Sputum stain for mycobacteria

Definition

Sputum stain for mycobacteria is a test to check for a type of bacteria that cause tuberculosis and other kinds of infection.

Alternative Names

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

How the test is performed

To obtain a sputum sample, you will be asked to cough deeply and spit the substance that comes up 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 you still don't produce enough sputum, you might have a bronchoscopy.

The specimen is spread on a microscope slide. The cells of the specimen are stained with dyes and 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, because sometimes the number of organisms is so low that they don't show up with staining, but they 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 makes the test more accurate if it's 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 the doctor suspects tuberculosis or other Mycobacterium infection.

Normal Values

Results are normal when no mycobacterial organisms are found.

What abnormal results mean

Abnormal results show that the stain is positive for:

• Mycobacterium tuberculosis
• Mycobacterium avium-intracellular
• 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 three days in a row.

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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