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What is an Arterial Blood Gas (ABG) test?

ABG test measures the levels of oxygen, carbon dioxide (CO2) and acidity of arterial blood. It can help determine how efficiently your lungs are able to mobilise oxygen in the bloodstream and remove CO2. Evaluation of lung function and detection of acid-base imbalance can further help in the diagnosis of metabolic disorders and kidney and lung conditions.

An ABG test generally measures the following:

  • pH
  • The partial pressure of oxygen (PaO2) dissolved in the blood
  • The partial pressure of CO2 (PaCO2) present in the blood
  • Oxygen content (O2CT) or the amount of oxygen in 100 millilitres (mL) of blood
  • Oxygen saturation (O2SAT) or the percentage of haemoglobin used in carrying oxygen. Haemoglobin present in red blood cells is the oxygen carrier in the blood
  • Bicarbonates: The metabolic component of the acid-base balance can be measured
  • Base excess/deficit or the amount of metabolic buffering agents in the blood. The buffering capacity of lungs can be assessed with this test
  1. Why is an ABG test performed?
  2. How do you prepare for an ABG test?
  3. How is an ABG test performed?
  4. What do Arterial Blood Gas test results indicate?

An ABG test is done:

  • To check for oxygen/CO2 or a pH imbalance
  • To check for respiratory problems
  • To check for metabolic or kidney disorders
  • In case of head or neck trauma
  • To monitor the effectiveness of the treatment in a person who is on ventilation (oxygen)
  • To assess the efficiency of lung treatment
  • To check for lung diseases, such as asthma, cystic fibrosis and chronic obstructive pulmonary disease (COPD)
  • To measure acid–base level in blood

No special preparation is needed for this test.

However, you must inform your doctor about the following:

  • If you are on any medications
  • If you are taking any anticoagulants (blood-thinning medicines)
  • If you are allergic to any medications

This is also known as the room air test. If a patient is receiving oxygen therapy, oxygen should be turned off 20 minutes prior to the test. In case the patient would not be unable to breathe without oxygen or the doctor has to check oxygen levels with the therapy, it is not turned off, and the amount of delivered oxygen is recorded. Talk to your doctor for a better understanding if you have any concern regarding the procedure, risks involved or the interpretation of results.

For an ABG test, a blood sample is usually taken from arteries, such as

  • Radial artery (inside of the wrist)
  • Femoral artery (in the groin)
  • Brachial artery (inside of the arm above the elbow crease)

The healthcare provider may rotate your hand to test the circulation before taking the blood sample. He/she will clean the needle injection site with alcohol and insert a needle into the vein, to draw the required amount of blood. After the test, the injection site will be pressed firmly, and a cotton ball or gauze pad will be placed over that area to prevent bleeding. The collected blood sample will be sent for examination.

Normal results: Standard values of ABG may vary among laboratories. Speak with your doctor about the interpretation of results. He/she can evaluate the results based on your overall health and other factors.

Elevation above the sea level is a major factor affecting test results.

Normal values at sea level are as follows:

  • pH: Normal pH range of blood is 7.35-7.45. A pH value of less than 7 is considered acidic and more than 13 is basic. Increased CO2 can make the blood acidic and its decreased amounts can make it basic
  • Partial pressure of oxygen: 70-100 mm Hg (10.5-13.5 kPa [kilopascal])
  • The partial pressure of CO2: 38-42 mm Hg (5.1-5.6 kPa)
  • Arterial blood pH: 7.38-7.42
  • Oxygen saturation: 94-100%
  • Bicarbonates: 22-28 milliequivalents per litre [mEq/L])

Abnormal results: Abnormal results can be an indication of a kidney, lung or metabolic disorder. It can also be due to head or neck injuries that can affect breathing.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor. 

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References

  1. Health Link [internet]. British Columbia. Arterial Blood Gases.
  2. Castro D, Keenaghan M. Arterial Blood Gas. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2019 Jan
  3. Science Direct (Elsevier) [Internet]; Mechanisms of hypoxaemia and the interpretation of arterial blood gases
  4. Larkin BG, Zimmanck RJ. Interpreting Arterial Blood Gases Successfully.. AORN J. 2015 Oct;102(4):343-54; quiz 355-7. PMID: 26411819
  5. J. Larry Jameson. Harrison's Principles of Internal Medicine, 20e. 20th edition. McGraw-Hill, 2014.
  6. Carl Burtis David Bruns. Tietz Fundamentals of Clinical Chemistry. 6th Edition; ISBN: 9781437719406
  7. Steven Weinberger Barbara Cockrill Jess Mandel. Principles of Pulmonary Medicine. 6th Edition; ISBN: 9781455725328