What Does the Anion Gap Tell You?

The anion gap is a measurement that tells you the acid levels in the blood. It is the difference between primary measured cations (sodium and potassium, which are positively charged ions) and the primary measured anions (chloride and bicarbonate, which are negatively charged ions) in the blood.

This test is most commonly performed in patients with symptoms consistent with a condition caused by high levels of acid in the blood, such as:

• Altered mental status
• Abnormal heartbeat 
• Unknown exposures
• Acute renal failure
• Acute illnesses
• Metabolic acidosis
• Shortness of breath
• Nausea/vomiting
• Confusion 

The anion gap is measured by a blood test. This blood test checks electrolytes in the blood, specifically sodium, potassium, and chloride. Electrolytes are positively or negatively charged, and the anion gap test measures these charges and determines if they are balanced.

The anion gap tells you if your electrolytes are unbalanced, which can cause changes in the acid levels in the blood. An anion gap result can be low, normal, or high.

A low anion gap (less than 6 mEq/L) may indicate:

• Low levels of albumin in the blood (hypoalbuminemia)
• Plasma cell disorder
• Monoclonal protein
• Bromide intoxication
• Normal variant

A normal anion gap (6-12 mEq/L) may indicate:

• Loss of bicarbonate (i.e., diarrhea)
• Recovery from diabetic ketoacidosis
• Ileostomy fluid loss
• Use of carbonic anhydrase inhibitors (acetazolamide, dorzolamide, topiramate)
• Renal tubular acidosis
• Arginine and lysine in parenteral nutrition
• Normal variant

An elevated anion gap (greater than 12 mEq/L; “mud pilers,” which is an acronym for the main causes of elevated anion gap) may indicate:

• Methanol
• Uremia
• Diabetic ketoacidosis
• Propylene glycol
• Isoniazid intoxication
• Lactic acidosis
• Ethanol ethylene glycol
• Rhabdomyolysis/Renal failure
• Salicylates