Proceeds from website advertising help sustain Lab Tests Online. AACC is a not-for-profit organization and does not endorse non-AACC products and services.

Blood Gases

Print this article
Share this page:
Also known as: Arterial Blood Gases; ABGs
Formal name: Arterial Blood Gas Analysis

At a Glance

Why Get Tested?

To evaluate lung function by measuring blood pH, oxygen (O2) and carbon dioxide (CO2); to monitor treatment for lung diseases; to detect an acid-base imbalance in your blood, which may indicate a respiratory, metabolic, or kidney disorder; to evaluate the effectiveness of oxygen therapy

When to Get Tested?

When you have symptoms of a respiratory problem such as difficulty breathing, shortness of breath, or rapid breathing; when you are being treated for a lung disease; when an acid-base imbalance is suspected; periodically when you have a condition that causes an acute or chronic oxygen shortage and you are on oxygen therapy; during certain surgeries to monitor your blood's O2 and CO2 levels

Sample Required?

Most often a blood sample collected from an artery, usually the radial artery in your wrist; sometimes a blood sample drawn from a vein in your arm; capillary blood from a heelstick may be used for babies.

Test Preparation Needed?

Typically, none; however, if you are on oxygen therapy, the O2 may either be turned off for 20 to 30 minutes before the collection for a "Room Air" test. If this cannot be tolerated, or if your health practitioner wants to check your oxygen levels with the O2 on, the amount of oxygen being taken will be recorded.

The Test Sample

What is being tested?

Blood gases are a group of tests that are performed together to measure the pH and the amount of oxygen (O2) and carbon dioxide (CO2) present in a sample of blood, usually from an artery, in order to evaluate lung function and help detect an acid-base imbalance that could indicate a respiratory, metabolic or kidney disorder.

A person's body carefully regulates blood pH, maintaining it within a narrow range of 7.35-7.45, not allowing blood to become too acidic (acidosis) nor too alkaline/basic (alkalosis). The body's regulation of acids and bases has two main components. The first component involves both metabolism and the kidneys: the cellular process of converting one substance to another for energy produces large amounts of acid that the kidneys help eliminate. The second component of regulating pH balance involves eliminating carbon dioxide (an acid when dissolved in blood) through the lungs. This respiratory component is also the way that the body supplies oxygen to tissues. The lungs inhale oxygen, which is then dissolved in the blood and carried throughout the body to tissues.

These processes of gas exchange and acid/base balance are also closely associated with the body's electrolyte balance. In a normal state of health, these processes are in a dynamic balance and the blood pH is stable. (For more on this, see Acidosis and Alkalosis).

There is a wide range of acute and chronic conditions that can affect kidney function, acid production, and lung function, and they have the potential to cause a pH, carbon dioxide/oxygen, or electrolyte imbalance. Examples include uncontrolled diabetes, which can lead to ketoacidosis and metabolic acidosis, and severe lung diseases that can affect CO2/O2 gas exchange. Even temporary conditions such as shock, anxiety, pain, prolonged vomiting, and severe diarrhea can sometimes lead to acidosis or alkalosis.

Blood gas analysis give a snapshot of a person's blood pH and O2 and CO2 content. The following components can be determined by blood gas analysis:

  • pH. A measure of the balance of acids and bases in the blood. Increased amounts of carbon dioxide and other acids can cause blood pH to decrease (become acidic). Decreased carbon dioxide or increased amounts of bases, like bicarbonate (HCO3-), can cause blood pH to increase (become alkaline).
  • Partial pressure of O2 (PaO2). Measures the amount of oxygen gas in the blood.
  • Partial pressure of CO2 (PaCO2). Measures the amount of carbon dioxide gas in the blood. As PaCO2 levels rise, blood pH decreases, making the blood more acidic; as PaCO2 decreases, pH rises, making the blood more basic (alkaline).
  • O2 saturation (O2Sat or SaO2). The percentage of hemoglobin that is carrying oxygen. Hemoglobin is the protein in red bloods cells that carries oxygen through blood vessels to tissues throughout the body.
  • O2 content (O2CT or CaO2). The amount of oxygen per 100 mL of blood.
  • Bicarbonate (HCO3-). The main form of CO2 in the body. It can be calculated from the pH and PaCO2. It is a measurement of the metabolic component of the acid-base balance. HCO3- is excreted and reabsorbed by the kidneys in response to pH imbalances and is directly related to the pH level. As the amount of HCO3- rises in the blood, so does the pH (becomes alkaline).
  • Base excess/base deficit. A calculated number that represents a sum total of the metabolic buffering agents (anions) in the blood. These anions include hemoglobin, proteins, phosphates, and HCO3- (bicarbonate, which is the dominant anion). Anions are regulated to compensate for imbalances in blood pH. The health practitioner will look at the HCO3- and base excess/deficit results to evaluate the total buffering capacity of the lungs and kidneys when deciding on a treatment to correct an imbalance.

How is the sample collected for testing?

Arterial blood is almost always used for blood gas analysis but, in some cases, as with babies, whole blood from a heelstick is collected instead. Blood may also be taken from the umbilical cord of a newborn. Since arterial blood carries oxygen to the body and venous blood carries waste products to the lungs and kidneys, the gas and pH levels will not be the same in both types of blood samples.

An arterial blood sample is usually collected from the radial artery in the wrist, located on the inside of the wrist, below the thumb, where the pulse can be felt. A circulation test called an Allen test will be done before the collection to make sure that there is adequate circulation in the person's wrist. The test involves compressing both the radial and the ulnar wrist arteries, then releasing each in turn to watch for "flushing," the pinking of the skin as blood returns to the hand. If one hand does not flush, then the other wrist will be tested. Blood can also be collected from the brachial artery in the elbow or the femoral artery in the groin, although these sample locations require special training to properly access.

In newborns that experience difficulty in breathing right after birth, blood may be collected from both the umbilical artery and vein and tested separately.

After an arterial blood draw, pressure must be firmly applied to the site for at least 5 minutes. Since blood pumps through the artery, the puncture will take awhile to stop bleeding. If someone is taking blood thinners or aspirin, it may take as long as 10-15 minutes to stop bleeding. The person collecting the sample will verify that the bleeding has stopped and will put a wrap around the wrist, which should be left in place for about an hour.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

Typically, no test preparation is needed. However, if someone is on oxygen therapy, the O2 may either be turned off for 20 to 30 minutes before the collection for a "Room Air" test or, if this cannot be tolerated or if the health practitioner wants to check oxygen levels with the O2 on, the amount of oxygen being taking will be recorded. This is usually expressed as fraction of inspired (inhaled) oxygen in percent (FiO2) or as liters of O2 flowing per minute.

The Test

Common Questions

Ask a Laboratory Scientist

Form temporarily unavailable

Due to a dramatic increase in the number of questions submitted to the volunteer laboratory scientists who respond to our users, we have had to limit the number of questions that can be submitted each day. Unfortunately, we have reached that limit today and are unable to accept your inquiry now. We understand that your questions are vital to your health and peace of mind, and recommend instead that you speak with your doctor or another healthcare professional. We apologize for this inconvenience.

This was not an easy step for us to take, as the volunteers on the response team are dedicated to the work they do and are often inspired by the help they can provide. We are actively seeking to expand our capability so that we can again accept and answer all user questions. We will accept and respond to the same limited number of questions tomorrow, but expect to resume the service, 24/7, as soon as possible.

Article Sources

« Return to Related Pages

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Blood Gases. (Updated Sept. 1, 2012.) Medline Plus Medical Encyclopedia. Available online at Accessed October 2013.

Venous Blood. The Free Medical Dictionary. Available online at through Accessed October 2013.

WebMD. Arterial Blood Gases. Available online at through Accessed October 2013.

GLOBALRPh. Arterial Blood Bases. Available online at through Accessed October 2013.

Healthline. Blood Gas Test. Available online at through Accessed October 2013.

Fraser, Anna and Ong, Yee Ean. Interpreting Arterial Blood Gases. Medscape. Available online at through Accessed October 2013.

National Heart, Lung, and Blood Institute. What is Oxygen Therapy? Available online at through Accessed October 2013.

Christine L. Snozek, PhD. Lab Tests Online adjunct board member.

Sources Used in Previous Reviews

Healthcare Product Comparison System (February 2000). Blood Gas/pH Analyzers. ECRI (Emergency Care Research Institute) [On-line serial]. Available online at through

MEDLINEplus (27 May 2001). Medical Encyclopedia: Cord Blood. U.S. National Library of Medicine, Bethesda, MD. MEDLINEplus. Available online at

MEDLINEplus (3 October 2001). Medical Encyclopedia: Arterial Stick. U.S. National Library of Medicine, Bethesda, MD. MEDLINEplus. Available online at

MEDLINEplus (3 October 2001). Medical Encyclopedia: Blood Gases. U.S. National Library of Medicine, Bethesda, MD. MEDLINEplus. Available online at

Product Showcase (2001). Product Showcase: Blood gas. RT Magazine, The Journal for Respiratory Care Practitioners [On-line journal]. Available online at through

Rhoades, J. (29 September1997) Arterial Blood Gases (ABG). About ( [On-line information]. Available online at through

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (1999). Mosby's Diagnostic and Laboratory Test Reference 4th Edition: Mosby, Inc., Saint Louis, MO.

Locke, J. (1996, Reviewed 2001) Blood Collection, Arterial Blood Gases (ABGs). Medical Laboratory Service Procedure Manual (Bozeman, MT). [ABG collection procedure]. Available E-mail:

Blood gases (2001) Blood gases. Cornell University, Cornell Veterinary Medicine [On-line information]. Previously available online at

Salvador F. Sena, PhD, DABCC. Associate Medical Director, Clinical Chemistry, Danbury Hospital, Danbury, CT, American Association for Clinical Chemistry member.

Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 117-125.

Clarke, W. and Dufour, D. R., Editors (© 2006). Contemporary Practice in Clinical Chemistry: AACC Press, Washington, DC. Pp 322, 469.

Dugdale, III, D. (Updated 2008 August 10). Blood gases. MedlinePlus Medical Encyclopedia [On-line information] Available online at Accessed February 2010.

Paxton, A. (2007 August). Blood gas analyzers—the old and the new. CAP Today [On-line information] Available online through Accessed February 2010.

Lehman, C. et. al. (Updated 2009 May). Metabolic Acidosis. ARUP Consult [On-line information] Available online at through Accessed February 2010.

Canham, E. and Beuther, D. (©2007). Interpreting Arterial Blood Gases. American College of Chest Physicians, Pulmonary and Critical Care Update [On-line information]. Available online at,3 through Accessed February 2010.

Priestley, M. and Huh, J. (Updated 2008 February 12). Respiratory Failure, eMedicine [On-line information] Available online at through Accessed February 2010.

McCarthy, K and Dweik, R. (Updated 2008 October 29). Pulmonary Function Testing. eMedicine [On-line information] Available online at through Accessed February 2010.

Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson RA and Pincus MR, eds. Philadelphia: 2007, Pp 83-84, 457-458.

MedlinePlus Medical Encyclopedia. Hemoglobin derivatives. Available online at Accessed February 2011. 

Mack, Elizabeth. Focus on Diagnosis: Co-oximetry . Pediatrics in Review. 2007; 28:73-74. doi:10.1542/pir.28-2-73. Available online at through Accessed February 2011.