Anion Gap Units: mEq/L vs mmol/L
The anion gap is reported in either milliequivalents per litre (mEq/L) or millimoles per litre (mmol/L). For the four ions that matter clinically — sodium, chloride, potassium, bicarbonate — the two units are numerically identical. Here is why, and where the distinction actually matters.
Why mEq/L equals mmol/L for monovalent ions
An equivalent (Eq) is the amount of a substance that carries one mole of charge. The conversion is simple: mEq/L = mmol/L × valence. For monovalent ions — sodium (Na⁺), chloride (Cl⁻), potassium (K⁺), and bicarbonate (HCO₃⁻), each carrying a charge of 1 — the valence is 1, so 1 mEq/L = 1 mmol/L. The two units are numerically interchangeable. A serum sodium of 140 mEq/L is also 140 mmol/L; a chloride of 104 mEq/L is also 104 mmol/L. Because the anion gap formula — Na⁺ − (Cl⁻ + HCO₃⁻) — only uses monovalent ions, the anion gap units you quote will be the same number whether the lab reports in mEq/L or mmol/L. You can confirm this with the anion gap calculator, which accepts either convention without conversion.
The valence table
| Ion | Valence (charge) | mEq/L ↔ mmol/L | Relevant to the anion gap? |
|---|---|---|---|
| Sodium (Na⁺) | 1 | 1 mEq/L = 1 mmol/L | Yes — major measured cation |
| Chloride (Cl⁻) | 1 | 1 mEq/L = 1 mmol/L | Yes — major measured anion |
| Potassium (K⁺) | 1 | 1 mEq/L = 1 mmol/L | Optional (K-inclusive variant) |
| Bicarbonate (HCO₃⁻) | 1 | 1 mEq/L = 1 mmol/L | Yes — major measured anion |
| Calcium (Ca²⁺) | 2 | 1 mmol/L = 2 mEq/L | No — minor unmeasured cation |
| Magnesium (Mg²⁺) | 2 | 1 mmol/L = 2 mEq/L | No — minor unmeasured cation |
| Albumin | ~−17 (polyvalent) | Reported as g/dL, not mEq/L | Yes — dominant unmeasured anion |
Conversion for divalent ions
For divalent ions such as calcium (Ca²⁺) and magnesium (Mg²⁺), the valence is 2, so the units diverge: 1 mmol/L = 2 mEq/L. A serum calcium of 2.5 mmol/L equals 5.0 mEq/L, and a magnesium of 0.8 mmol/L equals 1.6 mEq/L. These ions are deliberately excluded from the daily clinical anion gap formula because they are small and relatively constant, but they appear in extended quantitative frameworks such as the Stewart strong-ion difference, where every charged species contributes and unit conversion must be exact. Outside those research-grade calculations, the mEq/L-versus-mmol/L distinction for Ca²⁺ and Mg²⁺ is a non-issue for routine anion gap interpretation.
SI units vs conventional units
Global laboratory practice splits into two conventions. SI (Système International) units, used in most of Europe, Australasia, and Canada, report electrolytes in mmol/L. Conventional (US) units, used in the United States, report in mEq/L. For the monovalent ions, the numbers are identical and no conversion is needed — a sodium of 140 is 140 on both scales. The trap appears elsewhere on the metabolic panel: glucose, BUN, creatinine, and calcium use mg/dL in conventional units and mmol/L in SI units, and those do require conversion. The electrolyte panel reference page lists each analyte in both conventions.
What laboratories actually report
North American laboratories almost universally report Na⁺, Cl⁻, K⁺, and HCO₃⁻ (or total CO₂) in mEq/L. Most international laboratories report the same four values in mmol/L. The anion gap calculated from either report is the same number with the same unit label — the only question is which label the lab prints. When documenting a result in a chart that mixes conventions, label the unit explicitly (e.g. "AG 14 mEq/L") to avoid ambiguity. For anion gap unit conversion between the two systems, the practical rule is: no conversion is required, because the four relevant ions are all monovalent and the valence factor is 1.
Because Na⁺, Cl⁻, K⁺, and HCO₃⁻ all carry a single charge, the anion gap in mEq/L is numerically equal to the anion gap in mmol/L — the unit label is the only difference for routine clinical use.