Misclassification of Calcium Status Based on Albumin-Adjusted Calcium: Studies in a Tertiary Hospital Setting

BACKGROUND: Clinical laboratories measure total calcium and adjust for albumin concentrations to predict calcium status. We compared total and adjusted calcium (Adj-Ca) with ionized calcium (Ca²⁺) for correct assignment of calcium status. The effect of restriction of Adj-Ca reporting in patients with hypoalbuminemia was determined on the basis of frequency of misclassifications.

METHODS: Extraction of laboratory results was performed for 24 months. Adj-Ca was calculated from a modified Payne formula. A further prospective data set for 6 months was collected after stopping reporting of Adj-Ca for patients with an albumin <3.0 g/dL. The agreement between Ca²⁺ and Adj-Ca or total Ca was assessed with Cohen's kappa statistic.

RESULTS: In 5553 hospitalized patients, 13604 paired Ca²⁺ results were analyzed retrospectively. Prospective collection in 1113 paired samples was from 450 patients. Adj-Ca was a poor predictor of calcium status compared to the Ca²⁺ reference standard in both data sets (agreement 56.9% in the first, 65.6% in the second data set). Renal failure and low albumin concentrations were associated with worse agreement between Adj-Ca and Ca²⁺. Restriction of reporting of Adj-Ca to albumin concentrations >3.0g/dL improved correct classification of calcium status from 65.6% to 77.6% (P < 0.0001). Total Ca performed better than Adj-Ca for low albumin (<3.0g/dL) and performed similarly in samples with albumin >3.0g/dL.

CONCLUSIONS: Adj-Ca is unreliable for the classification of calcium status in hospital patients when compared to Ca²⁺. Adj-Ca overestimates calcium for patients with renal impairment and albumin concentrations <3.0g/dL. Restriction of reporting Adj-Ca for albumin below 3.0 g/dL reduces the number of misclassified patients.