Influence of magnesium sulfate-induced hypermagnesemia on the anion gap: role of hypersulfatemia.

Although hypermagnesemia purportedly lowers the anion gap (AG), we have shown previously that increases in the serum concentration of the unmeasured cation (UC) Mg due to therapeutic infusion of MgSO4 are not associated with AG reduction. To assess our hypothesis that increases in serum SO4 (unmeasured anion, UA) offset the effect of elevated serum Mg on the AG, we prospectively studied 11 patients receiving MgSO4 intravenously for toxemia of pregnancy. After 6 h of MgSO4 infusion, serum Mg increased by 2.1 +/- 0.2 (SE) mEq/l (p less than 0.001) without a significant decrease in the AG. Concomitantly, serum SO4 increased by 1.4 +/- 0.2 mEq/l. Comparison of the renal handling of SO4 versus Mg showed a higher fractional excretion of the former, probably accounting in part for the smaller increment of serum SO4 than of Mg. Comparison of the change in serum SO4 minus that of Mg indicated that, on the average, 70% of the observed 1.0 +/- 0.7 mEq/l reduction in AG was accounted for by the observed changes in the two pertinent unmeasured ions. A small decrement in serum Ca probably was a quantitatively minor factor tending to obviate a greater decrease in AG. We conclude that hypersulfatemia attenuates the reduction in AG that would otherwise accompany MgSO4-induced hypermagnesemia.