Blood Gas Tests

Other names: Arterial Blood Gases (ABGs)

The blood gas test measures the level of oxygen and carbon dioxide in the blood and the pH of the blood.

Why is the test used?

The blood gas test is used to measure how efficiently the lungs are exchanging gases and to test the pH of the blood. In most cases, the test is ordered when patients show symptoms of respiratory problems, such as:

  • Shortness of breath
  • Breathing difficulties
  • Rapid breathing

The test may also be carried out during a surgical procedure as a means of monitoring the patient’s condition; it may also be used as a means of assessing the condition of a patient who is having treatment using oxygen therapy.

The blood gas test may also be used when the doctor suspects a pH imbalance; symptoms may include breathing difficulties, nausea and vomiting.

How is the test performed?

The test is usually done by collecting a sample of blood from an artery in the wrist; the radial artery is usually used. Before the sample is collected, a test known as the Allen test will be done to check the patient’s circulation. This test will nearly always be carried out by a doctor because it can be difficult to get the needle into the artery. Once the sample has been collected, it will be bottled and labelled and then sent off for analysis.

After the test, a cotton wool pad will be applied to the site of the test to stem bleeding; this should continue for at least five minutes.

What do the test results mean?

Abnormal blood gas results may indicate the following problems:

  • The patient is not taking in enough oxygen
  • The patient is not getting rid of enough carbon dioxide
  • The kidneys are not functioning properly

The blood gas test is usually carried out alongside tests that measure the levels of the electrolyte, bicarbonate.

The results of the test may signify the following health conditions:

  • Metabolic acidosis: pH lower than 7.4, low bicarbonate, low PCO2: this may be caused by kidney failure, shock or diabetic ketoacidosis
  • Metabolic alkalosis: pH greater than 7.4, high bicarbonate, high PCO2: this may be caused by low levels of potassium in the blood or persistent vomiting
  • Respiratory acidosis: pH lower than 7.4, high bicarbonate, high PCO2: this may be caused by lung diseases
  • Respiratory alkalosis: pH higher than 7.4, low bicarbonate, high PCO2

Specific Blood Tests

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