Sputum tests

Sputum samples are used to diagnose active TB and to monitor the effectiveness of TB treatment. The sample is obtained by coughing up phlegm and spitting it into a small sterile container. Three to five early morning samples are usually required. If there is a problem obtaining the sputum, sometimes a fine mist of salt water is sprayed into the throat and this helps to bring up a sample.

Part of the sputum is stained and examined under a microscope to look for acid-fast bacilli (AFB), rod-shaped bacteria that can be seen and counted. A positive AFB smear indicates probable mycobacteria infection. A portion of the sample is also cultured. A positive AFB culture will identify the type of mycobacterium present.

A sputum test is usually ordered for anyone who has a persistent cough, is coughing up phlegm with blood, or has been recently exposed to someone with active TB infection.

If mycobacterium is present, the patient has infectious TB and needs to start treatment immediately. If hospitalisation is required, the person would need to be isolated in a room with negative air pressure.