Comparison Between Measured and Calculated Free Calcium Values at Different Serum Albumin Concentrations

INTRODUCTION: Free ionic calcium is the metabolically active component of total calcium (TCa) in blood. However, most laboratories report TCa levels that are dependent on serum albumin concentration. Hence, several formulae have evolved to calculate free calcium levels from TCa after adjustment for albumin. However, free calcium can directly be measured using direction selective electrodes rather than spectrophotometric methods used in autoanalyzers. OBJECTIVES: This study compares the levels of free calcium obtained by measurement by direct ion selective electrode (ISE) and the one calculated as a function of TCa by formulae. MATERIALS AND METHODS: A total of 254 serum samples submitted to clinical biochemistry laboratory of a tertiary care hospital were analyzed for total protein, albumin, and TCa by standard spectrophotometric methods and for free calcium by direct ISE. Three commonly used formulae viz. Orrell, Berry et al. and Payne et al. were used to calculate adjusted TCa. Calculated free calcium was obtained by taking 50% of these values. RESULTS: A significant difference (P < 0.05) was observed between calculated free calcium by all the three formulae and measured free calcium estimated by direct ISE using paired t-test and Bland–Altman plots. CONCLUSION: Formulae for predicting free calcium by estimating TCa and albumin lacks consistency in prediction and free calcium should be evaluated by direct measurement.