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Definition:
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Sputum stain for mycobacteria is a test to check for a type of bacteria that cause tuberculosis and other infections.
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Alternative Names:
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Acid fast bacilli stain; AFB stain; Tuberculosis smear; TB smear
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How the test is performed:
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This test requires a sample of sputum.
- To do this, you will be be asked to cough deeply and spit the substance that comes up from the lungs (sputum) into a container.
- You may be asked to breath in a mist of salty steam. This makes you cough more deeply and produce sputum.
- If you still don't produce enough sputum, you might have a bronchoscopy.
The test sample is examined under a microscope. Another test, called a culture, is done to confirm the results. A culture test takes a few days to get results. This sputum test can give your doctor a quick, early answer.
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How to prepare for the test:
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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.
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How the test will feel:
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There is no discomfort, unless a bronchoscopy needs to be performed.
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Why the test is performed:
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The test is performed when the doctor suspects tuberculosis or other Mycobacterium infection.
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Normal Values:
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Results are normal when no mycobacterial organisms are found.
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What abnormal results mean:
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Abnormal results show that the stain is positive for:
- Mycobacterium tuberculosis
- Mycobacterium avium-intracellular
- Other mycobacteria or acid-fast bacteria
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What the risks are:
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There are no risks, unless bronchoscopy is performed.
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Special considerations:
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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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References:
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Septimus EJ. Pleural effusion and empyema.In: Mandell GL, Bennett JE, Dolin R, eds.Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 65.
Murray PR, Witebsky FG. The clinician and the microbiology laboratory. In: Mandell GL, Bennett JE, Dolin R, eds.Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa: Elsevier Churchill Livingstone; 2009:chap 17.
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