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

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)+0.401 Ca(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.