Severe hypermagnesemia without reduction in the anion gap.

The serum anion gap (AG) is usually calculated by the equation: AG = Na - (Cl + HCO3), reflecting the difference between the total level of unmeasured anions (all anions other than chloride and bicarbonate) and that of unmeasured cations (all cations other than sodium). Thus, several recent reviews have cited hypermagnesemia as a cause of a reduction in the AG. Whether this might occur depends in part on the nature of the accompanying anion (if the etiology of the hypermagnesemia is an exogenous load of magnesium), as well as on the presence of concomitant changes in endogenous unmeasured anions. In the present paper, we describe an unusual patient whose severe hypermagnesemia was not associated with AG reduction. Although multiple factors, including lactic acidosis, might have countervailed the effect of her hypermagnesemia on the AG, there is no actual substantiation in the literature of an association between increased serum Mg and AG lowering. In conclusion, the finding of a normal AG has no documented negative predictive value, and the finding of a low AG has no documented positive predictive value for the diagnosis of hypermagnesemia.