A FURTHER PROSPECTIVE EVALUATION OF AN EQUATION TO PREDICT DAILY LITHIUM DOSE

Background: Recently, one prospective study confirmed the safety and a ccuracy of a lithium-dose prediction equation created by Zetin et al., but no definitive conclusion on the reliability of the equation has b een established as yet. Method: The authors applied the Zetin et al. e quation to 18 chronic male schizophrenic inpatients. Predicted doses t o reach the serum lithium concentration of 0.4 mmol/L were calculated and prescribed in the form of lithium capsules. At Weeks 1 and 3 after treatment initiation, morning blood samples were collected about 12 h ours after the last lithium dose for the measurement of serum lithium concentrations. Results: None of the 18 patients achieved the desired concentration (0.4 mmol/L) exactly. The mean +/- SD of serum lithium c oncentrations at Week 1 was 1.01 +/- 0.29 mmol/L (range, 0.2-1.5) and at Week 3 was 0.94 +/- 0.35 mmol/L (range, 0.2-1.8). Lithium concentra tions were lower than 0.4 mmol/L in only 1 patient and were higher tha n 0.4 mmol/L in the other 17 patients. The deviations from the expecte d value were significantly correlated with the renal function (blood u rea nitrogen and serum creatinine levels) but not with the neuroleptic doses administered to the patients. Moreover, our patients were relat ively older and weighed relatively less than the patients described in the previous prospective study. Conclusion: The Zetin et al. equation cannot always accurately predict a required lithium dose. Renal funct ion data, even when they range within normal values, may be useful to improve the accuracy of the equation, particularly in patients who are older or weigh less than the norm.