Prospective study comparing the outcome of a population-specific adjusted calcium equation to ionized calcium.

Background: Calcium circulates bound to albumin and changes in albumin concentration will therefore affect total calcium measurements. In order to mitigate this, correction factors are frequently used. The most widely used correction equation was described by Payne and colleagues in 1973. This equation was derived from well-defined hospitalized patients' data. Current clinical practice is consistent with the general application of the adjusted calcium equation irrespective of clinical setting. This study aims to assess the validity of this approach by the derivation of a community care-specific adjusted calcium equation ('community equation') and the comparison of its performance to a hospitalized patient equation and ionized calcium.

Method: Retrospective data were collected according to Payne's criteria from an inpatient and community care setting. Data were used to derive the two equations: the in-patient equation and community equation. The outcome of these equations was compared with ionized calcium obtained from 123 healthy participants.

Results: The community equation correctly identified the calcium status of 92% of the 123 healthy participants, while the inpatient equation identified 46% only. Regression analysis against ionized calcium showed a higher R for the community equation than for the inpatient equation. Furthermore, we have shown that mean albumin and calcium concentrations are significantly different between these two populations.

Conclusion: In this study, we found that the diagnostic accuracy of the adjusted calcium equation in ambulant patients was improved by the derivation of a population-specific equation for the community care setting.