Arterial Blood Gas (ABG) Testing and Results

Close up of doctor wearing surgical gloves, holding oxygen mask over patient in operating theater
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An arterial blood gas test, commonly known as an ABG, is performed on blood that is drawn from an artery. It is used to see how well the lungs are functioning and to determine the effectiveness of respiratory therapies, such as the use of a ventilator, CPAP, BiPAP or oxygen. A blood gas may also reveal the presence of kidney problems, but is not typically performed to diagnose problems with the kidneys.

The ABG is one of the most commonly performed tests before surgery, particularly in patients who have or are suspected to have breathing issues or lung disease. An ABG should be expected if the surgery is a long one, or if the patient will be on the ventilator for an extended period of time. This enables staff to know if the ventilator settings are appropriate for the patient.

Drawing an ABG

There are two ways to draw an ABG: an arterial line (a special type of IV line that is placed into an artery that allows arterial blood to be drawn without a needle) or a syringe to draw blood from an artery.

An arterial blood draw is more painful than a typical venous blood draw, and is usually performed on the wrist or the groin. After the blood is drawn, pressure may be held on the site for five minutes or longer to prevent bleeding from the artery. If a patient is expected to be on a ventilator for an extended period of time, an arterial line is typically placed to avoid repeated painful arterial sticks.

An arterial line allows for blood to be drawn from the artery without sticking the patient with a needle each time. Additionally, the arterial line allows for very accurate blood pressure monitoring that is continuous. 

The Test

An ABG looks at five different components of arterial blood:

  • pH: The pH of arterial blood should be between 7.35 and 7.45. Significant alterations in pH can indicate life-threatening problems that must be treated rapidly. A pH of 6.9 is typically considered the low end of survivable pH alterations.
  • Carbon Dioxide (PCO2): Determines if your body is able to rid itself of carbon dioxide appropriately or if carbon dioxide is being retained by the body.
  • Oxygen (PO2): Determines if your lungs are able to move oxygen into your blood appropriately.
  • Bicarbonate (HCO3): Low levels of bicarbonate in the blood can indicate issues with kidney function.
  • Oxygen Saturation (O2): Measured on a scale of 0-100 this indicates how much oxygen is making it to the tissues of the body. One hundred percent is perfect, and 97% or greater is expected in a healthy individual. Oxygen supplementation may be needed for low levels of saturation.

ABG Interpretation

Interpreting ABG results is a complex process and requires strong clinical skills in order to take into account an individual's overall condition. Something as simple as vomiting can change the results, just as a serious or life-threatening lung condition can cause a change in ABG tests.

In the hospital setting, these results are used to make changes to the settings on a ventilator or to determine if a patient needs respiratory support with a ventilator or oxygen. Results may include:

Metabolic Acidosis: Characterized by a low pH, low bicarbonate levels and low carbon dioxide, this condition can be caused by kidney issues, breathing too fast or breathing too deeply.

Metabolic Alkalosis: Elevated pH, bicarbonate and carbon dioxide typically indicate severe vomiting has altered the chemistry of the blood.

Respiratory Acidosis: A low pH, high bicarbonate and high carbon dioxide are often indicative of lung condition, such as pneumonia, or a disease such as COPD. May indicate a need for ventilator changes if the patient is on a ventilator.

Respiratory Alkalosis: A high pH, low bicarbonate level and low carbon dioxide typically indicate breathing that is too fast or too deep, such as when experiencing pain or during hyperventilation. May indicate a need for ventilator changes if the patient is on a ventilator.

A Word from Verywell

An arterial blood gas can be a very useful test, but the interpretation of this information is best left to the professionals. Results that would be alarming for one patient could be normal for another, and these test results can vary widely from hour to hour based on respiratory interventions that include providing oxygen or ventilator settings.   

Your healthcare provider should be able to tell you if the results were as expected and if the results indicate the patient is improving or needs more oxygen or even the support of a ventilator.  

Sources:

Blood Gases. Lab Tests Online.