What Is an Anion Gap Test?

Medically Reviewed by Zilpah Sheikh, MD on October 16, 2023
6 min read

The anion gap test measures anion gap (sometimes called serum anion gap) or 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.

Your doctor might order the anion gap test if you have symptoms of either acidosis (too acidic blood) or alkalosis (too basic blood).

Signs of acidosis or high blood acid may include:

  • Shortness of breath
  • Nausea or vomiting
  • Abnormal heartbeat
  • Confusion or fatigue

Signs of alkalosis or too basic blood may include:

  • Nausea or vomiting
  • Confusion or lightheadedness
  • Hand tremors
  • Numbness or tingling in the face, hands, or feet
  • Muscle spasms

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.

Your anion gap blood test measures two things in your blood: the acid-base balance (pH level) and the electrolyte balance. The anion gap test is the calculated difference between positively and negatively charged electrolytes in your blood to determine if your blood is too acidic or not acidic enough (too base). Your doctor may use various tests to determine your anion gap measurement. These tests are:

  • Comprehensive medical panel
  • Basic metabolic panel
  • Electrolyte panel

What are electrolytes?

Electrolytes are electrically charged minerals that help keep your blood pH level from becoming too acidic—or not acidic enough. The main electrolyte minerals in your body are bicarbonate, calcium, potassium, chloride, magnesium, phosphate, potassium, and sodium. Electrolytes come into our body through food and drinks.

Electrolytes can carry a positive or negative charge. The anion gap test measures how balanced these charges are. Maintaining the right balance of electrolytes helps your body perform muscle functions and many other processes. The result is a number that can help your doctor figure out if you have a condition that's throwing off your acid levels.

The anion gap test results will be normal, high, or low.

After your doctor reviews your anion gap test results, they will determine a diagnosis and treatment plan. In addition to the anion gap test result, your medical history and additional potential medical tests will determine your final diagnosis.

Normal anion gap

A normal anion gap range is typically 4-12 mEq/L. That means your blood pH level is healthy and balanced. A neutral pH level is 7.0. But because your blood is made of 90% water, blood is naturally slightly basic with a pH level of 7.35-7.45.

High anion gap

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:

  • Dehydration or overhydration. Both too much water or too little water in your body can disrupt your electrolyte balance. Dehydration can be caused by diarrhea, vomiting, excessive exercise, or other medical conditions.
  • Diarrhea. Diarrhea can lead to excessive dehydration, which drains your body of electrolytes. This may cause increased acidic levels in your blood.
  • Kidney disease . Your kidneys have an important role in regulating pH levels in your blood. The kidneys help remove acid from the blood through your pee. With kidney disease, their function is impaired, which causes the acidic levels in your blood to rise.
  • Diabetes. Acidosis in people with both types 1 and 2 diabetes is called diabetic ketoacidosis. This happens when low insulin prevents blood sugar from being used for energy. Instead, the liver breaks down fat for energy causing increased ketones in the blood.
  • Salicylate poisoning. Salicylate poisoning is caused by an overdose of aspirin. Beginning as respiratory acidosis, extra acid moves into the bloodstream causing metabolic acidosis.
  • Alcohol abuse.  Heavy alcohol use can cause alcoholic ketoacidosis. This means there's high levels of acidic ketones in your blood, which can cause acidosis.
  • Medications.Some medications may increase acidic levels in your blood. Work with your doctor to make sure they're aware of over-the-counter drugs you take when being prescribed medication for medical conditions.
  • Excessive exercise.  Sweating during exercise can cause dehydration and electrolyte loss, leading to acidosis.

Low anion gap

If your doctor says you have a low anion gap, you might have a lower than normal amount of acid in your blood. This is called alkalosis.

Alkalosis is common and sometimes 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 albumin. Albumin helps important vitamins, hormones, and enzymes move throughout the body. Low albumin can be a sign of:

  • Kidney problems. Healthy kidneys block albumin from entering urine. When albumin is leaked into the urine, it may be a sign of kidney disease.
  • Heart disease. Heart disease, when treated with diuretics, may lead to alkalosis and low albumin.
  • Certain types of cancer . Cancer may cause potassium levels in the blood to drop, causing alkalosis. Chemotherapy cancer treatments may also lead to lower potassium levels.
  • Liver disease. The acid—base balance in liver disease is complex. Your doctor may check look for respiratory alkalosis, metabolic acidosis, low albumin, and changes in your potassium levels.

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 will help you take charge of it.

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.

Anion gap blood tests show the acid—base, or pH level, in your blood. Knowing your anion gap level can help your doctor diagnose medical conditions that need treatment. Low or high anion gaps in your blood can stem from a wide variety of medical conditions. Therefore, your doctor may order additional testing to help you receive the correct diagnosis and best treatment possible.

  • What are the symptoms of a low anion gap?

Low anion gap symptoms include nausea; vomiting; confusion; lightheadedness; hand tremors; numbness or tingling in the face, hands, or feet; and muscle spasms.

  • What level of anion gap is concerning?

A healthy anion gap is typically 4-12 mEq/L. But this may vary depending on the lab testing.