How to Measure Blood Oxygen

Explore this Article

methods

1 Measuring Blood Oxygen by an Arterial Blood Gas Test

2 Measuring Blood Oxygen by Pulse Oximetry

Other Sections

Tips and Warnings

Related Articles

References

Medically reviewed by Tapan Abrol, MD

Last Updated: May 25, 2021 References

Experts say that your doctor might measure your blood oxygen levels to make sure your lungs are working correctly, to make sure a medical treatment is working, to check for sleep apnea, or to find out if you're healthy enough for exercise. Your doctor may do an arterial blood gas test or pulse oximetry test to measure how much oxygen is in your blood. Research shows that blood oxygen tests won't diagnose your condition, but they can help your doctor narrow down the cause of your symptoms. While arterial blood gas tests are typically more accurate, pulse oximetry may show your blood oxygen levels over a period of time. Fortunately, these tests are simple and easy.

Steps

Method 1

Method 1 of 2:

Measuring Blood Oxygen by an Arterial Blood Gas Test

1. 
   1
   Contact a health professional to receive an arterial blood gas test. Your doctor or other medical professional can accurately measure your blood oxygen level using advanced techniques and equipment. You may need to have your blood oxygen level tested prior to surgery or other medical procedures, or if you have certain conditions, such as:

   • Sleep apnea
   • A heart attack or congestive heart failure
   • Chronic obstructive pulmonary disease (COPD)
   • Anemia
   • Lung cancer
   • Asthma
   • Pneumonia
   • Cystic fibrosis
   • A current or possible need for mechanical ventilation to support your breathing
2. 
   2
   Prepare for the procedure. While the arterial blood gas test is common and quite safe, you will still want to prepare for the procedure. Talk to your doctor to make sure you understand the test, and ask questions any questions you may have about it. You can also help by letting your doctor know if:

   • You have or have had bleeding problems
   • You take blood thinners, such as aspirin or warfarin (Coumadin)
   • You are taking any medications
   • You have any known allergies to medications or anesthetics
3. 
   3
   Know the risks. The arterial blood gas test is a routine procedure, and there is little chance of serious issues developing as a result of it. Potential minor risks include:

   • A small bruise at the site where blood is drawn from the artery. Keeping pressure on the site for at least ten minutes after the needle is removed will lower the chances of bruising.
   • Feelings of lightheadedness, dizziness, or nausea while blood is being drawn from your artery.
   • Prolonged bleeding. This is a potential risk if you have a bleeding disorder or are taking blood-thinning medicines such as aspirin or warfarin.
   • A blocked artery. If the needle damages a nerve or artery, it may cause the artery to become blocked. This is a rare problem.
4. 
   4
   Have a health professional select the test site. To measure blood oxygen with this method, blood must be drawn from an artery. Usually, one in your wrist (the radial artery) is chosen, although blood can also be drawn from an artery in your groin (the femoral artery) or from your arm above the elbow (the brachial artery). A needle will be used to draw the blood for the sample.

   • You will be able to sit for the procedure, and your arm will be extended and will rest on a comfortable surface.
   • The health professional will feel your wrist to find your pulse and to check the blood flow of your arteries (a procedure called the Allen test).
   • If you use an arm for dialysis, or if there is infection or inflammation at the intended test site, another area will be used for the arterial blood gas test.
   • An artery is chosen for this procedure because it will allow the oxygen to be measured before it enters body tissues, giving a more accurate reading.
   • If you are currently on oxygen therapy, your doctor may have the oxygen shut off for twenty minutes prior to the test (unless you cannot breathe without the oxygen) to help get an accurate reading of your blood oxygen level.
5. 
   5
   Have a health professional take a blood sample. Once he or she has selected a test site, your health professional will prepare the site and use a needle to take a blood sample.

   • First, the skin on the test site will be cleaned with alcohol. You may be given a local anesthetic (by injection) to numb the area first.
   • The needle will be puncture your skin, and blood will fill the syringe. Make sure that you breathe normally while the blood is being drawn. If you were not given a local anesthetic, you may feel some slight pain during this step.
   • Once the syringe is full, the needle will be removed and gauze or a cotton ball will be put over the puncture site.
   • A bandage will be put over the puncture site. You should apply pressure to the site for five to ten minutes in order to stop any bleeding. If you are on any blood thinning medications or have bleeding problems, your health professional may instruct you to apply pressure for a longer time.
6. 
   6
   Follow post-procedure instructions. In the majority of cases, patients will recover from the minor discomfort of an arterial blood gas test quickly and without issue. However, you should be gentle at first with the arm or leg used for the blood draw. Avoid lifting or carrying objects for about twenty for hours after the test.

   • Contact your doctor if you have prolonged bleeding from the site, or any other unexpected issue.
7. 
   7
   Have the blood sample sent to a lab. Once the sample is collected, your health professional will send the sample to a lab to complete the test. When the sample arrives at a lab, technicians can use special equipment to measure the blood oxygen level of your sample.

   • The amount of time that passes before receiving the results of your arterial blood gas test will depend on which lab your sample is sent to. Your health professional will be able to give you this information.
   • In emergency situations, particularly if you are in a hospital, the results may be available within a few minutes. Ask your healthcare provider how long you can expect to wait to receive your results.
8. 
   8
   Interpret the results. The arterial blood gas test gives a reading of partial pressure of oxygen and carbon dioxide in your blood, which is more specific and useful to healthcare professionals than the percentages produced by pulse oximetry. Normal oxygen results are between 75-100mmHg (a unit used to measure pressure); normal carbon dioxide results are between 38-42mmHg. Your doctor will discuss the implications of your test results with you, including how your “normal” level may vary based on a number of factors, including:

   • Your elevation above sea level
   • The particular lab your sample was sent to
   • Your age
   • If you have a fever or low body temperature
   • If you have certain conditions, such as anemia
   • If you smoke just prior to the test

Method 2

Method 2 of 2:

Measuring Blood Oxygen by Pulse Oximetry

1. 
   1
   Contact a health professional to receive a pulse oximetry test. A pulse oximetry test can give the saturation of oxygen in your blood by transmitting light through your tissues. It You may need to have your blood oxygen level tested prior to surgery or other medical procedures, or if you have certain conditions, such as:

   • Sleep apnea
   • A heart attack or congestive heart failure
   • Chronic obstructive pulmonary disease (COPD)
   • Anemia
   • Lung cancer
   • Asthma
   • Pneumonia
   • Cystic fibrosis
   • A current or possible need for mechanical ventilation to support your breathing
2. 
   2
   Prepare for the procedure. The pulse oximetry method of measuring blood oxygen level is non-invasive, so there is usually little you need to do to prepare for the test. However, your doctor will still discuss the test with you and answer any questions you may have.

   • You may be asked to remove fingernail polish, if applicable.
   • Your doctor may give you other specific instructions for preparation, based on your medical condition and history.
3. 
   3
   Know the risks. There are very few risks associated with pulse oximetry. These are minimal, but include:

   • Skin irritation at the application site. This may occur with prolonged or repeated application of the probe sensor.
   • Inaccurate readings in cases of smoke or carbon monoxide inhalation.
   • Your doctor can let you know if there are any additional risks, based on your specific medical condition.
4. 
   4
   Have your health professional prepare the sensor. The sensor used to measure the oxygen level of blood by pulse oximetry is a clip-like device called a probe. The probe sensor contains a light source, a light detector, and a microprocessor. The light emitted from the source on one side of the clip passes through your skin and reaches the detector on the other side of the clip. The microprocessor does calculations based on the information received from the detector in order to calculate the oxygen level of your blood with a very small margin of error.
5. 
   5
   Have your health professional attach the sensor to your body. Usually, a finger, ear, or nose is chosen as the site to attach the sensor. The sensor will then use light to measure the oxygen level of your blood.

   • This method has the advantage of being painless and noninvasive, since no needles are involved.
   • However, it is not as accurate as an arterial blood gas test, so in some cases, both tests may need to be performed.
   • Your health professional cannot attach the sensor to an area with excessive movement or tremor, or with bruising. For example, if you have a dark bruise under your fingernail, your health professional may place the sensor on your ear instead.
6. 
   6
   Let the sensor perform a reading. The sensor’s microprocessor will compare the transmission of two wavelengths of light, red and infrared, as they pass through the relatively thin skin of your finger, ear or other site. Hemoglobin in your blood that has absorbed oxygen absorbs more infrared light, while hemoglobin lacking oxygen absorbs more red light. The sensor calculates the difference between these two values to provide information for deriving your blood oxygen level.
7. 
   7
   Remove the probe. If you are having your blood oxygen level measured for a one-time reading, then once the sensor has taken the necessary measurements and completed its calculations, the probe can be removed. In some cases (such as for certain congenital heart conditions), however, your doctor may require you to wear the probe for continuous monitoring. If you are asked to do this, remove the probe sensor only when your doctor tells you to.
8. 
   8
   Follow post-procedure instructions. Most of the time, there are no special restrictions following the pulse oximetry test, and you can immediately return to normal activities. Depending on your individual medical condition, however, your doctor may give you special post-procedure instructions.
9. 
   9
   Interpret the results. Once your doctor has the results of your pulse oximetry test, he or she will review them with you. An oxygen saturation level of about 95% is described as normal. Your doctor will discuss the implications of your test results with you, including how certain factors may alter the results of the test, including:

   • Decreased peripheral blood flow
   • Light shining on the oximetry probe
   • Movement of the test site area
   • Anemia
   • Abnormal warmth or coolness at the test site area
   • Sweating at the test site area
   • Recent injection of contrast dye
   • Smoking tobacco

Expert Q&A

Search

Add New Question

Ask a Question

200 characters left

Include your email address to get a message when this question is answered.

Submit

Tips

• There are proposed methods for taking pulse oximetry tests with smartphones or other consumer-grade electronic devices. These methods are still under development, so discuss them with your doctor first.
  Thanks

  Helpful 0 Not Helpful 0