Objective: Therapeutic drug monitoring for lithium therapy is important because of its narrow therapeutic range and non-specific nature of clinical findings of lithium toxicity. Methods such as ion-specific electrode (ISE), atomic absorption and flame emission have been used in lithium monitorization. The present study was performed to compare two analytical methods: the commonly used ISE assay (with Medica EasyLyte analyser) and the new spectrophotometric method developed by Thermotrace (Victoria, Australia) and applied to the Roche Modular P biochemistry autoanalyzer.

Material and Methods: Serum lithium levels were measured with spectrophotometric and ISE methods. The performances of both methods in measurement of lithium were evaluated.

Results: Between-run and within-run coefficient of variations (CV) for both assays were evaluated using commercial controls (Data-Trol Abnormal and Normal). Analytical sensitivities were defined as 0.2 mmol/L for ISE and 0.05 mmol/L for spectrophotometric method. The spectrophotometric method was found to be linear until 3.0 mmol/L. Dilutions were needed for higher levels. Interference studies were conducted with high and low levels of hemolysis, icterus, lipemia, and sodium. It has been observed that these factors did not affect the assay performance of spectrophotometric method.

Conclusion: Following relation was found between two methods: Spectrophotometric result = 0.942 ISE + 0.0094 (r=0.982). Although the 1/10 dilutions of samples, control and calibrator increased the manual work-load, it seemed that spectrophotometric measurement is safe alternative to ISE method in follow-up of lithium treatment.