The adjusted serum calcium concept--a reappraisal.

The relationship between serum calcium and albumin was studied using a data base containing biochemical results on approximately 687,000 samples. To exclude patients with established bone, liver and kidney disease, calcium and albumin values had to be accompanied by additional results falling within the laboratory reference range for urea, phosphate, globulin, alkaline phosphatase and aspartate transaminase activities. The regression coefficients of calcium on albumin changed significantly following a change of instrumentation, even though the analytical principles of the methods remained the same. In addition, using standard laboratory methods, the relationship between calcium and albumin deviated from linearity at albumin concentrations less than 30 g/L. Care is needed in the selection and clinical application of factors for the adjustment of serum calcium for changes in the concentration of circulating albumin. Using a large data base, an albumin-related reference range for serum calcium may be calculated, eliminating the need for adjusting calcium values.