Arterial Blood Gas (ABG)
What is an arterial blood gas (ABG) test?
An arterial blood gas (ABG) test is a blood test that requires a sample from an artery in your body to measure the levels of oxygen and carbon dioxide in your blood. The test also checks the balance of acids and bases, known as the pH balance, in your blood.
Your body normally tightly regulates the amount of oxygen and carbon dioxide in your blood, because low blood oxygen levels (hypoxemia) can lead to many serious conditions and damage to individual organ systems, especially your brain and heart.
Arterial blood gas tests can help healthcare providers interpret conditions that affect your respiratory system, circulatory system and metabolic processes (how your body transforms the food you eat into energy), especially in emergency situations.
There’s also a test known as a "blood gas analysis," which uses a sample of blood from anywhere in your circulatory system (artery, vein or capillary). An arterial blood gas (ABG) test only tests a blood sample from an artery in your body.
Other common names for an arterial blood gas test include:
- Blood gas test.
- Arterial blood gases.
- Blood gas analysis.
What is measured in an arterial blood gas test?
An arterial blood gas test usually includes the following measurements:
- Oxygen content (O2CT): This measures the amount of oxygen in your blood.
- Hemoglobin: This measures the amount of hemoglobin, the protein responsible for carrying oxygen to your cells, in your blood.
- Oxygen saturation (O2Sat): This measures how much hemoglobin in your blood is carrying oxygen. Hemoglobin is a protein in your red blood cells that carries oxygen from your lungs to the rest of your body.
- Partial pressure of oxygen (PaO2): This measures the pressure of oxygen dissolved in your blood. It helps show how well oxygen moves from your lungs to your bloodstream.
- Partial pressure of carbon dioxide (PaCO2): This measures the amount of carbon dioxide in your blood and how well carbon dioxide can move out of your body.
- pH: This measures the balance of acids and bases in your blood, known as your blood pH level. The pH of blood is usually between 7.35 and 7.45. If it’s lower than that, your blood is considered too acidic. If it’s higher than that range, your blood is considered too basic (alkaline).
- Bicarbonate (HCO3): This is calculated using the measured values of pH and PaCO2 to determine the amount of the basic compound made from carbon dioxide (CO2.)
When is an arterial blood gas (ABG) test performed?
Healthcare providers frequently order arterial blood gas (ABG) tests for the following settings or areas of medicine:
- Emergency medicine: Emergency medicine is the area of medicine that’s concerned with the care of illnesses or injuries requiring immediate medical attention.
- Anesthesiology: Anesthesiology is the area of medicine that’s concerned with the care of people before, during and after surgery. It encompasses anesthesia, intensive care medicine, critical emergency medicine and pain medicine.
- Pulmonology: Pulmonology is the area of medicine that deals with diseases involving your respiratory system.
Healthcare providers evaluate several conditions using an ABG, including:
- Acute respiratory distress syndrome (ARDS): This is a life-threatening lung injury that causes dangerously low oxygen levels in your blood. It’s caused by sepsis, COVID-19 and other conditions.
- Severe sepsis: Sepsis is a medical emergency caused by your body's response to an infection and can be life-threatening. Sepsis is the consequence of widespread inflammation in your body.
- Septic shock: Septic shock is a life-threatening condition that happens when your blood pressure drops to a dangerously low level after an infection.
- Hypovolemic shock: Hypovolemic shock is an emergency condition in which severe blood loss or other fluid loss makes your heart unable to pump enough blood to your body.
- Diabetes-related ketoacidosis (DKA): This is a serious and life-threatening complication that affects people with diabetes (mainly Type 1 diabetes) and those who have undiagnosed Type 1 diabetes. It causes your blood to become acidic.
- Renal tubular acidosis (RTA): This condition happens when your kidneys don’t remove acids from your blood into your urine as they should, causing your blood to become acidic.
- Acute respiratory failure: This happens when fluid builds up in the air sacs in your lungs, making it difficult for your lungs to release oxygen into your blood.
- Acute heart failure: This is a sudden weakening of your heart that limits its function. It requires emergency treatment to help manage and alleviate symptoms.
- Cardiac arrest: This happens when your heart suddenly stops beating. It can happen to individuals who may or may not have heart disease and requires immediate medical attention.
- Asthma attack: This is a sudden worsening of asthma symptoms caused by the tightening of muscles around your airways.
- Inborn errors of metabolism: These are rare genetic (inherited) conditions in which your body can’t properly turn food into energy. These conditions are usually caused by issues with or a lack of specific proteins (enzymes) that help break down parts of food.
Your provider may also perform an arterial blood gas test if you have any of the following lung conditions to make sure your treatment is working properly:
Who performs an arterial blood gas (ABG) test?
A healthcare provider called a respiratory therapist usually performs blood draws for arterial blood gas tests from an artery in your wrist, arm or groin. The respiratory therapist then processes the sample or sends it to a lab very quickly where medical laboratory scientists process the sample.
What happens before an arterial blood gas (ABG) test?
A respiratory therapist may perform a blood circulation test called an Allen test before taking a sample for an arterial blood gas test from your wrist. An Allen test involves holding your hand high with a clenched fist. The respiratory therapist will then apply pressure to the arteries in your wrist for several seconds. This simple test makes sure both of the arteries in your wrist are open and working properly.
If you’re on supplemental oxygen therapy, the respiratory therapist may turn off your oxygen for about 20 minutes before the blood draw. This is called a room air test. If you’re unable to breathe without supplemental oxygen, they won't do this test.
What should I expect during an arterial blood gas test?
Most blood tests take a blood sample from one of your veins. For an arterial blood gas test, a respiratory therapist will take a sample of blood from one of your arteries. This is because there are higher oxygen levels in blood from an artery than blood from a vein.
A respiratory therapist usually takes the sample from an artery inside your wrist known as the radial artery. Sometimes they may take a sample from an artery in your arm (brachial artery) or groin (femoral artery).
If a newborn needs an arterial blood gas test, a provider may take the sample from the baby's heel or umbilical cord.
Unfortunately, getting a blood sample from an artery is usually more painful than getting a sample from a vein. This is because arteries are deeper in your body than veins and are surrounded by nerves. You may feel light-headed, dizzy or nauseated while a provider takes blood from your artery.
An arterial blood gas test blood draw includes the following steps:
- You’ll sit in a chair or lie in a medical bed, and a respiratory therapist will look for an artery, usually in your inner wrist. They might use ultrasound imaging to help find an artery.
- Once they’ve located an artery, they’ll clean and disinfect the area.
- They’ll then insert a small needle into your artery to take a blood sample. You may feel a sharp pain as the needle goes into your artery.
- After they insert the needle, a small amount of blood will collect in a syringe.
- Once they have enough blood to test, they’ll remove the needle and hold a cotton ball or gauze on the site to stop the bleeding. They may apply pressure for five to 10 minutes or longer if you’re taking blood-thinning medication.
- They’ll place a bandage over the site, and you’ll be finished.
What should I expect after my arterial blood gas test?
You may experience some bruising and/or soreness at the site of the needle injection. Your provider may recommend that you avoid lifting heavy objects for 24 hours after the blood draw.
Are there any risks to an arterial blood gas test?
There’s little risk associated with getting an arterial blood gas test when the respiratory therapist performs the procedure correctly. Arteries vary in size from one person to another and from one side of your body to the other, so taking blood from some people may be more difficult than from others.
Risks associated with having an arterial blood gas blood draw are rare, but may include:
- Fainting or feeling lightheaded.
- Multiple punctures to locate your artery.
- Hematoma (blood building up under your skin).
- Excessive bleeding.
- Infection (a slight risk any time your skin is punctured or cut).
When should I know the results of my arterial blood gas (ABG) test?
Arterial blood gas (ABG) tests typically provide very quick results. Respiratory therapists and laboratory scientists commonly use automated blood gas analyzers, which provide results within 10 to 15 minutes.
Results and Follow-Up
What do the results of an arterial blood gas test mean?
Blood test reports, including arterial blood gas test reports, usually provide the following information:
- The name of the blood test or what was measured in your blood.
- The number or measurement of your blood test result.
- The normal measurement range for that test.
- Information that indicates if your result is normal or abnormal or high or low.
If your arterial blood gas test results are abnormal, it may mean you:
- Aren’t taking in enough oxygen.
- Aren’t getting rid of enough carbon dioxide.
- Have an imbalance in your blood pH (it’s too acidic or basic).
An arterial blood gas test can help diagnose certain conditions. If your results are abnormal, your healthcare provider may order additional tests, such as other blood tests and/or imaging tests, to confirm or rule out a diagnosis.
What are normal values for an arterial blood gas test?
Normal value ranges can vary slightly from lab to lab. When you get your blood test results back, there will be information that indicates what that lab’s normal ranges are for each measurement. If you have any questions about your results, be sure to ask your healthcare provider.
In general, normal values at sea level include:
- pH: 7.35-7.45.
- Partial pressure of oxygen (PaO2): 75 to 100 millimeters of mercury (mmHg).
- Partial pressure of carbon dioxide (PaCO2): 35 to 45 mmHg.
- Bicarbonate (HCO3): 22 to 26 milliequivalents per liter (mEq/L).
- Oxygen saturation (O2Sat or SaO2): 95 to 100%.
At altitudes of 3,000 feet (900 meters) and higher, the normal oxygen level is lower.
When should I call my doctor?
If you have a chronic lung condition, such as COPD or asthma, you’ll likely need to see your healthcare provider regularly and undergo arterial blood gas tests to make sure your treatment is working. If you develop concerning symptoms related to your lung condition, call your provider as soon as possible.
A note from Cleveland Clinic
Arterial blood gas tests are an effective diagnostic tool, especially in certain emergency situations. Healthcare providers also use this test to monitor certain health conditions. If you need to get an arterial blood gas test, know that it’s a slightly different process than a regular blood draw and it might hurt a little more. If you have any questions about the process or what to expect, don’t be afraid to ask your healthcare provider. They’re there to help you.
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