Evaluation of a new colorimetric assay for serum lithium

Using patient samples (n = 175) collected in our clinical chemistry laboratory, we have undertaken an analysis of a new colorimetric dry slide-based serum lithium (Li+) assay from Eastman Kodak for its Ektachem instrumentation series. Analyzer imprecision was acceptable, and good correlation was seen between the Ektachem assay and an ion-selective electrode (ISE)-based assay currently in use in our laboratory (r = 0.99). At all concentrations tested, potassium (K+), triglycerides, or bilirubin did not detectably interfere with the Ektachem determination of Li+. At concentrations within the reference range (135-145 mmol/L), sodium (Na+) did not affect the Ektachem Li+ determination. A slight Na(+)-dependent positive bias in the Li+ determination was evident at 157 mmol/L, but became clinically significant (> or = 0.2 mmol/L) only at physiologically extreme concentrations (> 188 mmol/L) of Na+. Very high concentrations (> 325 mg/dl) of hemoglobin also were found to cause a clinically significant positive bias. We conclude that the determination of Li+ by the Kodak Ektachem is precise, accurate, and adequately free from bias due to common interferents or other monovalent cations, and, therefore, is an acceptable alternative to currently available methods for the monitoring of serum LI+.