A reproducible sodium-lithium countertransport assay: the outcome of changing key laboratory parameters

This paper describes experimental conditions for reproducible measurement of sodium-lithium countertransport in red blood cells. The assay is sensitive to temperature (10% per degree C) and the condition of the red cells; it is affected little by changes in intracellular lithium over the range 2-7 mmol/1 or by red cell concentrations with laboratory hematocrits of 0.03 to 0.07. Repeated measurements of the same subjects give day-to-day coefficients of variation of 10% or less. The mean difference for interlaboratory comparisons is 11%.