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Sputum stain for mycobacteria

  • Definition
    • Sputum stain for mycobacteria is a test to check for a type of bacteria that cause tuberculosis and other infections.

  • Alternative Names
    • Acid fast bacilli stain; AFB stain; Tuberculosis smear; TB smear

  • How the Test is Performed
    • This test requires a sample of sputum.

      • To do this, 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 breathe in a mist of salty steam. This makes you cough more deeply and produce sputum.
      • If you still do not 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 answer.

  • 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 is 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 Results
    • 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
  • Risks
    • There are no risks, unless bronchoscopy is performed.

  • Considerations
    • To increase the accuracy of this test, it is sometimes done three times, often three days in a row.

      More sophisticated tests are sometimes used to stain sputum for mycobacteria. Check with your health care provider to see if these are available in the laboratory.

  • References
    • Murray PR. The clinician and the microbiology laboratory. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglass, and Bennett's Principles and Practice of Infectious Diseases. 6th ed. Philadelphia, PA: Elsevier Saunders; 2014:chap 16.

      Septimus EJ. Pleural effusion and empyema. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglass, and Bennett's Principles and Practice of Infectious Diseases. 6th ed. Philadelphia, PA: Elsevier Saunders; 2014:chap 70.