Reconsideration of "Albumin corrected total calcium" Determinations: Potential errors in the Clinical management of disorders of calcium metabolism.

BACKGROUND

The formula, referenced in major textbooks, for albumin corrected calcium [Calcium(alb)] may not accurately depict ionized calcium [ICa]. We evaluated the accuracy of unadjusted calcium [Calcium(Unadjusted)] and [Calcium(alb)], and developed a protocol for local laboratory adjustment of calcium for albumin.

METHODS

Laboratory data were obtained from an electronic health record. Assessments were accuracy, false positive, and false negative rates. Clinical reliability was defined in "error zones" for calcium [Ca]: Zone A = Ca(normal), ICa(low); Zone B = Ca(low), ICa(normal); and Zone C = Ca(normal), ICa(high), Zone D = Ca(high), ICa(normal).

RESULTS

A linear regression from 468 laboratory tests was used to derive a formula for "revised corrected calcium" [Calcium (revised)] over a range of albumin concentrations where, [Calcium (revised)] = plasma calcium (mg/dl) + [(4- Albumin (g/d)L)]*(plasma calcium (mg/dl)*0.052)]. [Calcium(alb]] vs [Calcium(Unadjusted)] decreased zone B errors 12%, [95%CI;8-15%], vs 44% [95%CI;37-50%], p < 0.001. However, [Calcium(alb]] vs [Calcium(Unadjusted)] increased zone A error (60%,[95%CI;42-78%], vs 7% [95%CI;1-13%], p < 0.001). [Calcium (revised)] decreased zone A errors (15%, [95%CI;6-24%]) vs [Calcium(alb) ] (60% [95%CI;42-78%], p < 0.001) and Zone D errors from 9% [95%CI;6-12%] to 2% [95%CI;1:5%, p < 0.001].

CONCLUSIONS

[Calcium(alb)] is unreliable in hypo- or hypercalcemia. We provide a protocol for locally derived correction of calcium for albumin.