Total Calcium and Adjusted Calcium: An Australasian Review of Current Evidence and Proposed Approaches

Abstract

To assess a patient’s calcium status, it is preferable to measure the ionised (unbound) calcium, the fraction which exerts a biological effect. This is routinely done in the critical care setting using a blood gas analyser. However, higher costs and manual handling requirements with the need for prompt sample processing limit the availability of ionised calcium measurement. In practice, total calcium is usually the first-line requested test, with clinicians and laboratories then using linear formulae to ‘adjust’ (or ‘correct’) the total calcium result for albumin concentrations. There is, however, frequent disagreement between ‘adjusted’ calcium and measured ionised calcium. Many inherent problems have been reported for applying adjustment equations to total calcium across different patient groups and clinical settings. This may impact patient diagnosis and treatment. Important considerations in the development of adjustment equations are calcium and albumin methods used, data collection (inclusion and exclusion criteria), and reference population (hospitalised vs ambulant patients, adults vs children, and people with specific diseases). There is much scope to improve and harmonise routine reporting and assessment of a patient’s calcium status in Australasia. As patients and clinicians move between health services and pathology providers, it is in the patient’s best interest that we have a consistent approach to calcium reporting that is valid and understood by all.