[Evaluation of Payne's formula for the correction of calcium: comparison with improved calcium and albumin measurement methods].

The ionized or free fraction of serum calcium is physiologically important for cellular function, but we most often measure total serum calcium. There are a number of correction formulas that can be used to estimate whether low total serum calcium can be attributed simply to low albumin or serum protein. In Japan, Payne's formula has been widely used to correct calcium concentration. However, there are some problems in the measurement methods of total calcium and serum albumin which were used to establish Payne's formula with respect to specificity, calibration curve and stability. Recently, improved measurement methods of calcium and albumin have been adopted at clinical laboratories. Here we evaluated Payne's formula by comparing it with improved measurement methods of total calcium and serum albumin. For the total calcium measurement, o-CPC (o-cresolphthaleincomplexone), CPZ(chlorophosphonazo) III, and enzymatic methods were used. For the serum albumin measurement, BCG (bromocresol green) and improved BCP(bromocresol purple) methods were used. The results of this comparison study suggest that the calcium correction equation is not affected by changes in total calcium concentration, but the assay used for albumin may affect the calcium correction equation. Using multiple linear regression, the following equations were derived: BCG between CPZ III [corrected Ca(mg/dL) = total Ca-0.76ALB + 3.2], and improved BCP between CPZ III [corrected Ca = total Ca-0.7ALB + 2.6]. These formulas are simplified respectively as [corrected Ca = total Ca + 0.8(4-ALB], and [corrected Ca = total Ca + 0.7 (4-ALB)]. We conclude that Payne's formula is valid with the BCG method, but with the improved BCP method, our formula is more suitable for correcting calcium.