RED-CELL EFFECTS ON LITHIUM MEASUREMENTS BY ION-SELECTIVE ELECTRODE

The possible interference of haematocrit was studied in an evaluation of the analytical performance of the NOVA 11 analyzer. For lithium con centrations between 0.56 and 0.68 mmol/L, the within-run imprecision w as less than 1.06%, and less than 2.8% in day-to-day imprecision, meas ured in quality control solutions. Comparison of whole blood with plas ma measurement showed satisfactory correlation with deviations up to 0 .06 mmol/L for haematocrits from 35 - 53% (y = 0.73 x + 0.12). Measure ment on a blood specimen not containing lithium, whose haematocrit was varied, allowed us to quantify a specific red cell interference. With samples from treated patients, and a sample spiked to various lithium concentrations, we obtained differences dependent on haematocrit and on lithium concentrations. In subtracting the specific red cell effect , we could systematically observe that apparent lithium levels diminis h as haematocrit increases. This observation should allow development of an appropriate algorithm for obtaining exact values on whole blood samples.