Plasma anion gap is a measurement of the balance between positively charged ions called cations and negatively charged ions called anions, within the plasma.
Its normal range is typically between 3 and 11 mEq/L, while anything below 3 mEq/L is considered abnormally low, and above 11 mEq/L is usually considered abnormally high, and.
Every single moment, trillions of cations and anions are floating around inside our blood vessels. For them to happily and stably coexist, the plasma has to be kept electrically neutral.
That means that the sum of all positive charge from cations has to equal the sum of all negative charge from anions.
The vast majority of cations are sodium Na+ ions, followed by potassium K+ ions, then calcium Ca2+ ions, then magnesium Mg2+ ions, and finally various positively charged proteins.
The majority of anions are chloride Cl− ions, followed by bicarbonate HCO3− ions, then phosphate PO43- ions, then sulfate SO42- ions, and finally some organic acids and negatively charged plasma proteins, like albumin.
So, to prove that there’s electroneutrality, let’s say we try to measure the concentration of the cations and anions in our plasma.
Unfortunately, not all of the ions are easy or convenient to measure. Specifically, among cations, usually just sodium Na+ is measured, which is typically around 137 mEq/L and among anions, chloride Cl− is measured, which is about 104 mEq/L, and bicarbonate HCO3− is measured, which is around 24 mEq/L.
So just counting up these three ions, there’s a difference, or “gap” between the sodium Na+ concentration and the sum of bicarbonate HCO3− and chloride Cl− concentrations in the plasma, which is 137 minus 128 (104 plus 24) or 9 mEq/L.
This is known as the anion gap, or in other words, how many more cations are there than anions.
Now just a few moments ago, we said that cations equal anions, so why does this gap even exist? Well, it’s because sodium Na+ accounts for the vast majority of cations in the plasma, but by measuring only chloride Cl− and bicarbonate HCO3−, we are ignoring a bunch of anions, including the anion component of several organic acids and negatively charged plasma proteins, like albumin.
In other words, this anion gap represents all these unmeasured, ignored negative charges out there, and normally, ranges between 3 and 11 mEq/L.
If the anion gap is high, it’s usually because there’s an unusually high amount of these unmeasured anions.
Calculating the anion gap is a useful diagnostic tool, because it can help identify potential causes of metabolic acidosis.
“Acidosis” refers to a process that lowers blood pH to less than 7.35 and “metabolic” refers to the fact that it’s caused by a decrease in the concentration of bicarbonate HCO3− ions.