Validity of "corrected" calcium values.

Individual values for the regression of plasma total calcium on albumin vary by more than can be accounted for by analytical imprecision alone; the range (-x +/- 2 S.D.) being 0.004--0.031 mmol/g. Individual regression coefficients (R) are constant with time. Furthermore, a large proportion of individual R values differ from a general population regression by more than can be accounted for by analytical error. Three adjusted calcium reference ranges were derived from the data. Two employed constant correction factors, the general population R value (0.013 mmol/g) and the mean of the individual R values (0.018 mmol/g). The reference range which utilised each individuals' own R value was smaller and therefore potential more useful than the ranges using constant factors.