What is an Anion Gap Blood Test?
An anion gap blood test is a way to check the acid-base balance (pH balance) of your blood. It tells you if your blood is too acidic or not acidic enough. The test uses the results of another blood test called an electrolyte panel. Electrolytes are electrically charged minerals in your body, such as sodium, potassium, and bicarbonate. They help control the acid-base balance of your blood.
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 in your body 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 that your blood acidity may not be normal. These symptoms may include:
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?
There is no special preparation 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 to follow.
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?
Several types of medical conditions can cause abnormal anion gap test results. Your provider will use the results of the anion gap test, your medical history, and other tests to make a diagnosis.
A high anion gap test result may mean that you may have acidosis (blood that is more acidic than normal). Acidosis may be caused by:
- Too much exercise
- Kidney diseases
- Diabetes (diabetic ketoacidosis)
- Certain medicines and poisons
A low anion gap test result may mean you have alkalosis (blood that is less acidic than normal). 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.
- ChemoCare.com [Internet]. Cleveland (OH): ChemoCare.com; c2002-2022. Hypoalbuminemia (Low Albumin) [cited 2022 Feb 4]; [about 2 screens]. Available from: http://chemocare.com/chemotherapy/side-effects/hypoalbuminemia-low-albumin.aspx
- Cleveland Clinic: Health Library: Diagnostics & Testing [Internet]. Cleveland (OH): Cleveland Clinic; c2022. Anion Gap Blood Test; [last reviewed 2021 Nov 11; cited 2022 Feb 4]; [about 15 screens]. Available from: https://my.clevelandclinic.org/health/diagnostics/22041-anion-gap-blood-test
- Evidence-Based Medicine Consult [Internet]. EBM Consult, LLC; Lab Test: Anion Gap; [cited 2017 Feb 1]; [about 2 screens]. Available from: http://www.ebmconsult.com/articles/lab-test-anion-gap
- Galla J. Metabolic Alkalosis. Journal of the American Society of Nephrology [Internet]. 2000 Feb 1 [cited 2017 Feb 1]; 11 (2): 369-75. Available from: http://jasn.asnjournals.org/content/11/2/369.full
- Kraut JA, Madias NE. Serum Anion Gap: Its Uses and Limitations in Clinical Medicine. Clinical Journal of the American Society of Nephrology [Internet]. 2007 Jan [cited 2017 Feb 1]; 2 (1):162–74. Available from: http://cjasn.asnjournals.org/content/2/1/162.full.pdf
- Kraut JA, Nagami GT. The serum anion gap in the evaluation of acid-base disorders: What are its limitations and can its effectiveness be improved?; Clinical Journal of the American Society of Nephrology [Internet]. 2013 Nov [cited 2017 Feb 1]; 8 (11):2018–24. Available from: https://www.ncbi.nlm.nih.gov/pubmed/23833313
- Lolekha PH, Vanavanan S, Lolekha S. Update on value of the anion gap in clinical diagnosis and laboratory evaluation. Clinica Chimica Acta [Internet]. 2001 May [cited 2016 Nov 16]; 307(1–2):33–6. Available from: https://www.ncbi.nlm.nih.gov/pubmed/11369334
- Merck Manuals [Internet]. Kenilworth (NJ): Merck & Co., Inc.; c2022. Consumer Version: Acidosis; [last full review 2021 Jul; cited 2022 Feb 4]; [about 3 screens]. Available from: https://www.merckmanuals.com/home/hormonal-and-metabolic-disorders/acid-base-balance/acidosis
- Merck Manuals [Internet]. Kenilworth (NJ): Merck & Co., Inc.; c2022. Consumer Version: Overview of Acid-Base Balance; [last full review 2021 Jul; cited 2022 Feb 4]; [about 3 screens]. Available from: https://www.merckmanuals.com/home/hormonal-and-metabolic-disorders/acid-base-balance/overview-of-acid-base-balance
- Merck Manuals: Professional Version [Internet]. Kenilworth (NJ): Merck & Co., Inc.; c2022. Acid-Base Disorders; [last full review 2021 Jul; cited 2022 Feb 4]; [about 6 screens]. Available from: http://www.merckmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/acid-base-disorders
- National Heart, Lung, and Blood Institute [Internet]. Bethesda (MD): U.S. Department of Health and Human Services; Blood Tests; [ cited 2022 Feb 4]; [about 14 screens]. Available from: https://www.nhlbi.nih.gov/health-topics/blood-tests
- Testing.com [Internet]. Seattle (WA): OneCare Media; c2022. Electrolytes and Anion Gap; [modified 2021 Nov9; cited 2022 Feb4]; [about 12 screens]. Available from: https://www.testing.com/tests/electrolytes-and-anion-gap/
- University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; c2022. Health Encyclopedia: Anion Gap (Blood); [cited 2022 Feb 4]; [about 4 screens]. Available from: https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=anion_gap_blood