At a Glance
Why Get Tested?
To determine if your lungs are functioning well enough to exchange oxygen and carbon dioxide if you are having symptoms of a respiratory problem; to determine if you have an imbalance in the amount of oxygen gas (O2) or carbon dioxide gas (CO2) in your blood or an acid-base imbalance, which may indicate a respiratory, metabolic, or kidney disorder
When to Get Tested?
When you have difficulty breathing, shortness of breath, or rapid breathing; when you have symptoms of an oxygen/carbon dioxide imbalance or an acid-base imbalance; 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 oxygen and carbon dioxide levels
Most often a blood sample collected from an artery, usually the radial artery in the wrist, which is located on the inside of the wrist, below the thumb, where you can feel your pulse; sometimes a blood sample drawn from a vein in the 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 or, if this cannot be tolerated or if the doctor 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 the arteries). The body carefully regulates blood pH, maintaining it within a narrow range of 7.35-7.45, not too acidic (acidosis) or too alkaline/basic (alkalosis).
The body's regulation of acids and bases has a component that involves metabolism and the kidneys. In the body, the process of converting one substance to another for energy (metabolism) produces large amounts of acid and the kidneys help to eliminate it. The body also regulates pH balance by eliminating carbon dioxide (an acid) through the lungs. This respiratory component is also the way 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 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 are a wide range of acute and chronic conditions that can affect kidney function, acid production, or lung function, and that 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 carbon dioxide/oxygen gas exchanges. Even temporary conditions such as shock, anxiety, pain, prolonged vomiting, and severe diarrhea can sometimes lead to acidosis or alkalosis.
Blood gas tests give a snapshot of the blood's pH and oxygen and carbon dioxide content. They directly measure:
- pH - a measure of the balance of acid and bases in the blood. Blood pH decreases, becoming more acidic, with increased amounts of carbon dioxide (PCO2) and other acids. Blood pH increases, becoming more alkaline, with decreased carbon dioxide or increased amounts of bases like bicarbonate (HCO3-).
- Partial pressure of O2 (PO2) - the amount of oxygen gas in blood.
- Partial pressure of CO2 (PCO2) - the amount of carbon dioxide gas in the blood. As PCO2 levels rise, blood pH levels decrease, becoming more acidic; as PCO2 decreases, pH levels rise, making the blood more basic (alkaline).
Calculations or measurements can also be done to give other parameters, such as:
- O2 saturation - 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.
- Bicarbonate (HCO3-) - the main form of CO2 in the body, it can be calculated from the pH and PCO2. 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, so does the pH.
- 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). The anions try to compensate for imbalances in the blood pH. The doctor will look at the HCO3- and base excess/deficit results to evaluate the total buffering capacity 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, such as for babies, whole blood from heelsticks is used. 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.
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 your 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 your 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. These sample locations require special training to properly access, so the collection is often performed by a doctor.
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 you are taking blood thinners or aspirin, it may take as long as ten to fifteen minutes to stop bleeding. The person collecting the sample will verify that the bleeding has stopped and will put a wrap around your wrist, which should be left in place for an hour or so.
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 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 or, if this cannot be tolerated or if the doctor wants to check your oxygen levels with the O2 on, the amount of oxygen being taking will be recorded. This is usually expressed as fraction of inspired oxygen in percent (e.g., 30% FIO2) or as liters of O2 flowing per minute.
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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
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 http://www.nlm.nih.gov/medlineplus/ency/article/003855.htm. Accessed February 2010.
Paxton, A. (2007 August). Blood gas analyzers—the old and the new. CAP Today [On-line information] Available online through http://www.cap.org. Accessed February 2010.
Lehman, C. et. al. (Updated 2009 May). Metabolic Acidosis. ARUP Consult [On-line information] Available online at http://www.arupconsult.com/Topics/RenalDz/MetabolicAcidosis.html# through http://www.arupconsult.com. 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 http://www.chestnet.org/accp/pccu/interpreting-arterial-blood-gases?page=0,3 through http://www.chestnet.org. Accessed February 2010.
Priestley, M. and Huh, J. (Updated 2008 February 12). Respiratory Failure, eMedicine [On-line information] Available online at http://emedicine.medscape.com/article/908172-overview through http://emedicine.medscape.com. Accessed February 2010.
McCarthy, K and Dweik, R. (Updated 2008 October 29). Pulmonary Function Testing. eMedicine [On-line information] Available online at http://emedicine.medscape.com/article/303239-overview through http://emedicine.medscape.com. 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 http://www.nlm.nih.gov/medlineplus/ency/article/003371.htm. 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 http://pedsinreview.aappublications.org/cgi/content/extract/28/2/73 through http://pedsinreview.aappublications.org. Accessed February 2011.
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 http://www.ecri.org/documents/453052.htm through http://www.ecri.org.
MEDLINEplus (27 May 2001). Medical Encyclopedia: Cord Blood. U.S. National Library of Medicine, Bethesda, MD. MEDLINEplus. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003403.htm.
MEDLINEplus (3 October 2001). Medical Encyclopedia: Arterial Stick. U.S. National Library of Medicine, Bethesda, MD. MEDLINEplus. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003422.htm.
MEDLINEplus (3 October 2001). Medical Encyclopedia: Blood Gases. U.S. National Library of Medicine, Bethesda, MD. MEDLINEplus. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003855.htm.
Product Showcase (2001). Product Showcase: Blood gas. RT Magazine, The Journal for Respiratory Care Practitioners [On-line journal]. Available online at http://www.rtmagazine.com/Articles.ASP?articleid=R0006D05 through http://www.rtmagazine.com.
Rhoades, J. (29 September1997) Arterial Blood Gases (ABG). About (http://about.com) [On-line information]. Available online at http://asthma.miningco.com/library/weekly/aa092997.htm through http://asthma.miningco.com.
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: email@example.com
Blood gases (2001) Blood gases. Cornell University, Cornell Veterinary Medicine [On-line information]. Available online at http://web.vet.cornell.edu/public/popmed/clinpath/cpPage/bgintro.htm through http://web.vet.cornell.edu.
Salvador F. Sena, PhD, DABCC. Associate Medical Director, Clinical Chemistry, Danbury Hospital, Danbury, CT, American Association for Clinical Chemistry member.