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Open Access Highly Accessed Research article

Derivation and internal validation of an equation for albumin-adjusted calcium

Matthew T James12, Jianguo Zhang1, Andrew W Lyon3 and Brenda R Hemmelgarn12*

Author affiliations

1 Department of Medicine, University of Calgary, Calgary, Alberta, Canada

2 Department of Community Health Sciences, University of Calgary, Calgary, Alberta, Canada

3 Calgary Laboratory Services, Calgary, Alberta, Canada

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Citation and License

BMC Clinical Pathology 2008, 8:12  doi:10.1186/1472-6890-8-12

Published: 27 November 2008

Abstract

Background

Previously published equations to adjust calcium for albumin concentration may vary depending on factors such as the type of reagents used. Albumin-adjusted calcium equations derived from laboratories using the bromocresol purple (BCP) albumin binding reagent have not been described.

Methods

The linear regression equation for the binding of calcium and BCP-albumin was derived in a cohort of 4613 outpatients, and the albumin-adjusted calcium equation was internally validated in a separate cohort of 1538 subjects. The performance of this equation was compared with a previously published equation (adjusted [Ca](mmol/L) = total [Ca](mmol/L) + 0.02 (40 - [albumin] (g/L)) in 343 subjects with albumin < 33 g/L (below reference range).

Results

The local adjustment equation was expressed by the relationship; adjusted [Ca](mmol/L) = total [Ca](mmol/L) + 0.012 (39.9 - [albumin](g/L)). The equation showed evidence of good internal validity (shrinkage value of adjusted r2 = -0.0059). Classification of calcium status differed between the two equations in 47 of 343 subjects with low serum albumin (weighted κ = 0.46; moderate agreement).

Conclusion

A locally derived and internally validated albumin-adjusted calcium equation differed from previously published equations and resulted in important differences in classification of calcium status in hypoalbuminemia patients. Individual laboratories should determine their own linear regression equation for calcium on albumin rather than relying on published formulas.