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Blood Gas Test

What Is a Blood Gas Test?

A blood gas test measures the amount of oxygen and carbon dioxide in the blood. It may also be used to determine the pH of the blood, or how acidic it is. The test is commonly known as a blood gas analysis or arterial blood gas (ABG) test.

Your red blood cells transport oxygen and carbon dioxide throughout your body. These are known as blood gases. As blood passes through your lungs, oxygen flows into the blood while carbon dioxide flows out of the blood into the lungs. The blood gas test can determine how well your lungs are able to move oxygen into the blood and remove carbon dioxide from the blood.

Imbalances in the oxygen, carbon dioxide, and pH levels of your blood can indicate the presence of certain medical conditions. These may include:

  • kidney failure
  • heart failure
  • uncontrolled diabetes
  • hemorrhage
  • chemical poisoning
  • a drug overdose
  • shock

Your doctor may order a blood gas test when you’re showing symptoms of any of these conditions. The test requires the collection of a small amount of blood from an artery. It’s a safe and simple procedure that only takes a few minutes to complete.

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Purpose

Why Is a Blood Gas Test Done?

A blood gas test provides a precise measurement of the oxygen and carbon dioxide levels in your body. This can help your doctor determine how well your lungs and kidneys are working.

This is a test that is most commonly used in the hospital setting to determine the management of acutely ill patients. It does not have a very significant role in the primary care setting, but may be used in a pulmonary function lab or clinic.

Your doctor may order a blood gas test if you’re showing symptoms of an oxygen, carbon dioxide, or pH imbalance. The symptoms can include:

  • shortness of breath
  • difficulty breathing
  • confusion
  • nausea

These symptoms may be signs of certain medical conditions, including asthma and chronic obstructive pulmonary disease (COPD).

Your doctor may also order a blood gas test if they suspect you’re experiencing any of the following conditions:

  • lung disease
  • kidney disease
  • metabolic disease
  • head or neck injuries that affect breathing

Identifying imbalances in your pH and blood gas levels can also help your doctor monitor treatment for certain conditions, such as lung and kidney diseases.

A blood gas test is often ordered along with other tests, such a blood glucose test to check blood sugar levels and a creatinine blood test to evaluate kidney function.

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Risks

What Are the Risks of a Blood Gas Test?

Since a blood gas test doesn’t require a large sample of blood, it’s considered a low-risk procedure. However, you should always tell your doctor about existing medical conditions that may make you bleed more than expected. You should also tell them if you’re taking any over-the-counter or prescription medications, such as blood thinners, that may affect your bleeding.

Possible side effects associated with the blood gas test include:

  • bleeding or bruising at the puncture site
  • feeling faint
  • blood accumulating under the skin
  • infection at the puncture site

Tell your doctor if you experience unexpected or prolonged side effects.

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Procedure

How Is a Blood Gas Test Performed?

A blood gas test requires the collection of a small sample of blood. Arterial blood can be obtained from an artery in your wrist, arm, or groin, or pre-existing arterial line if you are currently hospitalized. A blood gas sample can also be venous, from a vein or pre-existing IV or capillary, which requires a small prick to the heel.

A technician called a phlebotomist will first sterilize the injection site with an antiseptic. Once they find an artery, they’ll insert a needle into the artery and draw blood. You might feel a slight prick when the needle goes in, but the test itself isn’t painful. After the needle is removed, the technician will put a bandage over the puncture wound.

The blood sample will then be analyzed by a portable machine or in an on-site laboratory. The sample must be analyzed within 10 minutes of the procedure to ensure an accurate test result.

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Results

Interpreting the Results of a Blood Gas Test

The results of a blood gas test can help your doctor diagnose various diseases or determine how well treatments are working for certain conditions, including lung diseases. It also shows whether or not your body is compensating for the imbalance. Due to the potential for compensation in some values that will cause the correction of other values, it is essential that the person interpreting the result is a trained healthcare provider, with experience in blood gas interpretation. The test measures:

  • Arterial blood pH measures the amount of hydrogen ions in blood. A pH of less than 7.0 is called acidic, and a pH greater than 7.0 is called basic, or alkaline. A lower blood pH may indicate that your blood is more acidic and has higher carbon dioxide levels. A higher blood pH may indicate that your blood is more basic and has a higher bicarbonate level.
  • Bicarbonate is a chemical that helps prevent the pH of blood from becoming too acidic or too basic.
  • Partial pressure of oxygen is a measure of the pressure of oxygen dissolved in the blood. It determines how well oxygen is able to flow from the lungs into the blood.
  • Partial pressure of carbon dioxide is a measure of the pressure of carbon dioxide dissolved in the blood. It determines how well carbon dioxide is able to flow out of the body.
  • Oxygen saturation is a measure of the amount of oxygen being carried by the hemoglobin in the red blood cells.

In general, normal values include:

  • arterial blood pH: 7.38 to 7.42
  • bicarbonate: 22 to 28 milliequivalents per liter
  • partial pressure of oxygen: 75 to 100 mm Hg
  • partial pressure of carbon dioxide: 38 to 42 mm Hg
  • oxygen saturation: 94 to 100 percent

Your blood oxygen levels may be lower if you live above sea level.

The normal values will have a slightly different reference range if they are from a venous or capillary sample.

Abnormal results can be signs of certain medical conditions, including the ones in following table:

Blood pH Bicarbonate Partial pressure of carbon dioxide Condition Common causes
Less than 7.4 Low Low Metabolic acidosis Kidney failure, shock, diabetic ketoacidosis
Greater than 7.4 High High Metabolic alkalosis Chronic vomiting, low blood potassium
Less than 7.4 High High Respiratory acidosis Lung diseases, including pneumonia or COPD
Greater than 7.4 Low Low Respiratory alkalosis Breathing too fast, pain, or anxiety
 

Normal and abnormal ranges can vary depending on the lab because some use different measurements or methods to analyze blood samples. You should always meet with your doctor to discuss your test results in more detail. They’ll be able to tell you if you need more testing and if you’ll need any treatment.

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