What is being tested?

Blood gas tests tell your doctor if you have enough oxygen in your blood and whether your blood is too acidic (acidosis) or too alkaline (alkalosis). Blood gas tests directly measure:

• pH – a measure of the level of hydrogen ion (H+), which indicates the acid-base status of your blood. The pH of your blood decreases (becomes more acidic) with increased amounts of CO2 and other acids, and the pH increases (blood becomes more alkaline) with decreased CO2 or increased amounts of bases like bicarbonate (HCO3-).
• PO2 – the partial pressure of O2 (the amount of oxygen gas dissolved in blood).
• PCO2 – the partial pressure of CO2 (the amount of carbon dioxide gas dissolved in the blood). As PCO2 levels rise, blood pH levels will decrease, becoming more acidic. As PCO2 decreases, pH levels will rise, making the blood more alkaline.

Calculations or measurements also can be done to give other values, such as:

• O2 saturation – a calculation of how much oxygen is bound to haemoglobin in the red blood cells and available to be carried through the arteries to help the body’s cells burn fuel.
• HCO3- (bicarbonate) – the main form of CO2 in the body, 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 – negatively charged ions) in the blood; these anions include haemoglobin, 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 it used?

Blood gas measurements are used to evaluate the severity of an O2/CO2 or pH imbalance.

Your body will try to restore any imbalance by itself, but if you have an overwhelming sudden problem or a long-term drain on your body, you may need medical intervention (such as being given concentrated, pure oxygen or, in extreme cases, being put on a respirator to gain additional oxygen). If you are on oxygen therapy, your doctor may request blood gas tests to monitor the effectiveness of that therapy.

When is it requested?

Blood gas tests are ordered when you have symptoms of an O2/CO2 or pH imbalance, such as difficulty breathing or shortness of breath. Many conditions can cause an imbalance and while the blood gas tests do not tell your doctor the direct cause of your imbalance, they will point to either a respiratory or metabolic problem.

Blood gas measurements may be ordered if you are known to have a respiratory, metabolic, or kidney disease, especially if you experience respiratory distress, to evaluate the amount of oxygen in your blood and acid/base balance. They also may be requested for patients with head or neck trauma, as these injuries may affect breathing. Patients undergoing prolonged operations may have their blood gases monitored during the procedure, as may patients in intensive care who need a respirator to breathe for them.

Checking the blood gases from the umbilical cord of newborns may uncover respiratory problems as well as determine the acidity of the baby’s blood. Testing is usually only done if a newborn’s condition indicates that s/he may be having difficulty breathing.

What does the result mean?

Abnormal results of any of the blood gas components may mean that your body is not getting enough oxygen, is not getting rid of enough carbon dioxide, or that there is a problem acid and base balance in the blood. If left untreated, these conditions create an imbalance that could eventually be life threatening. The severity of the problem depends on whether the cause is acute (sudden onset and short term), or chronic (long-term), and whether or not your body is exhausted from trying to compensate for the imbalance. Your doctor will provide the necessary medical intervention to regain your body’s normal balance, but s/he must also address the original cause of the imbalance.

Blood gas imbalance and acid-base imbalance tend to fit into one of the following four disease states.

• Respiratory acidosis is characterised by a low pH and an increased PCO2 and is due to respiratory disease (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 drugs.
• Respiratory alkalosis is characterised by a raised pH and a decreased PCO2 and is due to over-ventilation (too much oxygen in and carbon dioxide out). This may occur when a person is in pain or has emotional distress and in some lung diseases.
• Metabolic acidosis is characterised by a low pH and decreased HCO3- and occurs when the blood is too acidic due to a metabolic/kidney disorder. Causes include shock, certain toxic substances and kidney failure.
• Metabolic alkalosis shows an elevated pH and increased HCO3-, seen in hypokalaemia (low blood potassium), with chronic vomiting (losing acid from the stomach) and sodium bicarbonate overdose.

Is there anything else I should know?

Arterial blood sample collection is usually more painful than normal collection of blood from a vein. You will experience moderate discomfort and the site will need to be pressed for some time to prevent further bleeding.

Common questions

• Can this test be done in a doctor's office?

Blood gas measurements are usually done in a hospital since the analysis should be done immediately after the blood has been collected and requires specialised equipment. Most GPs do not have such equipment in their surgeries.

• I've had pneumonia before and currently have asthma. Why has my doctor never requested this test on 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 your usual medications and pneumonia to antibiotics. Blood gas tests may be necessary if you have severe, sudden or long-term breathing problems. In these cases, blood gas tests are usually done in a hospital setting.
Is there any other way to measure my oxygen levels?
A pulse oximeter is a device used to continuously monitoring oxygen saturation without the use of a needle to obtain a blood sample. A small clip-like device called a 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 oxygen saturation, but their accuracy can be affected by the presence of abnormal forms of haemoglobin, like carboxyhaemoglobin (present in carbon monoxide poisoning), low pulse rate due to poor circulation, and very low levels of haemoglobin due to severe anaemia. Some coloured nail polishes may interfere with devices that clip on the finger. While pulse oximeters can be useful, they do not give any information about the patient’s acid-base status.

• Can I have a venous blood gas test instead of an arterial blood gas test?

Depending on your medical problem, it may be possible to have a venous blood gas test, but if your doctor wants to check your oxygen levels, it is necessary to have an arterial blood gas test.

• Why does my laboratory report also list carboxyhaemoglobin? What is it?

If your blood gases were measured using an instrument known as a co-oximeter, then your laboratory report may also list results for carboxyhaemoglobin and other altered forms of haemoglobin.

Carboxyhaemoglobin is an altered form or derivative of haemoglobin that forms when carbon monoxide binds to haemoglobin. Levels of carboxyhaemoglobin are often elevated with carbon monoxide poisoning, and a co-oximeter is used to measure carboxyhaemoglobin levels and to monitor oxygen therapy. Haemoglobin binds to carbon monoxide about 210 times more strongly than to oxygen. Binding to carbon dioxide significantly decreases haemoglobin's ability to carry oxygen through the body, which can lead to a serious, life-threatening condition.

Other altered forms of haemoglobin include sulfhaemoglobin (or sulfmethaemoglobin) and methaemoglobin, which may result from the ingestion of certain medicines or exposure to chemicals. These altered forms of haemoglobin, like carboxyhaemoglobin, cannot function properly to carry oxygen to tissues in the body and are commonly measured by a co-oximeter.

More information

RCPA Manual: Blood gas- arterial