Milliequivalents ↔ millimoles

What is this for?

Electrolyte orders in the United States are written in milliequivalents, while much of the world and most laboratory reports use millimoles. A milliequivalent measures charge; a millimole measures particle number. For a monovalent ion such as potassium the two are numerically identical, but for a divalent ion such as calcium or magnesium, 1 mmol carries 2 mEq of charge.

How to use it

1. Choose whether you are converting from mEq to mmol or the reverse.
2. Enter the amount.
3. Select the valence of the ion in question — not of the salt. Calcium in calcium chloride is divalent even though chloride is monovalent.

Worked example

A magnesium order is written for 20 mEq. The laboratory system requires the dose in millimoles. Magnesium is divalent.

Answer: 20 mEq ÷ 2 = 10 mmol. Reporting 20 mmol instead would represent a two-fold overdose.

Clinical pearls & pitfalls

• The single most common error is assuming a 1:1 relationship for every ion. It holds only for monovalent ions.
• Phosphate is the notable exception to valence-based conversion: because its charge varies with pH, phosphate products are labelled in millimoles of phosphorus, not milliequivalents. Never convert phosphate using a fixed valence.
• Milliequivalents describe the ion, not the salt. To get from a salt weight in milligrams to mEq you need the molecular weight of the salt as well — use the milliequivalents calculator for that.

Assumptions & limitations

• Assumes complete dissociation and a fixed valence. This does not hold for phosphate, and holds only approximately for partially dissociating salts such as magnesium sulfate.
• Does not convert from a mass in milligrams. That requires the molecular weight of the specific salt.

References

• Institute for Safe Medication Practices. Principles of Designing a Medication Label for Injectable Vials and Ampuls.
• American Society of Health-System Pharmacists. ASHP Guidelines on Preventing Medication Errors with Electrolyte Solutions.
• United States Pharmacopeia–National Formulary monographs for electrolyte injections (labelling in mEq and mmol).

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