In medicine, concentrations are often expressed as milliequivalents per liter (mEq/L) or millimoles per liter (mmol/L). But what’s the difference?
A mole is the International System of Units (SI) base unit for the amount of a substance (just like second for time, meter for length, kilogram for mass, etc.) There are ~6.022 x 1023 (Avogadro’s constant) quantifiable “things” (molecules, atoms, ions, etc.) in one mole of a sample. The same sample’s molar mass is determined by adding its constituents from the periodic table. For example, potassium chloride (KCl) has a molar mass of ~74.6 grams/mole (~39.1 g/mol for K, ~35.5 g/mol for Cl). A mmol is one-thousandth of a mole.
Similarly, a mEq is one-thousandth of an equivalent. Without delving into the chemistry definitions, mEq/L is often used for plasma electrolyte concentrations. mEq is related to the number (mmol) and electrical charge of ions in solution. Univalent ions like potassium (K+), sodium (Na+), and chloride (Cl–) have the same number of moles as equivalents in solution. For example, a normal plasma sodium range could be 135 – 145 mEq/L or mmol/L. In contrast, divalent ions like calcium (Ca2+) or magnesium (Mg2+) produce twice the equivalents (e.g., 1 mmol/L of calcium = 2 mEq/L).
In summary, “mEq/L” is most likely encountered when dealing with electrolyte concentrations. In the case of univalent ions, “mEq/L” can be replaced with “mmol/L.” A number without a unit is meaningless!
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