Sputum stain for mycobacteria

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

Definition
This is a test involving special staining and direct microscopic examination of a sputum specimen, used to check for the presence of Mycobacteria.

How 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. If sputum is not produced, sometimes bronchoscopy will be performed.

The specimen is spread on a microscope slide, which is called a smear. The cells of the specimen are stained with a dye, carbol-fuchsin, heat and acid are applied, and then it is counterstained with methylene blue.

The prepared specimen is then examined under the microscope. If the stain shows mycobacteria, the specimen may be placed in culture media. (Specimens are often cultured even if no mycobacteria are seen, since sometimes the numbers are so low that it is hard to see them.)

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 Values
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.

What the risks are
There are no risks, unless bronchoscopy is performed.

Special 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.

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