Blood Oxygen Level

What is a blood oxygen level test?

A blood oxygen level test, also known as a blood gas analysis, measures the amount of oxygen and carbon dioxide in the blood. When you breathe, your lungs take in (inhale) oxygen and breathe out (exhale) carbon dioxide. If there is an imbalance in the oxygen and carbon dioxide levels in your blood, it can mean your lungs aren't working well.

A blood oxygen level test also checks the balance of acids and bases, known as pH balance, in the blood. Too much or too little acid in the blood can mean there is a problem with your lungs or kidneys.

Other names: blood gas test, arterial blood gases, ABG, blood gas analysis, oxygen saturation test

What is it used for?

A blood oxygen level test is used to check how well your lungs are working and measure the acid-base balance in your blood. The test usually includes the following measurements:

Oxygen content (O2CT). This measures the amount of oxygen in the blood.
Oxygen saturation (O2Sat). This measures the amount of hemoglobin in your blood. 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 the 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 the blood.
pH. This measures the balance of acids and bases in the blood.

Why do I need a blood oxygen level test?

There are many reasons this test is ordered. You may need a blood oxygen level test if you:

Have trouble breathing
Have frequent periods of nausea and/or vomiting
Are being treated for a lung disease, such as asthma, chronic obstructive pulmonary disease (COPD), or cystic fibrosis. The test can help to see if treatment is working.
Recently injured your head or neck, which can affect your breathing
Had a drug overdose
Are receiving oxygen therapy while in the hospital. The test can help make sure you are getting the right amount of oxygen.
Have carbon monoxide poisoning
Have a smoke inhalation injury

A newborn baby may also need this test if he or she is having trouble breathing.

What happens during a blood oxygen level test?

Most blood tests take a sample from a vein. For this test, a health care provider will take a sample of blood from an artery. That's because blood from an artery has higher oxygen levels than blood from a vein. The sample is usually taken from an artery inside the wrist. This is called the radial artery. Sometimes the sample is taken from an artery in the elbow or the groin. If a newborn is being tested, the sample may be taken from the baby's heel or umbilical cord.

During the procedure, your provider will insert a needle with a syringe into the artery. You may feel a sharp pain as the needle goes into the artery. Getting a blood sample from an artery is usually more painful than getting blood from a vein, a more common type of blood test procedure.

Once the syringe is filled with blood, your provider will put a bandage over the puncture site. After the procedure, you or a provider will need to apply firm pressure to the site for 5–10 minutes, or even longer if you are taking a blood-thinning medicine.

Will I need to do anything to prepare for the test?

If your blood sample is taken from your wrist, your health care provider may perform a circulation test called an Allen test before taking the sample. In an Allen test, your provider will apply pressure to the arteries in your wrist for several seconds.

If you are on oxygen therapy, your oxygen may be turned off for about 20 minutes before the test. This is called a room air test. This won't be done if you are unable breathe without the oxygen.

Are there any risks to the test?

There is very little risk to having a blood oxygen level test. You may have some bleeding, bruising, or soreness at the spot where the needle was put in. Though problems are rare, you should avoid lifting heavy objects for 24 hours after the test.

What do the results mean?

If your blood oxygen level results are not normal, it may mean you:

Are not taking in enough oxygen
Are not getting rid of enough carbon dioxide
Have an imbalance in your acid-base levels

These conditions can be signs of a lung or kidney disease. The test can't diagnose specific diseases, but if your results are not normal, your health care provider will order more tests to confirm or rule out a diagnosis. If you have questions about your results, talk to your health care provider.

Is there anything else I need to know about blood oxygen level tests?

Another type of test, called pulse oximetry, also checks blood oxygen levels. This test doesn't use a needle or require a blood sample. In pulse oximetry, a small clip-like device with a special sensor is attached to your fingertip, toe, or earlobe. Since the device measures oxygen "peripherally"(in an outer area), the results are given as peripheral oxygen saturation, also known as SpO2.

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