What is an Anion Gap Blood Test?

An anion gap blood test gives a measurement of the acid-base balance (pH balance) in your blood. This lets you know whether your blood is too acidic or not acidic enough. The test also checks for an electrolyte imbalance in your blood. To do the measurement, an anion gap test uses the results of an electrolyte panel or other blood tests that measure your electrolytes. 

Electrolytes are minerals that have an electrical charge when they are dissolved in water or body fluids. You get them from the foods you eat and the fluids you drink. You have electrolytes in your blood, urine, tissues, and other body fluids. They help control the acid-base balance of your blood. A few of your body's electrolytes that are often measured in your blood include sodium, chloride, potassium, and bicarbonate. 

Some electrolytes have a positive electric charge. Others have a negative electric charge. The anion gap measures the difference—or gap—between the negatively charged and positively charged electrolytes in your blood. If the anion gap is too high, your blood is more acidic than normal. If the anion gap is too low, your blood isn't acidic enough. Both high and low results may be signs of a serious disorder that needs attention.

Other names: Serum anion gap

What is it used for?

The anion gap blood test shows whether your electrolytes are out of balance or if your blood is too acidic or not acidic enough. Too much acid in the blood is called acidosis. Too little acid in your blood is called alkalosis. Both conditions can be serious. 

Why do I need an anion gap blood test?

Your health care provider may order an anion gap blood test if you have symptoms of acidosis or alkalosis. Symptoms may vary depending on the underlying disease or condition.

Symptoms of acidosis may include:

• Shortness of breath
• Vomiting
• Confusion
• Fatigue
• Tachycardia (very fast heartbeat)

Symptoms of alkalosis may include:

• Arrhythmia (a problem with the rate or rhythm of your heartbeat)
• Irritability
• Hand tremor
• Muscle twitching
• Nausea, vomiting
• Numbness or tingling in the face, hands, or feet

What happens during an anion gap blood test?

The anion gap test is a calculation that's done with the results of an electrolyte panel, which is a blood test.

During a blood test, a health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.

Will I need to do anything to prepare for the test?

No special preparation is necessary for an anion gap blood test. If your provider has ordered more tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your provider will let you know if there are any special instructions.

Are there any risks to the test?

There is very little risk to having this test. You may feel slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

Normal results may vary by lab. Your anion gap results may be based on the results of an electrolyte panel, a basic metabolic panel (BMP), a comprehensive metabolic panel (CMP), or a renal function panel (RFP).

Several types of medical conditions can cause abnormal anion gap test results. Your provider will use the results of this test, your medical history, and other tests to make a diagnosis.

A high anion gap test result may mean that you may have acidosis, which may be caused by:

• Dehydration
• Diarrhea
• Too much exercise
• Kidney diseases
• Diabetes or diabetic ketoacidosis
• Salicylate poisoning, for example from too much aspirin
• Antifreeze poisoning 

A low anion gap test result may mean you have alkalosis. But this result is so rare that your provider will usually have you tested again to make sure the results are accurate.

The most common cause of a low anion gap is a low level of albumin, a protein in the blood. Low albumin may be a sign of not eating enough protein, kidney problems, liver disease, heart disease, or some types of cancer.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about an anion gap blood test?

The anion gap blood test provides information about the acid-base balance of your blood. This tells you about the health of many of your body functions. But there is a wide range of normal results and many possible causes of abnormal results. So, talk with your provider about what your test results mean and whether you need additional testing to make a diagnosis.