Sputum stain for mycobacteria

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Sputum test

This is a test to check for the presence of Mycobacteria. It involves taking a sputum (what you cough up) sample, staining it so the components become visible, and looking at it under a microscope.

Acid fast bacilli stain; AFB stain; Tuberculosis smear; TB smear

Obtain a coughed sputum sample. You are asked to cough deeply and expel any material that comes from the lungs (sputum) into a container. You may be asked to inhale a mist of salty steam in order to cough more deeply and produce sputum. If enough sputum is still not produced, sometimes bronchoscopy will be performed.  

The specimen is spread on a microscope slide. The cells of the specimen are stained with dyes to make the components visible.

The prepared specimen is then examined under the microscope. If the stain shows mycobacteria, the specimen may be placed in culture media, which encourages them to grow. (Specimens are often cultured even if no mycobacteria are seen, since sometimes the number of organisms is so low that they don't show up with staining, but eventually grow on the culture medium.)

It can help to drink a lot of fluids the night before the test. It enhances the accuracy of the test if done first thing in the morning.

There is no discomfort, unless a bronchoscopy needs to be performed.

The test is performed when tuberculosis or other Mycobacterium infection is suspected.

Review Date: 11/16/2005
Reviewed By: Monica Gandhi, M.D., M.P.H., Assistant Professor, Division of Infectious Diseases, UCSF, San Francisco, CA. Review provided by VeriMed Healthcare Network.