Anion Gap

The anion gap is calculated from routine electrolytes: sodium, chloride, and bicarbonate. It estimates whether there are “unmeasured” charged particles, mostly acids, circulating in the blood. These acids are often the key to diagnosing metabolic acidosis, a condition where the body’s fluids become too acidic.

To calculate the anion gap, the formula most often used is: AG = sodium – (chloride + bicarbonate).

Clinically, the anion gap is especially useful in separating different types of metabolic acidosis. A high anion gap acidosis suggests the presence of extra acids, such as lactic acid (from shock or severe infection), ketones (from diabetes or prolonged fasting), or toxins (like methanol or antifreeze). A normal anion gap acidosis, also called hyperchloremic acidosis, usually points toward bicarbonate loss from the gut or kidneys. In many real-world cases, the anion gap can also uncover mixed disorders, where more than one process is affecting acid–base balance.

That said, while the anion gap is a powerful tool, it is not perfect. The relationship between anion gap and specific acids, such as lactate, is not always exact. Other conditions, like multiple myeloma, may alter the gap because of unusual proteins in the blood. Correction formulas that adjust for albumin can help in certain situations, but even these have limitations.