INCREASED ABSOLUTE CALCIUM-BINDING TO ALBUMIN IN HYPOALBUMINEMIA

The amount of Ca bound to protein was measured in 30 patients with differing diseases and varying degrees of hypoalbuminemia. Total serum Ca increased directly with both serum albumin and ultrafilterable Ca concentrations. The estimated amount of Ca bound per gram of albumin varied inversely with the albumin concentration, decreasing from 2.0 to 1.0 mg Ca/g albumin as albumin concentration increased from 1.7 to 3.1 g/dl. Circulating parathyroid hormone (PTH) concentrations varied inversely with measured ultrafilterable Ca concentrations. The frequency of raised PTH concentrations decreased as serum albumin increased. Use of a conventional correction factor for albumin binding (0.88 mg Ca bound per gram of albumin) to calculate corrected total Ca led to major errors in estimating ultrafilterable Ca in these patients. The PTH concentrations in turn correlated with the degree of deviation between estimated and measured ultrafilterable Ca concentrations. Ionized Ca was low in 7 of 10 additional hypoalbuminemic patients studied whereas correction of total Ca for albumin indicated normocalcemia in all. Thus, correction of total Ca in patients with hypoalbuminemia by formulae which use a fixed binding ratio of Ca to albumin may give an erroneous impression of normocalcemia. The increase in Ca binding ratio during hypoalbuminemia needs to be considered during assessment of Ca status in these patients.