What Is an Anion Gap Test?

The anion gap test tells you how much acid is in your blood. Acid levels that are higher or lower than normal can tip your doctor off to a health problem and help them find the right treatment for you.

Too much acid in your blood can be a sign of short-term problems like dehydration and diarrhea, or long-term conditions like diabetes. It's not common, but too little acid may point to a serious problem like heart disease.

Why You May Need the Test

Your doctor might order the anion gap test if you have symptoms of high blood acid levels, like:

How the Test Is Done

You'll go to your doctor's office to get a blood test. Before your appointment, your doctor will tell you if you need to avoid food and certain drinks. Let them know if you're taking any medicines -- especially an antibiotic -- because that can affect your test results.

Your doctor will use a needle to take some blood from a vein in your arm. They'll send the blood sample to a lab for testing. You may have a bit of pain, soreness, or bruising in the area afterward.

What the Test Measures

The lab will take a close look at electrically charged minerals in your blood called electrolytes. These help keep your blood from becoming too acidic -- or not acidic enough.

Electrolytes can carry a positive or negative charge. The anion gap test measures how balanced these charges are. The result is a number that can help your doctor figure out if you have a condition that's throwing off your acid levels.

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What Your Results Mean

If your doctor says you have a high anion gap, you may have a higher than normal amount of acid in your blood. The doctor may call this "acidosis."

Acidosis can be a symptom of conditions like:

Acidosis can also be brought on by too much exercise.

If your doctor says you have a low anion gap, you might have a lower than normal amount of acid in your blood. This result isn't common, and sometimes it's due to a lab error. Your doctor may ask the lab to do the test again to double-check the numbers.

If you really do have a low anion gap, it could mean your blood doesn't have enough of a protein called abumin. That can be a sign of conditions like:

If your doctor diagnoses you with a condition based on a high or low anion gap result, they'll come up with a treatment plan that helps you take charge of it.

What to Ask Your Doctor

Once you get your test result, ask the doctor to explain exactly what it means. Keep in mind that even if your anion gap is high or low, that doesn't necessarily mean you have a serious health problem.

An anion gap number between 3 and 10 is considered normal. But the "normal" range can vary from person to person, and it may also depend on the methods your lab used to do the test.

WebMD Medical Reference Reviewed by Michael Dansinger, MD on January 17, 2020

Sources

SOURCES:

Clinical Journal of the American Society of Nephrology: "Serum Anion Gap: Its Uses and Limitations in Clinical Medicine."

MedlinePlus: "Anion Gap Blood Test."

University of Rochester Medical Center: "Anion Gap (Blood)."

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