The clinical and analytical aspects of albumin adjusted calcium and an assessment of the validity of harmonisation between laboratories

The routine measurement of the biologically active fraction of calcium 'ionised calcium' is fraught with difficulties. Therefore, an assessment of this fraction is estimated through a mathematical equation using the regression of albumin on calcium. In clinical practice, the most widely used equations for calcium estimation were derived by Payne et al (1973). To date this equation has been derived using hospitalised patients' data after excluding patients with diseases that may disturb calcium metabolism. Since the inception of their use, reports have described many inherent problems with applying a calcium equation to adjust total calcium measurements across different patient groups. However, no robust alternative approach or method for calcium measurement has been proposed by these critics. This study aims to elucidate the reasons behind suboptimal performance of the free calcium estimation (hereafter called adjusted calcium), causes for variability in adjusted calcium reporting as well as exploring new concepts to improve adjusted calcium performance such as the derivation and validation of a population specific equation for ambulant patients. For this purpose, three data sets were collected; first, a retrospective data set from 14 hospitals and communities representing various analytical platforms. These data were collected from all the methods that are commercially available for calcium measurement and were used to derive calcium equations, for hospitalised patients and ambulant patients. The second was a prospectively collected data set. This set was used to validate the newly proposed population specific equation for ambulant patients for a single analytical platform (Roche Cobas). The third data set includes an external quality data set that was collected to assess analytical and post analytical factors that impact on adjusted calcium classifications such as analytical performance goal of calcium and albumin and their reference intervals. Results from this study showed that in comparison to 'ionised calcium', the population specific equation performed better in ambulant patients than an equation derived from hospitalised patients. Replacing creatinine ≤ 200 Âμmol/L with estimated glomerular filtration rate (eGFR) > 60 mL/min/1.732 in the inclusion criteria prior to an equation derivation was without effect whereas replacing the calcium population mean with a mean of 2.4 mmol/L in the equation worsened the equation performance, leading to misclassification of calcium status. Reference interval studies for calcium and albumin showed a wide variation between various analytical platforms and methods. This finding was supported by the analytical performance studies. The Association of Clinical Biochemistry (ACB) proposed approach for data gathering (which includes total calcium values between 2.0 and 2.7 mmol/L) for equation calculation, appears promising and may lead to reduced adjusted calcium variability between laboratories. However, it has been shown that one of the major problems adding to increased adjusted calcium variability is related to analytical variability of the calcium and albumin methods. Albumin methods (Bromocresol green and Bromocresol purple) accounted for significant differences in the classification of adjusted calcium results which was found to be related to a difference in albumin concentrations measured by these assays. The mean albumin concentration of these assays was consistently higher for BCG than BCP methods irrespective of platform. Calcium methods showed a closer agreement with the exception of Siemens calcium assays, for which re-calibration should be seriously considered. This study showed a population specific equation concept can be a solution for improving the adjusted calcium equation in ambulant patients. The same concept opens the door for more sub-division and consideration of an age and gender related adjusted calcium equation. It has been found that standardising adjusted calcium equations and reducing variability in adjusted calcium results is a remote possibility without eliminating albumin and calcium calibration issues. Until these issues are resolved, laboratories should continue deriving their own equations according to Payne's described method. However, the introduction of a population specific equation in combination with the ACB recommended data collection approach is a step forward towards reducing variability in adjusted calcium reporting.