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Sputum stain for mycobacteria

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Illustrations

Sputum test
Sputum test

Alternative names    Return to top

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

Definition    Return to top

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    Return to top

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    Return to top

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    Return to top

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

Why the test is performed    Return to top

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

Normal Values    Return to top

The presence of no mycobacterial organisms is normal.

What abnormal results mean    Return to top

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    Return to top

There are no risks, unless bronchoscopy is performed.

Special considerations    Return to top

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.

Update Date: 1/19/2004

Updated by: Daniel Levy, M.D., Ph.D., Infectious Diseases, Greater Baltimore Medical Center, Baltimore, MD. Review provided by VeriMed Healthcare Network.

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