Evaluating a low anion gap: A practical approach

A “Gamblegram” showing the relative abundance of extracellular anions and cations. The serum cations are Na⁺ (sodium), K⁺ (potassium), Ca²⁺ (calcium), and Mg²⁺ (magnesium). The serum anions are (bicarbonate), Cl⁻(chloride), (hydrogen phosphate), (sulfate), OA⁻ (organic acids), and PR⁻ (proteins). In this chart and in this article, for consistency, we use milliequiva-lents per liter (mEq/L) as the unit of measurement.

Note: Laboratories report anion gap in either mEq/L or millimoles per liter (mmol/L): 1 mEq/L is equal to 1 mmol/L multiplied by the valence charge of the ion. Since the anion gap calculation involves only variables with a valency of +1 or −1 (sodium, bicarbonate, chloride), the value in mEq/L will be identical to the value in mmol/L.