Are mEq/L and mmol/L the Same Unit?
For monovalent ions (valence = 1) such as sodium, potassium, and chloride, mEq/L and mmol/L are numerically identical and can be used interchangeably. However, for divalent ions (valence = 2) such as calcium and magnesium, 1 mmol/L equals 2 mEq/L because the milliequivalent accounts for ionic charge 1, 2, 3.
Understanding the Distinction
Monovalent Ions (Valence = 1)
- Sodium (Na⁺), potassium (K⁺), and chloride (Cl⁻) have a valence of 1, meaning 1 mmol = 1 mEq 1.
- Clinical guidelines consistently use these units interchangeably for these electrolytes—for example, hyperkalemia is defined as K⁺ >5.0 mEq/L or >5.0 mmol/L without distinction 1, 2, 3.
- When calculating osmolality, electrolyte concentrations expressed as mEq/L equal their osmolarity as mOsm/L for monovalent ions 4.
Divalent Ions (Valence = 2)
- Magnesium (Mg²⁺) and calcium (Ca²⁺) have a valence of 2, so the conversion differs: 1 mmol/L = 2 mEq/L 1.
- For magnesium, a level <1.3 mEq/L (or <0.65 mmol/L) is considered low 1.
- This distinction is critical when interpreting laboratory values and calculating replacement doses for divalent cations.
Clinical Application in Guidelines
Potassium Management
- The American Heart Association and European Society of Cardiology classify hyperkalemia using both units interchangeably: mild (>5.0 to ≤5.5 mEq/L or mmol/L), moderate (>5.5 to ≤6.0 mEq/L or mmol/L), and severe (>6.0 mEq/L or mmol/L) 1, 2, 3.
- Treatment thresholds reference both units without conversion—for example, ECG changes typically manifest when potassium exceeds 6.5 mmol/L (or 6.5 mEq/L) 1.
Glucose and Other Non-Electrolytes
- Glucose and urea are measured in mmol/L or mg/dL, not mEq/L, because they are non-ionized substances 1, 4.
- The conversion for glucose from mg/dL to mmol/L uses a factor of 18 (e.g., 180 mg/dL ÷ 18 = 10 mmol/L) 1.
Osmolality Calculations
- When calculating plasma osmolality, sodium concentration in mEq/L equals its contribution in mOsm/L because it is monovalent 4, 5.
- The simplest and most accurate formula is: Posm = 2[Na⁺] + glucose (mmol/L) + urea (mmol/L), where sodium is measured in mmol/L (or mEq/L interchangeably) 4, 5.
Common Pitfalls to Avoid
- Do not assume mEq/L and mmol/L are always identical—verify the valence of the ion in question 1, 4.
- For divalent cations (calcium, magnesium), always convert appropriately: divide mEq/L by 2 to get mmol/L, or multiply mmol/L by 2 to get mEq/L 1.
- Do not use mEq/L for non-electrolytes such as glucose, urea, or ethanol—these are measured in mmol/L or mg/dL only 1, 4.
- When reviewing international literature, be aware that some European sources use mmol/L exclusively, while North American sources may use mEq/L for electrolytes, but both refer to the same value for monovalent ions 1.