What Is an Arterial Blood Gas Test (ABG)?
An arterial blood gas (ABG) test measures oxygen and carbon dioxide levels in your blood. It also measures your body’s acid-base (pH) level, which is usually in balance when you’re healthy.
You may get this test if you’re in the hospital or if you have a serious injury or illness.
The test gives your doctor clues about how well your lungs, heart, and kidneys are working. You’ll probably get other tests along with it.
Every cell in your body needs oxygen to live. When you breathe in (inhale) and breathe out (exhale), your lungs move oxygen into your blood and push carbon dioxide out. That process, called gas exchange, provides the oxygen we (and all of our cells) need to survive.
If you are having a hard time breathing, your doctor may use an arterial blood gas (ABG) test to help figure out what the problem is.
Why Is an Arterial Blood Gas Test Done?
Your doctor may ask for an arterial blood gas test to:
- Check for severe breathing and lung problems such as asthma, cystic fibrosis, chronic obstructive pulmonary disease (COPD), or obstructive sleep apnea
- Check how treatments for your lung problems are working
- Check whether you need extra oxygen or other help with breathing
- Check your acid-base balance. You can have too much acid in your body from kidney failure, a severe infection, specific toxic ingestions, or complications of diabetes (DKA).
What Is Measured in an Arterial Blood Gas Test?
An arterial blood gas tests a few important blood components. These measures help your doctor check your lung function, oxygen levels, and kidney health.
Oxygen content (O2CT) and oxygen saturation (O2Sat). Oxygen content measures the amount of oxygen in your blood, while oxygen saturation measures how much hemoglobin (a protein in the blood) is carrying oxygen — the main way oxygen is transported throughout the body.
Partial pressure of oxygen (PaO2). Your blood contains a small amount of dissolved oxygen that hemoglobin doesn’t transport. Partial pressure of oxygen measures the pressure of dissolved oxygen in your blood and how well oxygen travels from your lungs to your blood.
Partial pressure of carbon dioxide (PaCO2). This measures the amount of dissolved carbon dioxide in your blood and how well your body can get rid of it as part of gas exchange.
pH. pH measures blood acidity levels. Lots of hydrogen ions (a chemical component that holds electricity) makes blood more acidic. Less hydrogen ions make it more basic. 7.35 to 7.45 is the very narrow normal range of blood pH. If it’s lower, your blood is too acidic. If it’s higher, your blood is too basic.
Bicarbonate (HCO3). Bicarbonate, a form of carbon dioxide, is a byproduct of metabolism (chemical reactions in your body to make energy). Bicarbonate blood level is calculated using pH and PaCO2 levels.
Arterial Blood Gas Test Procedure
You are likely to have an arterial blood gas test in a hospital, but your doctor may be able to do it in their office.
Your doctor or another health care worker will use a small needle to take some of your blood, usually from your wrist. Sometimes they take it from an artery in your groin or on the inside of your arm above your elbow.
Before the arterial blood gas test, they may apply pressure to the arteries in your wrist for several seconds. The procedure, called the modified Allen test, checks that blood flow to your hand is normal.
In order to prepare for an arterial blood gas test, tell your doctor about all medications, supplements, and vitamins you’re taking.
If you’re on oxygen therapy but are able to breathe without it, they might turn off your oxygen for 20 minutes for a “room air” test before the blood gas test.
You may have a few minutes of discomfort during or after the test. Collecting blood from an artery typically hurts more than drawing it from a vein. Arteries are deeper than veins, and there are sensitive nerves nearby.
You also may feel lightheaded, faint, dizzy, or nauseated while your blood is drawn. To lower the chance of bruising, you can gently press on the area for a few minutes after the needle comes out.
Is an Arterial Blood Gas Test Painful?
A specially trained health care provider collects an arterial blood sample from your artery. Arteries, which are surrounded by nerves, are located deeper in your body than veins. So you can expect more pain from an arterial blood sample, compared to a typical blood draw from your vein.
You might feel dizzy, lightheaded, or nauseous while a health care provider collects an arterial blood gas. Your provider will likely have you lie down during the procedure, which may help you relax. Focus on slow, deep breaths, which can help distract from the pain and ensure an accurate result. Holding your breath or crying changes your normal breathing and may give an inaccurate test result.
If you’re critically ill and hospitalized, your doctor may give you an indwelling arterial catheter — a tube that stays put in your artery to continuously measure blood pressure and collect ABGs painlessly, without having to puncture your skin.
Arterial Blood Gas Test Results
Results of your arterial blood gas test are usually available in less than 15 minutes. But your doctor can’t diagnose you based on an arterial blood gas test alone. So you’ll probably get other tests, too.
Your arterial blood gas test results may show whether:
- You are getting enough oxygen
- Your lungs are exhaling enough carbon dioxide
- Your kidneys are working properly
- You have a serious infection effecting perfusion (blood delivery) to your organs
The numbers for normal results vary. Some components will have normal values, while others may be too high or too low. Your doctor will interpret your ABG results as they relate to your medical history and your current condition. Your ABG results help your doctor diagnose and treat your condition.
Normal values for the different ABG components are:
- O2CT: 15%-23% per 100 mL of blood
- SaO: 95%-100%
- PaO2: 75-100 mmHg
- PaCO2: 35-45 mmHg
- pH: 7.35-7.45
- HCO3: 22-26 mEq/L
Arterial Blood Gas Test Risks
Any test that involves using a needle carries some risks, including bleeding, infection, and bruising. You might have some soreness where the needle went in your skin.
Takeaways
An arterial blood gas test measures the levels of gas — oxygen and carbon dioxide — in your blood, as well as your blood’s pH. It’s usually performed in the hospital if you’ve been seriously injured or are critically ill. This test checks your lung and kidney function and how well oxygen is being delivered to the blood. Your doctor may use an arterial blood gas test to diagnose and monitor severe respiratory conditions, evaluate how well oxygen therapy is working, and check the pH (or acid level) of your blood.
Arterial Blood Gas Test FAQs
What are the six steps of an ABG analysis?
The arterial blood gas test includes several components. Your doctor compares, or analyzes, these measures to get a big picture of your body’s oxygenation and acid-base balance. These are the six steps of an ABG analysis:
- Analyze pH: Lower than 7.35 means your blood is too acidic; higher than 7.45, your blood is too basic.
- Analyze the PaCO2: Abnormal CO2 levels contribute to pH imbalances.
- Analyze the HCO3: Your kidneys produce this byproduct of gas exchange.
- Match the PaCO2 or HCO3 with your pH: Comparing these values helps your doctor figure out whether a respiratory or metabolic issue is disrupting your blood’s pH.
- Assess for compensation: Comparing values helps your doctor know whether your body is using its respiratory or metabolic (energy processing) systems to compensate — or correct — an abnormal blood pH.
- Analyze your PaO2 and SaO2 for low oxygen.
What is a normal HCO3 level for an ABG test?
A normal HCO3 level for an ABG test is 22-26 mEq/L.
What are blood gasses?
Your body's natural gas exchange results in dissolved oxygen and carbon dioxide (gasses) in your blood.
What happens if your arterial blood gas test result is low?
There are several components of an arterial blood gas test. Some values may be low while others are high. Your doctor compares these numbers to figure out if your body is getting enough oxygen, your blood’s acid-base level, and if your body’s respiratory or metabolic systems are correcting these imbalances.
