Failure of Total Calcium Corrected for Protein, Albumin,and pH to Correctly Assess Free Calcium Status*

Abstract

The clinical effectiveness of published algorithms in correcting serum total calcium (Ca_r) for the effects of protein, albumin, and pH was tested. Corrected calcium (Ca_c) values obtained by 13 of these methods were compared with values of measured free calcium (Ca_F) in 55 samples from normal controls and 404 samples from patients with various disorders of calcium metabolism. Three criteria were used to compare either Ca_c or Ca_r with measured Ca_F: 1) the correlation coefficient, 2) the average absolute deviation from measured Ca_F of the values of Ca_F predicted by the linear regression of Ca_F on each Ca_c, and 3) the number of samples in which Ca_c or Car gave a differentimpression of normality than measured Ca_F. Application of the 13 published algorithms produced varied results, but none produced substantially better agreement between Ca_c and Ca_F than was found between Ca_r and Ca_F. The application of additional algorithms derived by multiple linear regression using our data base gave slightly better results than any of the published algorithms, but many values of Ca_c remained whichwere disparate from the measured value of Ca_p. Correction of measured total calcium by using other concurrently obtained chemistry values does not seemto adequately predict calcium status as measured by free calcium.