What Are Arterial Blood Gases?

Taking blood sample
Arterial Blood Gases. SCIENCE PHOTO LIBRARY / Getty Images
In This Article

Arterial blood gases (ABGs) is a diagnostic test performed on blood taken from an artery that provides a glimpse of how much oxygen and carbon dioxide are contained in your blood, along with your blood's pH level. ABG tests are used to evaluate the respiratory function and kidney function and give an overall look into the body's metabolic state.

Purpose of Test

Every cell in your body requires oxygen to live. Inhaling and exhaling brings oxygen into your body and pushes carbon dioxide out; a process called gas exchange. However, certain conditions may affect the function of this gas exchange, leading to imbalances throughout the body systems.

Measuring ABGs can inform you how efficiently the lungs provide oxygen to the body and subsequently remove carbon dioxide. ABGs also measure blood pH and the integrity of the body's acid-base balance. In total, an ABG test measures five different markers:

  • Partial pressure of oxygen (PaO2): The pressure of oxygen dissolved in the blood, measuring how well oxygen can flow from the lungs into the blood.
  • Partial pressure of carbon dioxide (PaCO2): The pressure of carbon dioxide dissolved in the blood, measuring how efficiently carbon dioxide can move out of the body.
  • Arterial blood pH: The amount of hydrogen ions in the blood. A pH of less than 7.0 is considered acidic, and a pH of greater than 7.0 is considered alkaline.
  • Blood oxygen saturation: The amount of oxygen carried by the hemoglobin in the red blood cells.
  • Bicarbonate (HCO3): a chemical buffer that helps stabilize the blood pH.

Analysis of blood gases helps evaluate a person's respiratory and metabolic status. An ABG can also be a helpful metric to determine the effectiveness of certain therapies and treatments, such as supplemental oxygen or medication.

This test is most commonly performed in a hospital setting on patients who are very ill and at risk of respiratory failure, but it may also be used in a pulmonary function facility or lab.

Your physician may order an ABG test if you exhibit symptoms of an oxygen/carbon dioxide imbalance, including:

  • Shortness of breath
  • Difficulty breathing
  • Confusion
  • Dizziness
  • Nausea

Your doctor may also order an ABG test if you have sleep apnea, a heart condition, kidney issues, asthma, cystic fibrosis, chronic pulmonary obstructive disease (COPD), or other conditions affecting breathing and lung function.

ABG is commonly ordered alongside a blood glucose test and a creatinine test to help determine kidney function.

Risks and Contraindications

The ABG test is a standard blood test and a very safe procedure. If performed correctly, very low risk is involved with an ABG, aside from the normal risk associated with a blood draw. Because the test is normally performed on an artery, which is typically located deeper within the body than veins, there may be some slight pain.

The most common occurrence of risk includes that of excess bleeding or bruising at the puncture site, and there is a slight risk of circulation impairment.

Additional slight risks may include:

  • Feeling faint or dizzy
  • Hematoma (buildup of blood under the skin)
  • Infection (a risk anytime the skin barrier is broken)

Considerations

If you're currently taking supplemental oxygen therapy, your oxygen levels must stay consistent (without assistance) for at least 20 minutes before giving blood for an ABG test. Also be sure to tell your healthcare provider if you're currently taking any blood thinners such as warfarin or aspirin, or even supplements such as fish oil.

During the Test

There is no special preparation needed for an ABG test. After cleaning the area with an antiseptic, a needle will be used to collect a small amount of blood from either the radial artery in your wrist, the brachial artery in your arm or the femoral artery in your groin. You may feel a slight prick when the needle breaks the skin. As arteries are slightly thicker than veins (they have more smooth muscle layers), an arterial blood draw may hurt a bit more than a venous blood draw, but the pain should fade away quickly after the test.

After the blood is withdrawn, direct pressure will be applied to the site for several minutes to stop the bleeding. The blood will then be sent to the laboratory (usually on-site) for quick analysis, as ABGs must be read within 10 minutes for an accurate result.

Interpreting Results

An analysis of blood gases helps evaluate a person's respiratory and metabolic status. Because the body may naturally overcompensate for deficiencies in certain areas, the provider reading your ABG test results should be well-trained in interpreting blood gases.

Normal ABG values fall within the following ranges:

  • Partial pressure of oxygen (PaO2): 75 to 100 millimeters of mercury (mm Hg)
  • Partial pressure of carbon dioxide (PaCO2): 38 to 42 millimeters of mercury (mm Hg)
  • pH: 7.38 to 7.42
  • Oxygen saturation (SaO2): 95% to 100%
  • Bicarbonate - (HCO3): 22 to 28 milliequivalents per liter (mEq/liter)

Note: At altitudes above 3,000 feet, the oxygen saturation level may be lower.

Abnormal values may be signs of certain medical conditions, as seen in the following table.

Abnormal ABG Values
Condition pH Bicarbonate PaCO2
Metabolic acidosis < 7.4 Low Low
​Metabolic alkalosis > 7.4 High High
Respiratory acidosis < 7.4 High High
Respiratory alkalosis > 7.4 Low Low
       

An abnormal result basically means that your body is out of acid-base balance, and your body fluids may become too acidic or too alkaline, and less able to effectively support normal functioning.

Metabolic acidosis may be caused by kidney failure or severe diarrhea, while metabolic alkalosis may be caused by chronic vomiting or the use of steroids.

In contrast, respiratory acidosis means you're holding onto too much carbon dioxide, likely due to a lung condition such as COPD. Respiratory alkalosis means you have too little carbon dioxide, thanks to a wide variety of potential causes, such as anxiety, pregnancy, or liver disease.

A Word From Verywell

The results from an ABG test should be discussed in detail with your medical provider, who can take a look at your personal medical history to help determine any potential cause for why your body may be in a state of pH imbalance, then take certain steps to help you breathe easier.

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