Arterial Blood Gas Test

An arterial blood gas (ABG) test measures the oxygen and carbon dioxide balance in your blood. It also measures the pH or acid-base balance in your blood. ABG tests are used to evaluate lung and kidney functions and the body's metabolic state.

This article discusses the arterial blood gas test, its purpose, and what the results mean.

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What Is the ABG Test?

Every cell in your body requires oxygen to live. Inhaling and exhaling bring in oxygen and push carbon dioxide out, a process called gas exchange. Certain conditions may affect this process, leading to imbalances throughout the body.

Analysis of blood gases helps your healthcare provider understand your respiratory and metabolic status. Your healthcare provider 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 healthcare provider 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.

An ABG can also be a helpful metric to determine the effectiveness of certain therapies and treatments, such as supplemental oxygen or medication.

What Is an Arterial Blood Gas Test Used For?

ABGs can indicate 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 7.35-7.45 is considered normal.
• Blood oxygen saturation (SaO2): The amount of oxygen carried by the hemoglobin in the red blood cells
• Bicarbonate (HCO3): A chemical buffer that helps stabilize blood pH

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. Some of the conditions it can help evaluate include:

• Acute respiratory distress syndrome (ARDS)
• Sepsis and septic shock
• Acute heart failure and cardiac arrest
• Asthma attack
• Renal tubular acidosis
• Diabetic ketoacidosis (DKA)
  

During the Test

There is no special preparation needed for an ABG test. After cleaning the area with an antiseptic, a needle is used to collect a small amount of blood from either the radial artery in your wrist or the femoral artery in your groin.

You may feel a slight prick when the needle breaks the skin. Since 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, since ABGs must be read within 10 minutes for an accurate result.

What Does an Arterial Blood Gas Test Tell You?

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:

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

Abnormal ABG values may be signs of certain medical conditions. An abnormal result basically means that your body is out of acid-base balance. As a result, 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, which can be due to a wide variety of potential causes, such as anxiety, pregnancy, or liver disease.

Risks of the ABG Test

The ABG test is a standard blood test and a very safe procedure. If performed correctly, there's very low risk involved, aside from the normal risks associated with any 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 complication is excess bleeding or bruising at the puncture site.

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. Tell your healthcare provider if you're currently taking any blood thinners such as warfarin or aspirin, or even supplements such as fish oil.

Summary

The ABG test helps your healthcare provider evaluate your lung and kidney function. The test measures the balance between oxygen and carbon dioxide in the blood. It also measures your blood's pH levels.

The ABG test is most often used on people who are critically ill. It may also be used to test lung function for people with health conditions like sleep apnea, asthma, and cystic fibrosis.

Abnormal ABG results can indicate health problems like kidney failure or a lung condition such as COPD.