Methodological assessment of assays for red cell sodium concentration and sodium-dependent lithium efflux

Assays for red cell sodium concentration and sodium-dependent lithium efflux were studied from a methodological point of view. The technical errors for sodium concentration--based on concurrent analysis of randomized blind duplicate samples--was 2.5% and for lithium efflux 6.4%. Based on repeated measures in the same individual, the assays were stable over time and reproducible; individual differences at a mean interval of 16 days were not significantly different. A wait of two hours from time of phlebotomy to analysis yielded a slight (3.8%) but significant fall in the sodium concentration; no further change was found at four hours. No change occurred in the lithium efflux over time. A standardized breakfast containing 664 mg sodium did not affect the measurements when fasting and three-hour postprandial levels were compared. The ratios of intra- to inter-individual variance were small--0.04 for red cell sodium concentration and 0.09 for sodium-stimulated lithium efflux. These assays appear to be reproducible and stable and can be applied in large scale field trials.