Evaluation ofThree First-Generation I -Selective Electrode Analyzers for Lithium: Systematic Errors, Frequency of Random Interferences, and Recommendations Based on Comparison with Flame Atomic Emission Spectrometry

Ion-selective electrode analyzers for measuring lithium (Lu ISE) in serum became available in early 1987. We compared results for patients’ samples from three of them vs results from flame atomic emission spectrometry (FAES). Within-run and day-to-day imprecision ranged from 0.01 to 0.03 mmol/L and 0.01 to 0.04 mmol/L, respectively. Comparing U/ISE results ( with the FAES results (x) gave the following equations: y = 1.063x 0.035 for AMDEV’s Lytening 2, y = 1.020x + 0.038 for NOVA’s Model 11, and y = 1.030x 0.027 for AVL’s Model 985. Unexplained positive errors >0.2 mmol/L were observed for two of the 90 patients’ samples, but only a few additional excessively high values were seen in 3000 patients’ samples run subsequently (Lytening 2). Causes of error in clinical Li/ISE measurements are still unclear; simply characterizing them as “matrix effects” does not correct the underlying analytical problem. An increase in pH from loss of CO2 gave low results on two of the three Li/ISE analyzers but did not change FAESresults. Trimethylammonium bicarbonate used in a reconstitution solution caused extremely high Li/ISE results but did not change FAES results. Performance specifications to help reduce and correct these errors are recommended.