INTRODUCTION: One commonly used equation which continues to be widely mentioned in text books and hence familiar to clinical people is total calcium + 0.02 (40 - albumin). This equation was derived using cresophthalein complexone and bromocresol green (BCG) methods for measuring serum total calcium and serum albumin respectively. However this equation maybe invalid when applied to calcium and albumin results generated by alternative assays. Hence we aim to derive an albumin-adjusted calcium equation specific to our laboratory's total calcium and albumin methodologies.
MATERIALS AND METHODS: A total of 3,175 adult University Malaya Medical Centre (UMMC) patients deemed free of any calcium metabolism disorders were selected and divided into two groups for derivation and validation. Simple linear regression associating total calcium and albumin was constructed from the data in the derivation group. The new albumin-adjusted calcium equation was validated in the validation group. Differences in calcium status classification following adjustments based on existing and new albumin-adjusted calcium equation was compared in a 469 hypoalbuminaemic patients.
RESULT: The new albumin adjusted calcium equation was: total calcium + 0.014 x (39-albumin). Of the 469 hypoalbuminemic patients, 78 were classified differently based on new equation. Based on the new equation, 55 normocalcemic patients were classified as hypocalcemic and 22 were classified as normocalcemic instead of hyperclacaemic.
CONCLUSION: Based on the newly derived albuminadjusted calcium equation 17% of patients had different adjusted calcium classifications. This could potentially impact in the management. It is recommended that laboratories derive equations specific to their calcium/albumin methods and analytical platforms.
* Title and MeSH Headings from MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.