What Is the Partial Pressure of Oxygen (PaO2) Test?

A PaO2 test measures partial pressure of oxygen, or PaO2—the oxygen pressure in arterial blood. The PaO2 reflects how well oxygen is able to move from the lungs to the blood. It is often altered by severe illnesses, with the PaO2 test results used to guide treatment.

The PaO2 is one of the components measured in an arterial blood gas (ABG) test, which also reports oxygen (O2) saturation, bicarbonate (HCO3), the partial pressure of carbon dioxide (CO2), and the pH level in red blood cells.

This article explains the purpose and risks of a PaO2 test. It describes what to expect before, during, and after the test, as well as how the results are interpreted.

Purpose of PaO2 Test

The PaO2 test can be used to assess the effects of breathing problems on oxygen supply, especially in a hospital setting or during an episode of severe respiratory distress. The results are often used to determine whether emergency treatment—such as oxygen supplementation or mechanical breathing support—are needed.

PaO2 values can also be used (along with other tests) to help diagnose a number of chronic medical conditions or events like:

• Chest or lung trauma
• Chronic lung diseases such as asthma, cystic fibrosis, or chronic obstructive pulmonary disease (COPD)
• Congestive heart failure
• Heart attack
• Loss of consciousness
• Sudden shortness of breath

In some situations, such as when oxygen therapy or ventilation (mechanical breathing support) has been started, a repeat PaO2 test is used to assess whether a respiratory condition is improving or worsening and whether the treatment should be adjusted.

Risks and Contraindications

A PaO2 test has very few risks associated with it. Because the test uses blood obtained with an arterial puncture instead of blood drawn from a vein, there is a slightly higher risk of bleeding or bruising.

If you're taking a blood thinner or have a bleeding disorder, your medical team may take special precautions while you undergo this test, such as monitoring for bleeding from your puncture site for a longer period of time.

Before the PaO2 Test

A PaO2 test as part of an outpatient diagnostic evaluation requires an appointment. However, the test can be done urgently during a respiratory emergency.

Often, the test is administered during a hospital stay or to monitor treatment or the progression of an illness.

Timing

Drawing blood for this test takes about five minutes, but you should allot about an hour if you're having the test done in an outpatient setting. This will give you time to register, wait for your turn, and ensure that the puncture site isn't bleeding.

Location

Outpatient PaO2 tests are done at a healthcare provider's office or at a clinic or lab where blood draws are performed. If you have this test in a hospital or during an emergency, you will be able to remain in a hospital bed during the arterial blood draw.

What to Wear

Dress comfortably for a PaO2 test. Often, blood is drawn from the radial artery, which runs along the inner surface of the wrist. So it's a good idea to wear either a short-sleeve shirt or a shirt with sleeves that are easy to pull up.

Food and Drink

You can eat and drink whatever you want prior to this test because your diet will not affect the results.

Cost and Health Insurance

Check with your healthcare provider or health insurer about whether your plan will cover the cost of the test and, if relevant, the cost of your co-pay.

If you have time, you may wish to shop around and compare prices. They can vary considerably.

During a PaO2 Test

Whether you're in a hospital setting or having your PaO2 measured as an outpatient, the procedure will be performed by a healthcare provider or a specialized nurse or technician.

Pre-Test

First, you'll have your arterial pulse checked. While blood is usually drawn from the radial artery in your wrist, this is not always ideal. If your pulse is weak due to severe illness or blood loss, the femoral artery in your groin may be used instead.

Throughout the Test

The skin around the intended puncture site will be cleaned, usually with an alcohol pad. A small needle, which is attached to a tube, will then be inserted into the artery in your wrist.

The pressure or pain is a bit uncomfortable—more uncomfortable than the pain felt when you have blood drawn from a vein. However, most people consider the pain bearable.

Typically, blood pumps rapidly through the arteries. This is why the volume of blood needed for your test can be collected quickly. Once the blood is collected, the needle is removed and cotton or gauze is placed over the puncture site.

After the PaO2 Test

After your PaO2 test, you will have a bandage placed over your puncture wound. You might also need to have gauze wrapped around your wrist for a few hours. Your wrist may feel a bit sore for several days after the test.

Many people are able to resume normal activities soon after having a PaO2 test. Still, it's best to avoid heavy lifting with the arm used for the blood draw until a few days after your procedure. Ask your healthcare provider to be sure.

Managing Side Effects

If you feel sore, you can place an ice pack on your wrist. Ask your healthcare provider if you can use mild pain relievers for soreness and pain. Be careful with respect to medications that are also blood thinners (like aspirin or ibuprofen). They can trigger additional bleeding.

When to Call Your Healthcare Provider

If you experience any of the following, be sure to let your healthcare provider know immediately:

• Numbness, tingling, or diminished sensation in your hand or fingers
• Pale or bluish color in your hand or fingers
• Persistent bleeding
• Severe wrist, hand, or arm pain
• Swelling of your fingers, hand, or arm
• Weakness of your fingers, hand, or arm

Interpreting Results

Each breath delivers air to the alveoli in your lungs. There, oxygen and carbon dioxide are transferred between the lungs and the blood. Because oxygen pressure is higher in the alveoli than it is in the adjacent capillaries (tiny blood vessels), it flows into the capillaries.

When the body is functioning normally, PaO2 is between 75 and 100 mmHg (at sea level). A result in this range means a sufficient amount of oxygen is flowing from the alveoli to the blood. If your PaO2 is below the normal range, that is not the case.

Lung diseases and breathing problems can increase the risk of developing low PaO2. Talk to your healthcare provider for their interpretation of the results.

Factors Affecting PaO2 Levels

A number of factors can lower your PaO2 levels, including:

• The partial pressure of oxygen in the air you inhale: At higher altitudes (such as in mountainous areas), the decrease in atmospheric pressure reduces oxygen availability—and oxygen pressure in your lungs.
• A neurological condition, such as Guillain-Barre Syndrome or Amyotrophic Lateral Sclerosis (ALS)
• Damage to the lungs due to trauma or cancer
• A reduction in the concentration of hemoglobin in your blood cells: Having iron-deficiency anemia means that your red blood cells are not able to effectively carry oxygen molecules
• Obesity

All of these conditions diminish the oxygen pressure in your lungs, as reflected in the arterial oxygen pressure as measured by PaO2.

Follow-Up

You may need to have a follow up PaO2 test, especially if your symptoms don't improve. You may also need a follow-up test when it's time to change your oxygen supply or ventilation assistance.

Often, when respiratory support is decreased due to improvement, PaO2 is rechecked to confirm that breathing and oxygenation remain at optimal levels, even with less support.

Summary

The PaO2 test is used to evaluate respiratory diseases and conditions that affect the lungs. The results are often used to determine whether emergency treatment—such as oxygen supplementation or mechanical breathing support—is needed.

The test can also shed light on the body's lung and kidney function and its general metabolic state, meaning the physical and chemical processes in the body that use or convert energy.

If oxygen therapy is being used, the PaO2 can tell a physician how well it seems to be working. Like other lab tests, the PaO2 test is over in no time.

A Word From Verywell

While this test is quick to administer, it can be mildly uncomfortable. This is because blood is drawn from an artery, not a vein (like you're probably used to). Even if you experience throbbing or slight bruising, try to remember that these side effects are short-lived.