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
Sample Required?
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.
The Test
How is it used?
Blood gas measurements are used to evaluate your oxygenation and acid/base status. They are typically ordered if you have worsening symptoms of an acid/base imbalance, difficulty breathing, or shortness of breath. Blood gases may be ordered along with other tests, such as electrolytes to determine if an electrolyte imbalance is present, glucose to evaluate blood sugar concentrations, and BUN and creatinine tests to evaluate kidney function.
If you are on continuing supplemental oxygen therapy, blood gases may be used to monitor the effectiveness of that treatment.
When is it ordered?
Blood gas tests are ordered when you have symptoms of an oxygen/carbon dioxide or pH imbalance, such as difficulty breathing, shortness of breath, nausea or vomiting.
Blood gas measurements may be ordered when you are known to have a respiratory, metabolic, or kidney disease and are experiencing respiratory distress.
When you are "on oxygen" (ventilation), you may have your blood gases measured at intervals to monitor the effectiveness of treatment.
Blood gases may also be ordered when you have head or neck trauma, injuries that may affect breathing. When you are undergoing prolonged anesthesia – particularly for cardiac bypass surgery or brain surgery – you may have your blood gases monitored during and for a period after the procedure.
Checking the blood gases from the umbilical cord of newborns may uncover respiratory problems as well as determine the baby's acid/base status. Testing is usually only done if a newborn's condition indicates that he or she may be having difficulty breathing.
What does the test result mean?
Abnormal results of any of the blood gas components may mean that:
- you are not getting enough oxygen
- you are not getting rid of enough carbon dioxide
- there is a problem with kidney function
The results of the PO2 component of the tests for blood gases relates to how much oxygen you are able to breathe in and the amount of oxygen in your blood. Low levels may mean you are not getting enough oxygen while results that are within normal range usually mean your oxygen intake is sufficient.
The results of the other components of the tests for blood gases are interrelated and the results must be considered together. Certain combinations of results, if abnormal, may indicate a condition that is causing acidosis or alkalosis:
- Respiratory acidosis is characterized by a lower pH and an increased PCO2 and is due to respiratory depression – not enough oxygen in and carbon dioxide out. This can be caused by many things, including pneumonia, chronic obstructive pulmonary disease (COPD), and over-sedation from narcotics.
- Respiratory alkalosis, characterized by a raised pH and a decreased PCO2, is due to over ventilation caused by hyperventilating, pain, emotional distress, or certain lung diseases that interfere with oxygen exchange.
- Metabolic acidosis is characterized by a lower pH and decreased HCO3-; the blood is too acidic on a metabolic/kidney level. Causes include diabetes, shock, and renal failure.
- Metabolic alkalosis is characterized by an elevated pH and increased HCO3- and is seen in hypokalemia, chronic vomiting (losing acid from the stomach), and sodium bicarbonate overdose.
Combinations of results that may be seen in certain conditions are summarized below:
| pH result | Bicarbonate result | PCO2 result | Condition | Common causes |
|---|---|---|---|---|
| Less than 7.4 | Low | Low | Metabolic acidosis | Kidney failure, shock, diabetic ketoacidosis |
| Greater than 7.4 | High | High | Metabolic alkalosis | Chronic vomiting, low blood potassium |
| Less than 7.4 | High | High | Respiratory acidosis | Lung diseases such as pneumonia, COPD |
| Greater than 7.4 | Low | Low | Respiratory alkalosis | Hyperventilation, pain, anxiety |
If left untreated, these conditions can create an imbalance that can eventually be life-threatening. Your doctor will provide the necessary medical intervention for you to regain your body's normal balance, but the underlying cause of the imbalance must also be addressed.
Is there anything else I should know?
Arterial blood sample collection is usually more painful than regular venipuncture. You will experience moderate discomfort, and a compress is required for some time to prevent any bleeding from the site.
Sometimes mixed venous blood taken from a central line is used in particular situations, such as in cardiac catheterization labs and by transplant services. Careful interpretation of the results is required. Peripheral venous blood, such as that taken from a vein in the arm, is of no use for oxygen status.
Common Questions
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Article Sources
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.
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Sources Used in Previous Reviews
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Salvador F. Sena, PhD, DABCC. Associate Medical Director, Clinical Chemistry, Danbury Hospital, Danbury, CT, American Association for Clinical Chemistry member.
