Lithium dosing equations: are they accurate?

Several equations are used to predict lithium doses necessary to attain therapeutic serum concentrations. A number of studies have evaluated these equations; however, few equations were compared simultaneously.

To assess the accuracy and precision of published dosing equations in predicting daily lithium doses and to evaluate if pertinent laboratory tests were performed prior to initiation.

A retrospective analysis was performed of patients who received lithium at the Medical University of South Carolina Institute of Psychiatry between July 2010 and July 2012. Using dosing equations, expected lithium doses were calculated based on corresponding serum concentrations identified in patient charts. Expected doses were then compared with actual lithium doses. The primary end point was to assess the accuracy and precision of dosing equations using mean differences in daily lithium doses and standard deviations. Secondary end points included presence of pertinent laboratory tests and use of concomitant interacting drugs .

Of 155 patients identified, 59 were eligible for analysis. Equations developed by Abou-Auda et al and Pepin et al did not predict doses that were significantly different from actual doses. Conversely, equations by Jermain et al, Terao et al, and Zetinet al did predict statistically different doses.

Abou-Auda et al developed a predictive lithium dosing equation that was more accurate than equations developed by Jermain et al, Terao et al, and Zetin et al and more precise than the Pepin et al equation. Further study evaluating the influence of equations on clinical outcomes is warranted.