A new formula for correction of total calcium level into ionized serum calcium values in very elderly hospitalized patients

Abstract

Ionized calcium (Ca 2+) seems to be the best measure of active serum calcium but, in France, numerous laboratories do not have Ca 2+ analyzers so that numerous clinicians use Payne's formula to obtain adjusted calcium (Ca Ad) values. In frail very elderly patients with protein/energetic malnutrition and very low concentrations of albumin, “correction” with Payne's formula usually gives false hypercalcemic results, so that hypocalcemia may be seriously underdiagnosed. Two hundred and ninety-four patients of 80 years and older with serum albumin level <35 g/l were included in four French hospitals for elderly people. Biological measurements were standardized in order to determine Ca 2+ and total calcium (Ca T) in accordance with approved guidelines. Ca Ad was calculated with Payne's formula whereas the dependence of Ca 2+ with serum protein, albumin and Ca Ad was investigated by linear regression, the goodness-of-fit of each equation with the measure of Ca 2+ being studied. Taking into account serum protein and albumin levels, multiple linear regression gave the equation: Ca 2+ (mmol/l) = 0.188 − 0.00469 protein (g/l) + 0.0110 albumin (g/l) + 0.401Ca Ad with r 2 = 0.442. The relative difference between the measure and the value given by the equation did not depend upon the center, and the correlation between measured and computed values of Ca 2+ was better, for any group, with our formula than with Payne's formula. When Ca 2+ was expressed with Ca T instead of Ca Ad, albumin term was no longer significant and the new equation was: Ca 2+ (mmol/l) = 0.592 − 0.00449 protein (g/l) + 0.410 total calcium (mmol/l) with r 2 = 0.438. We propose an alternative to direct measurement of Ca 2+ with a simple formula usable in geriatric units, which are often deprived of high-performance equipment.