The anion gap is a value that’s calculated using the results of an electrolyte blood test.
Electrolytes are elements and compounds that occur naturally in the body and control important physiological functions. Calcium, chloride, magnesium, and sodium, among others, are electrolytes.
Electrolytes have an electrical charge — some are positive and others are negative. They help to control the balance of acids and bases in your body.
The anion gap value is the difference between the negatively and positively charged electrolytes. If the calculated value for the anion gap is too high or too low, it may be a sign of a disorder.
If your doctor suspects that you have an electrolyte imbalance in your blood, they’ll order an anion gap blood test.
Symptoms may include:
- shortness of breath
- nausea or vomiting
- edema (accumulation of fluid)
- abnormal heartbeat
The anion gap value is reported in units of milliequivalents per liter (mEq/L). Normal results generally fall between 3 and 10 mEq/L. However, normal ranges may vary by lab.
A high anion gap value means that your blood is more acidic than normal. It may indicate that you have acidosis. Conditions that can cause acidosis (and therefore a high anion gap value) include:
- diabetic ketoacidosis, a condition requiring immediate medical attention
- overdose of salicylates, such as aspirin
- uremia (urea in the blood)
- ethylene glycol (antifreeze) poisoning
Having a low anion gap value is very rare. The most common causes of low anion gap results may include the following.
If your test indicates a low anion gap value, your doctor may order a second test to account for laboratory error.
Because the anion gap is calculated from the results of the electrolyte panel, accurate measurement of the individual electrolytes is necessary. A published review found that out of 67,000 calculations of anion gap, a low anion gap was calculated less than 1 percent of the time. Within this small percentage, over 90 percent of the results were due to laboratory error in calculating one of the electrolyte values.
Hypoalbuminemia means that there are low levels of a protein (albumin) in your blood. Albumin is one of the most abundant proteins in circulation, so a drop in the level of this protein would affect the anion gap.
If your doctor suspects hypoalbuminemia, they may order a blood test to assess the albumin levels in your blood.
Lower-than-normal albumin can be caused by the following conditions:
Monoclonal and polyclonal gammopathy
This condition refers to an overabundance of proteins (immunoglobulins) in your blood. Immunoglobulins, also known as antibodies, play a critical role in your immune system. There are many types of immunoglobulins, but one type, called IgG, is positively charged. Overproduction of IgG can sometimes lead to a low anion gap value.
Monoclonal gammopathies can be associated with conditions such as multiple myeloma. Polyclonal gammopathies are often associated with various inflammatory diseases.
Your doctor may order a blood test to assess the levels of immunoglobulins in your blood. They may also order a serum or urine protein electrophoresis test to help monitor and diagnose your condition.
There are a few more rare causes of low anion gap. These include:
- Bromide intoxication. Bromide is present in some sedative drugs, medication for myasthenia gravis, and some herbal medications. High concentrations of bromide can lead to neurologic or dermatologic symptoms. Although negatively charged, bromide can interfere with the calculation of chloride. This affects the calculation of the anion gap and gives a falsely low anion gap result.
- Lithium. Lithium is positively charged and is sometimes prescribed for treatment of bipolar disorder. In high concentrations, it can lower the anion gap.
- Increase in other positively charged ions. A large increase in other positively charged ions, such as calcium and magnesium, can also lower the anion gap.
Treatment for low anion gap is focused on treating the underlying cause.
If your test results come back indicating a low anion gap, your doctor may want to repeat the test to account for laboratory error. Once a low anion gap has been confirmed, your doctor will order additional tests to determine the underlying cause of the result.
If you’re taking medication that can lead to a low anion gap, such as lithium or medication containing bromide, you can talk to your doctor about adjusting the dosage, if possible.
If you seek out and get the treatment that you need for the underlying cause, the outlook for having a low anion gap is good. Following the proper treatment, your anion gap value should normalize.