What Is an Anion Gap?

Indication of Acid-Base Balance in the Blood

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Normal blood contains electrically charged particles. An anion gap is a difference between the positively charged particles and the negatively charged particles, called cations (sodium Na+ and potassium K+) and anions (chloride Cl- and bicarbonate HCO3-). These charged particles contribute to a blood’s pH.

Our health is largely tied to the body’s ability to keep blood within a neutral pH range. 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 results are reported as part of a comprehensive metabolic panel.

The anion gap test, along with a physical exam, a focused history from the patient, and additional tests, helps healthcare professionals provide specific treatments, especially when patients present with a sudden illness such as altered mental status or acute renal failure.

Blood tests to be analyzed for anion gap
Yoshiyoshi Hirokawa / DigitalVision / Getty Images

Purpose of the Anion Gap 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 an 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 this test if you experience symptoms of an electrolyte imbalance—such as fatigue, shortness of breath, nausea and vomiting, rapid heart rate, or low blood pressure.

During the Test

The anion gap is not ordered as a separate test. This value is derived from the measurements of blood electrolytes. The test you will see ordered would be an electrolyte panel, basic metabolic panel (BMP), or comprehensive metabolic panel (CMP).

No specific preparation is needed before the test. It requires a venous blood draw, which may be done at the bedside if you are in the hospital or emergency room. If you are having this done as an outpatient, you may be sent to the lab for a blood draw.

A healthcare professional uses a needle to puncture your skin, access the vein, and draw a small sample of blood into a tube. The entire blood drawing process is usually less than five minutes long. The test is relatively safe. You may have slight pain or bruising at the injection site.

The tube of blood is then sent to the laboratory for analysis. The anion gap is calculated from the values obtained for the electrolytes.

The most common method of evaluation of metabolic acidosis involves the Henderson-Hasselbalch equation and the Lewis model interpretation of biological acidosis, which evaluates the plasma concentration of hydrogen ions.

Interpreting Results

A high anion gap means you have more acid in your blood than normal. 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.

Although there are differences between laboratories and assays, the normal anion gap 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.

What to Ask Your Doctor

A low or high anion gap does not necessarily mean that you have a serious health problem. The normal range varies from person to person, so make sure you ask your doctor to interpret the results of the test for you. 

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 could be limited to supportive measures, such as intravenous (IV) fluids and respiratory support. Alkalization therapy is reserved for individuals with severe acidosis (pH less than 7.1 and bicarbonate less than 6 mEq/L). 

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