The hypocalcemic effect of inorganic sulfate infusions

Inorganic sulfate is a divalent anion that forms a soluble ion-pair complex with serum calcium, but the extent to which infusions of sulfate salts may depress the concentration of ionized calcium has never been quantitated. In a study of 9 patients who received sodium sulfate infusions as part of a standard diagnostic workup for their renal tubular acidosis, we observed a decrease in mean ionized calcium (adjusted to pH 7.40) from 1.15 +/- 0.01 to 1.04 +/- 0.02 mmol/l (p less than 0.01). The changes in ionized calcium were highly correlated with those in serum sulfate (r2 = 0.95; p less than 0.01). Quantitatively, an increase of 1 mmol/l in serum sulfate was associated with a decrease of 0.017 mmol/l in ionized calcium, a result that is in close agreement with in vitro data based on simple salt solutions. Diagnostic sulfate infusions should be used with caution in any patient predisposed to hypocalcemia.