concentration in millimoles per liter) 3.6 x (age in years) + 7.2 x (weight in kilograms) - 13.7 x (blood urea nitrogen [BUN] in milligrams per deciliter). When the equation was applied to 30 patients, the mean +/- SD of deviations from th e expected concentration was 0.15 +/- 0.30 mmol/L, and 19 patients (63%) ha d deviations of less than 0.20 mmol/L. On the other hand, when the equation see forth by Zetin and associates was applied to the same patients, the me an +/- SD of deviations from the expected concentration was 0.52 +/- 0.42 m mol/L, and only 6 patients (20%) had deviations of less than 0.20 mmol/L. A lthough it is necessary to measure BUN levels before starting lithium, this equation may be simpler and more accurate than that offered by Zetin and a ssociates.