An evaluation of the Ektachem serum lithium method and comparison with flame emission spectrometry

We evaluated the performance of a new colorimetric method in dry chemistry for serum lithium (Li) assay using an Ektachem E500 analyser. Imprecision results were acceptable and the linearity was verified for concentrations within the range of 0.2-3.9 mmol l-1, i.e. y(measured) = 1.02x(calculated) + 0.07, r = 0.99. The Ektachem Li assay was unaffected by potassium (K), calcium (Ca) and magnesium (Mg) at all concentrations tested. Significant interference was caused by sodium (Na) and haemoglobin. Statistically and clinically significant interference was caused by high concentrations of Na (213 mmol l-1) with a bias of 0.20 mmol l-1 (p = 0.02) and by high levels of haemoglobin (280 mumol l-1) with a bias of 0.20 mmol l-1 (p = 0.01). Comparison of serum Li results from 80 patient samples assayed using the Ektachem method with those obtained using the IL943, a flame atomic emission spectrometry (FAES)-based method, gave a regression line equation: Ektachem = 0.95xFAES + 0.13, r = 0.96. The data revealed a mean difference of 0.10 mmol l-1 between the Ektachem and FAES results that was statistically significant (p = 0.01), but clinically negligible. We conclude that the Kodak method provides reliable Li serum results and represents a valid alternative to the FAES method.