Health Guide

Partial pressure arterial oxygen/fraction inspired oxygen ratio

What is this test?

The ratio of partial pressure arterial oxygen and fraction of inspired oxygen is a comparison between the oxygen level in the blood and the oxygen concentration that is breathed. This helps to determine the degree of any problems with how the lungs transfer oxygen to the blood[1]. A sample of arterial blood is collected for this test[2].

What are other names for this test?

  • PaO2/FiO2 ratio

Why do I need this test?

Laboratory tests may be done for many reasons. Tests are performed for routine health screenings or if a disease or toxicity is suspected. Lab tests may be used to determine if a medical condition is improving or worsening. Lab tests may also be used to measure the success or failure of a medication or treatment plan. Lab tests may be ordered for professional or legal reasons. The following are possible reasons why this test may be done:

  • ARDS - Acute respiratory distress syndrome
  • Transfusion related acute lung injury

When and how often should I have this test?

When and how often laboratory tests are done may depend on many factors. The timing of laboratory tests may rely on the results or completion of other tests, procedures, or treatments. Lab tests may be performed immediately in an emergency, or tests may be delayed as a condition is treated or monitored. A test may be suggested or become necessary when certain signs or symptoms appear.

Due to changes in the way your body naturally functions through the course of a day, lab tests may need to be performed at a certain time of day. If you have prepared for a test by changing your food or fluid intake, lab tests may be timed in accordance with those changes. Timing of tests may be based on increased and decreased levels of medications, drugs or other substances in the body.

The age or gender of the person being tested may affect when and how often a lab test is required. Chronic or progressive conditions may need ongoing monitoring through the use of lab tests. Conditions that worsen and improve may also need frequent monitoring. Certain tests may be repeated to obtain a series of results, or tests may need to be repeated to confirm or disprove results. Timing and frequency of lab tests may vary if they are performed for professional or legal reasons.

How should I get ready for the test?

Before having blood collected, tell the person drawing your blood if you are allergic to latex. Tell the healthcare worker if you have a medical condition or are using a medication or supplement that causes excessive bleeding. Also tell the healthcare worker if you have felt nauseated, lightheaded, or have fainted while having blood drawn in the past.

Do not exercise before this test[2].

How is the test done?

Before an arterial blood draw, the healthcare worker may check blood circulation to your arm or leg. Tell the healthcare worker if you use supplemental oxygen as this may affect the test results.

An artery in the wrist is the most common site to draw arterial blood. However, an artery at the inside bend of the arm or groin may be used. The area will be swabbed clean with antiseptic. Anesthetic may be used to numb the skin over the area where the blood will be drawn. A needle is used to puncture the skin. You will be asked to hold very still while your blood is collected. When enough blood is collected, the needle will be removed.

If you have an existing arterial catheter, a blood sample may be collected from that catheter.

How will the test feel?

The amount of discomfort you feel will depend on many factors, including your sensitivity to pain. Communicate how you are feeling with the person doing the test. Inform the person doing the test if you feel that you cannot continue with the test.

If you receive anesthetic to numb the skin before an arterial blood draw, you may feel discomfort at the location where the anesthetic needle is inserted into the skin. You may feel discomfort as the needle used to draw your blood is inserted into the skin, and you may have cramping at the site during the procedure. Inform the person doing the test if you feel faint or nauseated or if the discomfort is severe.

If blood is collected from an existing catheter inserted into an artery, you will feel little or no discomfort.

What should I do after the test?

If blood was drawn from an artery in your arm or leg, cotton will be placed over the site and held firmly for at least five minutes to stop the bleeding. Bleeding may continue beyond five minutes if you are using medications or supplements that thin your blood or have a medical condition that causes excessive bleeding. When bleeding has stopped, a bandage will be placed firmly over the site and should be left on for 30 to 60 minutes. You should rest for at least 15 minutes after the test. Avoid heavy use of the arm or leg from which the blood was drawn for 24 hours after the blood is collected.

If blood was drawn from an existing catheter inserted into an artery, there are no special instructions for you after this test.

What are the risks?

Arterial blood: During an arterial blood draw, a hematoma (blood-filled bump under the skin) or slight bleeding from the puncture site may occur. After a blood draw, a bruise or infection may occur at the puncture site. The person doing this test may need to perform it more than once.

If you have a medical condition, or are using a medication or supplement that causes excessive bleeding, you are at a higher risk of bleeding from the puncture site. Rarely, damage to the artery that affects blood flow to the arm or the leg may occur. Talk to your healthcare worker if you have any concerns about the risks of this test.

What are normal results for this test?

Laboratory test results may vary depending on your age, gender, health history, the method used for the test, and many other factors. If your results are different from the results suggested below, this may not mean that you have a disease. Contact your healthcare worker if you have any questions. The following is considered to be a normal result for this test:

  • Adults: Room air partial pressure arterial oxygen (PaO2) 80-100 mmHg (11-13 kPa) [3]
  • Fraction inspired oxygen (FiO2) in air: 21%
  • Normal range (PaO2/FiO2): = 400

What might affect my test results?

  • Results increased in [2]:
    • Oxygen enriched air
    • Exercise
    • Angiomas of the brain
  • Results decreased in [2]:
    • Advancing age
    • High altitude (eg, hypoxemia)
    • Decreased cardiac output
    • Carbon monoxide poisoning
    • Anesthesia
    • Near drowning

What follow up should I do after this test?

Ask your healthcare worker how you will be informed of the test results. You may be asked to call for results, schedule an appointment to discuss results, or notified of results by mail. Follow up care varies depending on many factors related to your test. Sometimes there is no follow up after you have been notified of test results. At other times follow up may be suggested or necessary. Some examples of follow up care include changes to medication or treatment plans, referral to a specialist, more or less frequent monitoring, and additional tests or procedures. Talk with your healthcare worker about any concerns or questions you have regarding follow up care or instructions.

Where can I get more information?

Related Companies

  • American Lung Association -
  • Society of Critical Care Medicine -
  • National Heart, Lung and Blood Institute -


[1] Toy P, Popovsky MA, Abraham E, et al: Transfusion-related acute lung injury: definition and review. Crit Care Med 2005; 33(4):721-726.

[2] Tietz NW (Ed): Clinical Guide to Laboratory Tests, 3rd ed. W. B. Saunders, Philadelphia, PA, 1995.

[3] Kratz A, Ferraro M, Sluss PM, et al: Case records of the Massachusetts General Hospital: laboratory values. N Engl J Med 2004; 351(15):1549-1563.

Last Updated: 6/12/2013
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