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.