title: Comparison Between Measured and Calculated Free Calcium Values at Different Serum Albumin Concentrations.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  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.
  
  
  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.
  
  
  CONCLUSIONS
  Formulae for predicting free calcium by estimating TCa and albumin lacks consistency in prediction and free calcium should be evaluated by direct measurement.
authors.affiliation: Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh, India.
authors.forename: Altaf Ahmad
authors.initials: AA
authors.lastname: Mir
authors.affiliation: Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh, India.
authors.forename: Bela
authors.initials: B
authors.lastname: Goyal
authors.affiliation: Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh, India; Department of Laboratory Medicine, All India Institute of Medical Sciences, New Delhi, India.
authors.forename: Sudip Kumar
authors.initials: SK
authors.lastname: Datta
authors.affiliation: Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh, India.
authors.forename: Saidaiah
authors.initials: S
authors.lastname: Ikkurthi
authors.affiliation: Department of Biochemistry, Postgraduate Institute of Medical Education and Research, Chandigarh, India.
authors.forename: Arnab
authors.initials: A
authors.lastname: Pal
keywords: Calculated calcium; ISE; free calcium