About the Test
Purpose of the test
The purpose of a blood gases test is to evaluate the amount of oxygen and carbon dioxide and the acid-base balance, or pH, in the blood. Blood gases testing is primarily used for diagnosis and monitoring of different medical conditions.
Diagnosis is the process of finding the cause of a person’s health problems. Measurement of blood gases can aide in the diagnosis of health problems like:
- Lung and breathing disorders: When you breathe, your lungs transfer oxygen from the air into the blood and pull carbon dioxide out of the blood so that it can be exhaled. This process, called gas exchange, is fundamental to health and bodily function. A blood gases test can help detect abnormal oxygen or carbon dioxide levels that can be linked to breathing problems or lung diseases.
- Acid-base imbalances: The body has several systems to maintain a healthy acid-base balance, and a blood gases test can help show whether those systems are working correctly. This can also offer information about the body’s production and use of energy, which is known as its metabolism.
- Kidney problems: The kidneys play an important part in regulating the tight acid-base balance of the body, so a blood gases test can provide evidence of possible kidney disease or disrupted kidney function.
Monitoring is testing that is done after an initial diagnosis to observe how a patient’s condition changes over time. A blood gases test may help assess whether treatment is working in people with breathing disorders, kidney disease, metabolic problems, or other acid-base imbalances.
What does the test measure?
A blood gases test includes multiple measurements using a sample of blood taken from either an artery or a vein. Typical measurements in a blood gases test include:
- Partial pressure of oxygen (O2): Also known as oxygen tension, this measures how well oxygen is being transferred into the blood.
- Partial pressure of carbon dioxide (CO2): Also known as carbon dioxide tension, this measures the level of carbon dioxide in the blood.
- Oxygen saturation (O2 Sat): This is an assessment of the amount of oxygen in the blood that is based on measuring levels of hemoglobin. Hemoglobin is a protein found inside red blood cells that is responsible for carrying oxygen throughout the body.
- Acidity: This is the pH level of your blood. It is measured from 0-14 with 0 being the most acidic and 14 being the most basic.
- Bicarbonate (HCO3-) concentration: Bicarbonate is an electrolyte, which is a type of mineral involved in managing the body’s acid-base balance. Most of the carbon dioxide in the blood is stored in the form of bicarbonate, so this measurement helps reflect carbon dioxide levels.
In a blood gases test, a blood sample can either be taken from an artery or from a vein. Arteries carry oxygen-rich blood away from the heart to your tissues, while veins bring low-oxygen blood back to the heart. The source of the blood sample can influence the way measurements of a blood gases test are interpreted.
The specific measurements on your test report may change based on whether the blood sample came from your artery or your vein. The following table shows the differences in some key abbreviations for measurements on arterial or venous blood gases tests:
| Arterial Blood Gases Test | Venous Blood Gases Test | |
|---|---|---|
| Source of Blood Sample | Artery | Vein |
| Partial Pressure of Oxygen | PaO2 | PvO2 |
| Partial Pressure of Carbon Dioxide | PaCO2 | PvCO2 |
| Oxygen Saturation | SaO2 | SvO2 |
Although not universal, some arterial blood gases tests include measurements of hemoglobin as well as altered forms of the hemoglobin protein. Examples of these potential additional measurements include:
- Methemoglobin: This is a type of hemoglobin that can alter the ability of the blood to carry oxygen through the body.
- Carboxyhemoglobin: This abnormal form of hemoglobin attaches to carbon monoxide and can interfere with oxygen’s ability to travel in the blood.
- Oxyhemoglobin: This defines the amount of oxygen-carrying hemoglobin in the blood.
- Deoxyhemoglobin: This is the form of hemoglobin without oxygen in the blood.
When should I get a blood gases test?
Your doctor is most likely to recommend a blood gases test if they believe you may have a problem affecting your acid-base balance, your oxygen levels, and/or your carbon dioxide levels.
Symptoms that can be associated with acidosis, which is when there is too much acid in the body, include:
- Rapid breathing
- Shortness of breath
- Mental confusion
- Fatigue
Symptoms that may arise from alkalosis, which is when there is too much base in the body, include:
- Confusion
- Nausea and vomiting
- Hand tremors
- Muscle twitching or spasms
- Lightheadedness
- Numbness in the hands, feet, or face
Blood gases testing may also be performed if there are signs or symptoms that may be tied to impaired lung function. Your doctor may recommend blood gas testing as a type of pulmonary function test if:
- You have difficulty breathing, shortness of breath, or other respiratory problems
- You have suffered an injury to your head or neck that could affect your breathing
- You have experienced carbon monoxide or smoke inhalation
If you are receiving therapy for an acid-base imbalance or treatment for a breathing disorder, a blood gases test can be part of how your doctor checks the status of your health. This can also provide insight into whether treatment, such as supplemental oxygen therapy, is helping your condition.
It is important to talk with your doctor about any health changes or symptoms that you are experiencing. Your doctor can then address whether testing blood gases is appropriate in your situation.
Finding a Blood Gases Test
How to get tested
Measuring blood gases requires obtaining a sample of blood. Most often, this blood is taken from an artery, but in certain situations it can be drawn from a vein.
Blood gases testing is prescribed by a health care professional, and the procedure to withdraw a blood sample is done in a hospital or doctor’s office.
Can I take the test at home?
There are no at-home test kits for blood gases that are commercially available at this time.
Most blood gases testing is done by drawing blood from an artery, a procedure that requires medical training to reduce the risks of unwanted side effects or complications. This procedure is normally done in a hospital or medical office.
How much does the test cost?
Several factors can affect the cost of a blood gases test including:
- Whether the blood sample is from an artery or vein
- The specific measurements included in the test
- Where the test is performed
- Whether you have health insurance coverage
The types of charges involved in blood gases testing may include fees for office visits, the procedure to draw your blood, and the analysis by the laboratory. If the test is recommended by your doctor and you have insurance, some or all of these charges may be paid for by your insurance provider. However, you should talk with your doctor or insurance plan to find out about any out-of-pocket costs such as deductibles or copays.
Taking a Blood Gases Test
The blood sample for a blood gases test can come from either an artery or a vein, but most often the blood is drawn from an artery.
The artery from which blood is most frequently taken, especially for people who are taking this test as an outpatient, is the radial artery in the wrist. For people who are in the hospital, blood can be drawn from other arteries. Hospitalized patients with an arterial catheter can have blood drawn through the catheter.
Before the test
Usually there are no special preparations required for a blood gases test. Before testing, though, talk to your doctor if:
- You use oxygen therapy: Depending on your situation, you may be asked to adjust your supplemental oxygen intake in the 20 minutes before the test.
- You take blood-thinning medications: These medications can increase bleeding, which your health care team may need to account for when preparing to draw blood from an artery.
During the test
The procedure for a blood gases test varies depending on whether you are having a blood sample taken from an artery or a vein.
Arterial blood gases test
Most arterial blood draws take a sample from the artery in your wrist. While seated, your arm is positioned with your wrist facing upward. The technician or nurse may briefly apply and remove pressure to your arteries in order to test your circulation. They sterilize the skin around the puncture site and may give you a mild topical anesthesia to reduce pain during the procedure.
The technician palpates your artery with their fingers and then inserts a pre-prepared needle into the artery. The required amount of blood is withdrawn, the needle is removed, and then pressure is applied directly to the puncture site for at least five minutes to prevent bleeding.
It is normal to feel minor pain when the needle is inserted. The pain is typically greater for an arterial blood draw than a venous blood draw, but the discomfort is not usually long-lasting.
A similar process is used for a blood draw from other arteries, but your body positioning will change depending on the specific artery that is being accessed during the procedure.
In some situations, ultrasound may be used to help guide needle placement, especially for patients with a weaker pulse or arteries that are more difficult to locate.
Venous blood gases test
A venous blood gases test involves drawing a sample of blood from a vein. This procedure is similar to blood draws for most laboratory tests and usually takes blood from a vein on the inner part of the elbow.
The technician may apply an elastic band to the upper arm to increase blood flow in the veins, but this is removed about one minute before blood is actually drawn. The puncture site is cleaned with an antiseptic, and a needle is inserted into the vein. After the required amount of blood is withdrawn, the needle is removed.
This procedure is usually over in less than a few minutes. There may be a sting when the needle is inserted and taken out, but this pain usually goes away quickly.
After the test
There are some differences in post-test protocols for blood gases tests based on how the blood sample is taken.
Arterial blood gases test
If your blood is drawn from an artery, you can expect pressure to be applied to the puncture site for a few minutes in order to prevent complications associated with excess bleeding. Afterward, a bandage may be applied as well.
When the blood draw is over, you may be monitored for any complications. For most people, the only lasting effects are some soreness and bruising near the puncture site. However, any worsening pain, skin color changes, or signs of infection should be reported to your health care team immediately.
You can return to most normal activities after a blood draw from your artery, but you may be advised to avoid heavy lifting for 24 hours.
Venous blood gases test
If you have a blood draw from a vein, pressure may be applied to the puncture site with a bandage or cotton swab. You can usually resume normal activities once the test is over.
You may have some light pain or bruising. Other effects are uncommon, but you should inform your doctor of any persistent or worsening effects.
Blood Gases Test Results
Receiving test results
Results from a blood gases test are usually available within a few hours to a few days.
Analysis of your blood sample is often done rapidly, as many laboratories perform blood gases tests within minutes after the sample is received by the lab to ensure that results are accurate.
Sometimes you may get results shortly after this laboratory analysis is done. However, your doctor may need time to review and interpret the test results, so full results may not be immediately available.
A test report showing the results of each measurement of the blood gases test may be provided to you by mail or made accessible through an online health portal.
Interpreting test results
A blood gas test must be interpreted carefully for several reasons:
- There are multiple test measurements
- Those measurements involve interconnected bodily systems and processes
- Individual laboratories may use different methods or devices to conduct the test
- Measurements can vary based on whether the blood sample is taken from an artery or a vein
- Results can be affected by other factors such as body temperature and altitude
For these reasons, there is no universal range of normal levels for blood gases tests. Instead, each laboratory defines the reference range that it considers normal, and your doctor interprets that information in the context of your situation to assess the significance of the test result.
Abnormal results on any part of a blood gases test may be an indication of disruption of normal breathing. They can also reflect problems in the body’s system of regulating its acid-base balance.
Too much acid in the blood is known as acidosis, and too much base is called alkalosis. Both the lungs and kidneys are central to maintaining your acid-base balance, so finding acidosis or alkalosis on a blood gases test can be a sign of a problem affecting one or both of these organs.
When the lungs are not functioning properly, the body may have too little oxygen and too much carbon dioxide. Lack of oxygen and excess carbon dioxide can cause serious and even life-threatening damage to tissues throughout the body. In addition, carbon dioxide is acidic, so buildup of carbon dioxide can disrupt the acid-base balance.
It is also possible for abnormal breathing to contribute to having too much oxygen and too little carbon dioxide in the blood. For example, this can happen as a result of very rapid breathing, known as hyperventilation, that can be caused by several kinds of medical problems.
Lung diseases, disrupted breathing, and head and neck injuries are examples of conditions that may affect the lung’s ability to properly regulate oxygen and carbon dioxide, which can cause abnormal results on a blood gases test.
The kidneys help to maintain an acid-base balance by removing excess bicarbonate from the body through the urine. As a result, disorders that affect kidney function can cause acidosis or alkalosis that may be detected on a blood gases test.
Many different health conditions and even some medications can affect how the body regulates oxygen, carbon dioxide, and acidity, so blood gases tests are often one of several tests that may be needed to conclusively diagnose an underlying health problem.
It is essential to talk with your health care provider about your test results. They can explain specific findings on the test and what they mean in the context of your health.
Are test results accurate?
Blood gas tests are generally reliable if standard procedures are followed in preparing and analyzing the blood sample. But no test is perfect, and certain factors can affect test accuracy and interpretation:
- Altitude: Oxygen levels change at different altitudes, so it is normal for people to have lower blood oxygen levels if they are tested at altitudes over 3,000 feet.
- Body temperature: Oxygen and carbon dioxide levels as well as blood pH can change based on body temperature. Laboratories often show results for a body temperature of 37 degrees celsius, which may not be the same as a person’s actual body temperature at the time of testing.
Arterial blood gas tests are generally considered to be more informative than venous blood gas tests.
Do I need follow-up tests?
If your blood gases test is abnormal, it is common to have follow-up testing. In general, changes to your oxygen levels, carbon dioxide levels, or acid-base balance are caused by underlying problems that cannot be identified by the blood gases test alone.
As a result, you may need pulmonary function tests to assess your lung health, laboratory tests to evaluate your metabolism and kidney function, or other tests that can help determine the reason for abnormal results on a blood gases test.
Your health care provider is in the best position to explain the most relevant follow-up tests for you based on your test results, symptoms, and overall health.
Questions for your doctor about test results
The following list of questions may help you understand the meaning of blood gases testing when you review your test results with your doctor:
- Were there any abnormal levels on my blood gases test?
- What are the possible explanations for any abnormal results?
- Do you suggest repeating this test now or in the future?
- Are there other follow-up tests that you recommend? What are the benefits and downsides of those tests?
Comparing and contrasting arterial and venous blood gases tests
An arterial blood draw is generally preferred for testing blood gases. Testing blood from an artery is necessary to assess how well oxygen is being transferred to the blood. Although blood from a vein can be used to measure acidity and carbon dioxide levels, the inability to measure oxygen transfer is an important limitation. A venous blood gases test may also be less accurate during significant acid-base imbalances.
An arterial blood sample, though, is typically more difficult to obtain. An arterial blood draw has more steps, is often more painful for patients, and carries a higher risk of complications when compared to a venous blood sample.
Furthermore, some patients may not be able to have blood drawn from their artery because of preexisting conditions that can make the procedure unsafe. Extra precautions may also be necessary in certain patients, such as people with abnormalities in how their blood clots.
For any given patient, a doctor will carefully review the benefits and risks of these two approaches to blood gases testing to determine which is most appropriate in their case.
Comparing and contrasting a blood gases test and pulse oximetry
Pulse oximetry is another type of test designed to evaluate blood oxygen levels. This is a noninvasive test that involves clipping a small device on a finger or earlobe. The device contains a sensor that checks your oxygen saturation.
Because it is noninvasive and rapid, pulse oximetry can often be done at home and does not carry the risks associated with a blood draw. In addition, the sensor can be worn for an extended period, allowing for continuous observation of oxygen levels.
The result provided by pulse oximetry is known as peripheral oxygen saturation (SpO2). In contrast, an arterial blood gases test measures arterial oxygen saturation (SaO2). Results from pulse oximetry generally correlate to arterial blood gases testing but may be up to 5% higher or lower. However, other factors including skin pigmentation and use of nail polish can affect the accuracy of pulse oximetry.
Other limitations of pulse oximetry include an inability to measure carbon dioxide levels or excess amounts of oxygen in the arteries.
In many cases, pulse oximetry may be used to monitor oxygen saturation, and if a more precise measurement is needed, the doctor can prescribe a blood gases test. For some conditions, this approach can reduce the number of patients who need arterial blood draws.
Comparing and contrasting a blood gases test and pulmonary function tests
Pulmonary function tests are a collection of tests that can evaluate how well the lungs are working. An arterial blood gases test is one type of pulmonary function test because it provides information about how effectively the lungs are delivering oxygen to the blood. However, there are many different types of pulmonary function tests that assess other specific aspects of lung health.
Comparing and contrasting a blood gases test and a lung diffusion capacity test
Like a blood gases test, a lung diffusion capacity test is a type of pulmonary function test. Both tests help evaluate how well the lungs are moving oxygen into the blood. However, there are important differences in how these tests are performed and interpreted.
A lung diffusion capacity test does not involve a blood draw. Instead, the test requires breathing in a harmless gas from a chamber and then breathing out through a mouthpiece. The amount of gas you inhale and exhale is measured, and the difference shows how well the lungs transferred that gas to the blood.
Another important difference is that a lung diffusion capacity test does not measure acidity or specific oxygen and carbon dioxide levels. In this way, a blood gases test gives detailed information that is not available from a lung diffusion capacity test.
