Evaluation of three simple methods for predicting therapeutic lithium doses

The bias and accuracy of three simple methods for predicting lithium doses were assessed in this prospective study. In each patient, we computed the predicted doses (PDs) of lithium by applying three formulas. The actual dose (AD), the lowest lithium dose that was enough to produce a serum lithium concentration higher than 0.80 mmol/l, was determined. The PD computed by each formula was then compared with the AD to identify the one with the least bias and the greatest accuracy in predicting therapeutic lithium doses. As mean prediction error (ME) is a convenient measure of bias, the prediction error (PE) of each comparison was computed by subtracting the AD from the PD. Accuracy was assessed as a function of the root-mean-squared prediction error (rMSE). Seventeen psychiatric inpatients participated in this study. The 95% confidence interval of ME of a proposed formula [Zetin, M., Garber, D., De Antonio, M., Schlegel, A., Feureisen, S., Fieve, R., Jewett, C., Reus, V., Huey, L.Y., 1986. Prediction of lithium dose: a mathematic alternative to the test-dose method. Journal of Clinical Psychiatry 47, 175-178] was across zero (-15.48 to 133.72). In addition, the rMSE of that formula was also the lowest one (152.66 mg/day). The method proposed by Zetin et al. (1986) is the least biased and the most accurate way to predict therapeutic lithium doses.