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

Ionized calcium (Ca2+) seems to be the best measure of active serum calcium but, in France, numerous laboratories do not have Ca2+ analyzers so that numerous clinicians use Payne's formula to obtain adjusted calcium (CaAd) 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 Ca2+ and total calcium (CaT) in accordance with approved guidelines. CaAd was calculated with Payne's formula whereas the dependence of Ca2+ with serum protein, albumin and CaAd was investigated by linear regression, the goodness-of-fit of each equation with the measure of Ca2+ being studied. Taking into account serum protein and albumin levels, multiple linear regression gave the equation: Ca2+ (mmol/l) = 0.188 − 0.00469 protein (g/l) + 0.0110 albumin (g/l) + 0.401CaAd with r2 = 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 Ca2+ was better, for any group, with our formula than with Payne's formula. When Ca2+ was expressed with CaT instead of CaAd, albumin term was no longer significant and the new equation was: Ca2+ (mmol/l) = 0.592 − 0.00449 protein (g/l) + 0.410 total calcium (mmol/l) with r2 = 0.438. We propose an alternative to direct measurement of Ca2+ with a simple formula usable in geriatric units, which are often deprived of high-performance equipment.