What Is an Anion Gap?

Your overall health depends in part on your body’s ability to keep blood within a neutral pH range, meaning it is not too acidic or basic. A blood test called a serum anion gap test is used by healthcare providers to diagnose conditions caused by an imbalance between the acids and bases in the blood.

The anion gap blood test, along with other diagnostic tests, can help healthcare professionals provide specific treatments, especially when individuals experience a sudden illness such as altered mental status or acute kidney failure.

Purpose of the Anion Gap Blood Test

An anion gap test can help diagnose metabolic acidosis, a life-threatening condition that occurs when your blood becomes too acidic. It can also detect alkalosis, when your blood is not acidic enough. It is important to find the cause and treat it as soon as possible. 

Metabolic acidosis may occur due to:

• Ketone build-up as a consequence of uncontrolled diabetes (diabetic ketoacidosis)
• Ingestion of certain medications, or toxicity from methanol, aspirin overdose, or antifreeze
• Severe infections
• Excessive diarrhea (extreme loss of bicarbonate)
• Medical conditions such as cancer
• Kidney disease, including proximal renal tubular acidosis—a condition in which the kidneys do not reabsorb enough bicarbonate and therefore it is lost in the urine
• Respiratory distress

Lactic acid builds up following vigorous exercise (lactic acidosis), but this is not typically enough to cause changes in the body's pH.

Indications

You might need an anion gap blood test if you experience symptoms of an electrolyte imbalance such as:

• Fatigue
• Shortness of breath
• Nausea
• Vomiting
• Rapid heart rate
• Low blood pressure

During the Anion Gap Blood Test

The anion gap blood test usually takes less than five minutes and doesn't require any specific preparation.

During the anion gap blood test:

• A healthcare professional uses a needle to puncture your skin and access a vein.
• They then draw a small sample of blood into a tube.
• The tube of blood is sent to the laboratory for analysis and the anion gap is calculated.

The anion gap blood test is relatively safe and may be done at a hospital, emergency room, or in lab. You may have slight pain or bruising at the injection site.

The anion gap is not usually ordered as a separate test. It tends to be ordered as part of an electrolyte panel, basic metabolic panel (BMP), or comprehensive metabolic panel (CMP).

What Does It Mean When Your Anion Gap Is Low or High?

A low anion gap means you have a lower amount of acid in your blood than normal, but this result is uncommon and usually due to a lab error. A low anion gap test result may be caused by alkalosis.

A high anion gap test result means you have more acid in your blood than normal. A high anion gap test result may be caused by acidosis.

Although there are differences between laboratories and assays, the anion gap normal range has traditionally been set between 8 mEq/L to 12 mEq/L. But, there is a wide range of normal values—often 8 to 10 mEq/L—thus an increase in anion concentration can be present in the absence of an increased anion gap.

In some cases, a false normal anion gap can occur due to severe kidney disease or certain medications. A normal anion gap with a high bicarbonate level can be one indication that the result is a false normal.

Treatment

Correcting the underlying cause of the high anion gap (metabolic acidosis) is the primary way to mitigate potential serious health consequences.

In mild to moderate acidosis, treatment may include supportive measures, such as intravenous (IV) fluids and respiratory support. Alkalization therapy may be used for individuals with severe acidosis (pH less than 7.1 and bicarbonate less than 6 mEq/L). 

To fix a low anion gap, treatment will depend on the underlying cause.

Treatment options may include:

• Oxygen therapy if your oxygen level is low
• Chloride and potassium if alkalosis is due to a mineral or electrolyte loss