The effect of different analytical platforms and methods on the performance of population-specific adjusted calcium equation

Abstract

Background A recent attempt to improve the diagnostic value of adjusted calcium addressed a primary care-specific adjusted calcium equation, but validated the new equation for Roche Cobas, BCG and NM-BAPTA methods only. In this study, we aim to validate a population-specific equation for other methods and platforms. Method We collected retrospective patient data-sets from 15 hospital laboratories using a range of commercially available analytical platforms and methods for calcium and albumin measurements. Raw data-sets were collected and filtered according to Payne’s criteria, and separate adjusted calcium equations were derived for hospitalized and primary care patients. Results Mean albumin and calcium results were significantly higher in primary care populations ( P < 0.0001). The prevalence of hypocalcaemia using adjusted calcium ranged between 6% and 44% for inpatient data-sets and was higher in users of BCG methods. The application of community-specific adjustment equation to primary care data-sets reduced the prevalence of hypocalcaemia (mean 1.7%, range 0.8–3.7%). Conclusion We demonstrated that the use of a community-specific calcium adjustment equation to a primary care population reduces both the percentage and the variation of hypocalcaemia between different laboratories.

In reply to Roberts

“…However, our data from 15 clinical laboratories show significantly lower means, with an inpatient population mean for calcium of 2.2 mmol/L and a primary care calcium population mean of 2.3 mmol/L. 2 We also showed that the calcium population mean has a major impact on the outcome of the adjusted calcium equation. 2 When the normalized equation performance was tested against ionized calcium in 123 subjects, as described in our population-specific equation paper, 3 the normalized equation overestimated calcium status in healthy individuals and failed to identify any of the nine hypocalcaemic subjects.…”

“…2 We also showed that the calcium population mean has a major impact on the outcome of the adjusted calcium equation. 2 When the normalized equation performance was tested against ionized calcium in 123 subjects, as described in our population-specific equation paper, 3 the normalized equation overestimated calcium status in healthy individuals and failed to identify any of the nine hypocalcaemic subjects. The same finding was observed when the normalized equations for 15 hospitals were compared with inpatient equations derived using Payne's criteria (Figure 1).…”

Calcium – An adjustment too far or not far enough?

“…This issue of the Annals sees the publication of two papers by Jassam et al. 1 , 2 which, together with their two previously published papers, 3 , 4 make a short series on the broad theme of calcium albumin adjustment equations.…”

Adjustment of calcium and Pathology Harmony

“…I read with interest the recent papers by Jassam et al., 1 , 2 regarding the use of population-specific adjusted calcium equations. In particular, the factors which may affect the performance of locally derived adjustment equations are investigated in more detail revealing many potential pitfalls.…”