Biological and analytical variation of commonly determined blood constituents in healthy blood donors.

The relative contributions of analytical error and non-systematic biological variation to the reference range for 19 commonly-determined blood chemistry values have been estimated during the course of studying 1000 healthy blood donors. Estimates of non-systematic biological variations are arrived at by determining the variation found in donors matched for factors known to affect blood chemistry such as age and sex, and deducting the within-batch analytical error which applied. Comparison of the within-batch and between-batch analytical precision with the biological variation showed that the percentage increases in the normal range attributable to between-batch analytical error varies between 1 and 29%. This contribution is particularly large in the case of the electrolytes and amounts to no less than 29% in the case of calcium. Apart from well-known relationships a remarkable lack of correlation between the concentration of the different constituents in health is demonstrated.