# Why do we use mEq/L instead of mmol/L to quantify physiological electrolytes?

I was looking at a table in my Guyton and Hall Medical Physiology Textbook, specifically in the membrane potential chapter, and noticed that the concentrations were written in that table using the units mEq/L, I was wondering, why wasn’t the units mmol/L used? What advantage does mEq/L confer in the given context? Does it have to do with the Nernest Equation calculations? (Note: I am aware of mEq/L being the number of univalent counter ions needed to react with a given species of ions, if there is a better definition, please do inform me).

• Pretty weird to learn that it’s archaic... my textbook is the 13th edition, released in 2016. Commented Apr 25, 2022 at 7:43
• Wikipedia is wrong. This unit of measure is used every day in clinical practice all over the United States. Source: the electronic health record used in half of all hospitals in the US. Commented Apr 25, 2022 at 12:30
• @IanCampbell Ah okay! Then it might be a region specific thing, haven’t seen it in Europe :) Commented Apr 25, 2022 at 13:31
• @Narusan There is certainly a push to move more towards SI units. But once the entire population learns glucose in `mg/dL`, it's hard to switch. I recall asking this question about why we use mEq/L to my instructors in school and don't think I got a very satisfactory answer. Commented Apr 25, 2022 at 14:25
• The origins are somewhat archaic as far as I know..that doesn't mean they aren't still commonly used. Archaic in this case is meaning more like...1900s, though, not ancient Greece. Commented Apr 25, 2022 at 16:51

Good question! When calculating electrolyte activity, mEq is more accurate, because electrolyte activity depends on the amount of charge present.

But we still use mmol/L, because mEq = mmol when the valence of the electrolyte is 1. All cellular life has electrolyte valences of 1. And medically, only 2 units are traditionally used for clinical chemistry : mmol/L and mg/dL. For now, I disregard international units, IU or U, which is a third style.

Ionized calcium and magnesium, are technically not considered electrolytes. So your screenshot is using mEq to keep the units the same, as you need more math to go from mEq to mmol. Consider the electrolyte value of sodium in mEq, then Google this value in mmol, and you shall see they are equal! The same does not hold true for ionized calcium.

• Hello. Answers on Medical Sciences (even correct ones) are expected to include references to back up claims and information in them. Please edit your answer to include your sources. Here is a list of reliable sources to get you started in case you need them. Commented Sep 27, 2023 at 13:02