Health Encyclopedia
Sputum stain for mycobacteria
Definition
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.
Alternative Names
Acid fast bacilli stain; AFB stain; Tuberculosis smear; TB smearHow the Test is Performed
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.)
How to Prepare for the Test
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.
How the Test Will Feel
There is no discomfort, unless a bronchoscopy needs to be performed.
Why the Test is Performed
The test is performed when tuberculosis or other Mycobacterium infection is suspected.
Normal Results
The presence of no mycobacterial organisms is normal.
What Abnormal Results Mean
Abnormal results show that the stain is positive for Mycobacterium tuberculosis, Mycobacterium avium-intracellular, or other Mycobacteria or acid-fast bacteria.
Risks
There are no risks, unless bronchoscopy is performed.
Considerations
To increase the accuracy of this test, it is sometimes done three times, often on three consecutive days.
There are more sophisticated tests that are sometimes used to stain sputum for mycobacteria. Check with your health care provider to see if these are available in the laboratory.
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.
