People who are experiencing breathing problems or some other symptoms of low oxygen levels in the blood are usually recommended to undergo arterial blood gas tests.
This test helps a doctor diagnose diseases affecting the respiratory system of the body or the underlying cause of such symptoms. Here, you will learn about why an arterial blood gas test is recommended, what are the associated risks, and more.
What is an arterial blood gas test?
An arterial blood gas test is a blood test used to estimate the balance of carbon dioxide and oxygen in a patient’s blood to assess the health of the lungs. This test requires a sample from an artery of the body and involves an analysis of the pH levels of the patient’s blood.
Under normal circumstances, the body regulates the carbon dioxide and oxygen balance effectively on its own, but this balance may be disturbed if there is a problem related to the respiratory system, circulatory system, and metabolic processes. A low level of oxygen in the blood—hypoxemia—can damage organ systems, affect the heart and brain, and lead to severe complications. Other names for this test are systemic arterial blood gas analysis (ABG) or PaO2, PaCO2, pH, and oxygen saturation test.
A variation of the ABG test is called blood gas analysis, in which the blood sample can be taken from either a vein, capillary, or artery. However, an ABG test is only performed with the samples taken from the artery.
What does an arterial blood gas analysis test measure?
By using the arterial blood gas analysis test, doctors can examine the following:
- Oxygen level or the concentration of oxygen in a patient’s blood
- Partial pressure of oxygen (PaO2) or the pressure of oxygen dissolved in the patient’s blood
PaO2 shows how well the oxygen moves from the lungs to the bloodstream. - Oxygen saturation, also known as O2Sat, or how much haemoglobin is transferring oxygen in the blood
- Haemoglobin (the protein that transfers oxygen to the cells) amount in the blood
- Carbon dioxide partial pressure, also known as PaCO2, or the concentration of carbon dioxide in the blood
PaCO2 shows the efficiency of the removal of carbon dioxide from the body. - pH levels of the blood
- Bicarbonate (HCO3) levels or the level of a basic compound produced with carbon dioxide by employing the determined values of PaCO2 and pH
When and why is an ABG test recommended?
The doctor will order an ABG test for one of the following three reasons:
- To treat a serious injury or illness that requires immediate medical care for emergency medicine
- To prepare for surgery (anaesthesiology)
- To check for any diseases of the respiratory system (pulmonology)
Patients who have trouble getting enough oxygen into their system due to a lung disease need an ABG test in order to determine the exact cause of impaired gas exchange and low oxygen levels. An ABG test is also useful in determining if the patient, who is using supplemental oxygen, is getting enough extra oxygen or not.
Additionally, people who have kidney diseases, kidney failure, chronic vomiting, diabetes, or other conditions usually have a problem with the pH balance of their blood, and they are recommended an ABG test.
The doctor will recommend an ABG test if the patient is experiencing any of the symptoms listed here:
- Shortness of breath
- Extreme fatigue or tiredness
- Confusion or disorientation
- Chronic nausea or vomiting
- Smoke-inhalation injury
- Undergoing treatment for near-drowning accident or decompression sickness
- Drug overdose
- Neck injury resulting in trouble while breathing
- Pre-existing conditions such as chronic obstructive pulmonary disease (COPD), asthma, Guillain-Barre syndrome
- Burns in the airways
In addition to an ABG test, the doctor might recommend a test for checking the levels of glucose and electrolytes, other blood tests, chest X-ray, lung volume measurements, and lung diffusion capacity test, etc.
Risks of the test
An ABG test is relatively safe and does not cause problems to the patient. However, there is a little risk associated with drawing blood correctly from the patient’s artery. This risk arises because the size of the arteries varies from person to person, and some people have especially thin arteries, which make it difficult to draw blood. Other rare but probable risks associated with an ABG test include the following:
- Light-headedness or fainting
- Haematoma (blood building up under the skin)
- Excessive bleeding
- Infection
- Multiple punctures and bruises in the process of locating the artery
ABG test preparation and procedure
Patients are recommended to wear comfortable clothes with uncovered arms, preferably, to allow access to the healthcare provider for drawing blood. No other special preparation is required before this test except for informing the healthcare provider about medications and/or any supplemental oxygen the patient is taking.
A healthcare provider could conduct a bloody circulation test—Allen test—before a sample is taken to ensure that both the arteries in the wrist are working properly. They may also turn off supplemental oxygen 20 minutes before taking the sample to ensure accurate results. After locating the radial artery in the wrist, the healthcare provider will clean the area and get a sample from this area. Sometimes, the brachial artery in the arm or femoral artery in the groin provide a sample. While performing the test on a new-born baby, they will draw the blood sample from the umbilical cord or the baby’s heel.
The healthcare provider may also use ultrasound to locate the artery in the inner wrist. They may apply pressure on the small incision for a while or ask the patient to do so to stop bleeding. Usually, the incision is covered with a band-aid. The procedure of an ABG test is slightly more painful than taking a sample from the vein, as the arteries, which are surrounded with nerves, lie deeper in the body. It is normal for the patient to feel lightheaded, nauseated, or dizzy during the sample collection. The patient must inform the healthcare provider or their doctor if they experience any severe symptoms after the test.
Interpreting the results
The test results can be delivered to the patient or the concerned doctor as early as under 15 minutes after taking the sample. However, additional tests are necessary to make an accurate diagnosis. The results of the ABG test can indicate whether the patient is getting enough oxygen, their lungs are exhaling adequate carbon dioxide, their kidneys are working properly, and/or they have a serious infection affecting the perfusion to the organs. Abnormal ABG test results usually indicate that the patient isn’t getting in sufficient oxygen and isn’t eliminating an adequate amount of carbon dioxide or that the patient’s pH levels in their blood are not in balance. The results can slightly differ with the lab as well.
It is important to clear all the doubts about the results and care plan with the doctor. Listed below are the normal results values for an ABG test:
- Partial pressure of oxygen (PaO2): 75 to 100 mmHg
- Partial pressure of carbon dioxide (PaCO2): 35 to 45 mmHg
- Oxygen saturation: 95% to 100%
- Bicarbonate: 22 to 26 mEq/L
- pH: 7.35 to 7.45
It is important to note that normal oxygen levels at the altitudes of 3000 feet or 900 metres are lower.
Cost of the test
On average, the price of an ABG test can range from INR 1000 to INR 2000, depending on the city and some other factors. If the budget is a constraint, the patient is recommended to check with hospitals run by the government or NGOs or talk to their health insurance provider about cost coverage if the test is not required urgently. It is important to note that urgent medical care must not be delayed to avoid unfavourable outcomes.
