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
Blood Gases
Blood Gases
Also Known As
Blood Gases
Formal Name
Blood Gases
At a Glance
Why Get Tested?
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 healthcare practitioner wants to check your oxygen levels with the O2 on, the amount of oxygen being delivered will be recorded.
Accordion Title
Common Questions
- How is it used?
Blood gas measurements are used to evaluate a person's lung function and acid/base balance.
They are typically ordered if someone is having worsening symptoms of a respiratory problem, such as difficulty breathing or shortness of breath, and a condition such as asthma or chronic obstructive pulmonary disease (COPD) is suspected. Blood gases may also be used to monitor treatment for lung diseases and to evaluate the effectiveness of supplemental oxygen therapy.
Blood gases can also be used to detect an acid-base imbalance, which can occur in kidney failure, heart failure, uncontrolled diabetes, severe infections, and drug overdose. They 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.
- When is it ordered?
A blood gas analysis is ordered when someone has symptoms of an oxygen/carbon dioxide or pH imbalance, such as difficulty breathing, shortness of breath, nausea, or vomiting. It may also be ordered when someone is known to have respiratory, metabolic, or kidney disease and is experiencing respiratory distress.
When someone is "on oxygen" (ventilation), blood gases may be measured at intervals to monitor the effectiveness of treatment. Other treatments for lung diseases may also be monitored with blood gases.
Blood gases may also be ordered when someone has head or neck trauma, which may affect breathing, and when someone is undergoing prolonged anesthesia – particularly for cardiac bypass surgery or brain surgery – to monitor blood gases during, and for a period after, the procedure.
Checking blood gases from the umbilical cord of a newborn may uncover respiratory problems as well as determine acid/base status. Testing is usually only done if a newborn is having difficulty breathing.
- What does the test result mean?
Normal values will vary from lab to lab. They are also dependent on elevation above sea level as a person's blood oxygen level will be lower if he or she lives higher than sea level.
Results from an arterial blood gas analysis are not diagnostic; they should be used in combination with the results of other tests and exams to evaluate someone for a respiratory, metabolic, or kidney problem.
Abnormal results of any of the blood gas components may indicate one or more of the following issues:
- A person is not getting enough oxygen
- A person is not getting rid of enough carbon dioxide
- There is a problem with a person's kidney function
A low partial pressure of oxygen (PaO2) suggests that a person is not getting enough oxygen, while results that are within normal range usually mean that oxygen intake is sufficient.
All other components of the blood gas analysis (pH, PaCO2, HCO3-) 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. These may include the following:
- Respiratory acidosis is characterized by a lower pH and an increased PaCO2 and is due to respiratory depression (not enough oxygen taken in and carbon dioxide removed). 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 PaCO2, 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-, causing the blood to be too acidic for proper metabolic/kidney function. 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.
Examples of test results associated with the above conditions are summarized below:
pH result Bicarbonate result PaCO2 result Condition Common causes Less than 7.35 Low Low Metabolic acidosis Kidney failure, shock, diabetic ketoacidosis, intoxication with methanol, salicyate, ethanol Greater than 7.45 High High Metabolic alkalosis Chronic vomiting, low blood potassium, heart failure, cirrhosis Less than 7.35 High High Respiratory acidosis Narcotics, lung diseases such as asthma, COPD, airway obstruction, pneumonia, myasthenia gravis Greater than 7.45 Low Low Respiratory alkalosis Hyperventilation, pain, anxiety, brain trauma, pneumonia, certain drugs (salicylate, catecholamines) If left untreated, these conditions can create an imbalance that can eventually become life-threatening. A healthcare practitioner can provide the necessary medical intervention to regain normal acid/base 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 a bit more painful than regular venipuncture. You may experience moderate discomfort and a compress may be 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 because it has decreased oxygen content due to the fact that it is composed of blood returning to the heart.
- Can this test be done in a doctor's office?
Blood gas analysis, performed by trained personnel, is usually done in a hospital, emergency room, surgical center, ambulance, or large laboratory setting because it should be performed immediately after sample collection and specialized equipment is required. Most doctors' offices do not have such capabilities.
- I've had pneumonia before and currently have asthma. Why has my doctor never ordered this test for me?
Most cases of pneumonia or asthma can be diagnosed by symptoms and monitored by listening to your chest sounds or by examining the results of spirometry tests or chest x-rays. Most of the time, asthma will respond to prescribed medications and pneumonia will respond to rest and possibly antibiotics. Blood gas analysis may be necessary, however, if you have severe or acute breathing problems or prolonged, chronic ones. In these cases, blood gas analysis is usually done in an emergency room or hospital setting.
- Is there any other way to measure my oxygen levels?
A pulse oximeter is a noninvasive way (no needlestick or blood sample required) of continuously monitoring O2 saturation. A small clip-like device (sensor) is attached to the end of the finger or earlobe. The sensor reads light that is transmitted through the skin. Pulse oximeters are useful for monitoring trends in O2 saturation, but their accuracy can be affected by the presence of abnormal forms of hemoglobin, like carboxyhemoglobin (see below), low blood pressure due to poor perfusion (pumping of blood into an organ or tissue), and very low levels of hemoglobin due to severe anemia.
- Why does my lab report also list carboxyhemoglobin? What is it?
If your blood gases were measured using an instrument known as a co-oximeter, then your lab report may also list results for carboxyhemoglobin and other altered forms of hemoglobin. A co-oximeter is a blood gas analyzer that can measure concentrations of hemoglobin derivatives (like carboxyhemoglobin) in addition to the usual blood gas measurements. A co-oximeter is not always used, so these values are not reported for all blood gas analyses.
Carboxyhemoglobin is an altered form or derivative of hemoglobin that forms when carbon monoxide binds to hemoglobin. Levels of carboxyhemoglobin are often elevated with carbon monoxide poisoning, and a co-oximeter is used to measure carboxyhemoglobin levels and to monitor oxygen therapy. Hemoglobin binds to carbon monoxide about 210 times more strongly than to oxygen. Binding to carbon monoxide significantly decreases hemoglobin's ability to carry oxygen through the body, which can lead to a serious, life-threatening condition.
Other hemoglobin derivatives include sulfhemoglobin (or sulfmethemoglobin) and methemoglobin, which may result from the ingestion of certain medicines or exposure to chemicals. These altered forms of hemoglobin, like carboxyhemoglobin, cannot function properly to carry oxygen to tissues in the body and are commonly measured by a co-oximeter.