A to Z of medical tests
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When and why

This test assesses whether the body is short of oxygen and is part of the investigation into symptoms of breathlessness, cough and fever.

A small sample of blood is taken from an artery either at the wrist or in the groin where a pulse can be felt. This procedure can cause some pain and discomfort but the use of a local anaesthetic should reduce this.

How it will help

The result tells the doctor how well the lungs are functioning and gives an indication of the severity of the underlying problem (such as pneumonia). This information will help decide what is the best treatment and may determine whether the patient needs to go to the intensive care unit.

Taking blood gases is not always necessary and for many situations (e.g. in initial assessment and routine monitoring) is now replaced by using a pulse oximeter which involves attaching a probe to a finger and measuring the level of oxygen saturation in the blood. Although this is a simple non-invasive procedure it does not provide all the information which analysis of arterial blood gases can.