AFB (Acid-Fast Bacilli) Stain

What it is:
The AFB stain is a special microscopic staining test used to detect acid-fast bacteria, primarily Mycobacterium tuberculosis, the organism that causes tuberculosis (TB). The test can be performed on sputum, body fluids, or tissue samples.

Uses

• Detect Mycobacterium tuberculosis and other acid-fast organisms

• Support diagnosis of pulmonary or extrapulmonary tuberculosis

• Monitor response to anti-tuberculosis therapy

• Identify other acid-fast bacteria (e.g., Mycobacterium leprae causing leprosy)

Symptoms That May Lead to the Test

• Persistent cough lasting more than 2–3 weeks

• Coughing up blood (hemoptysis)

• Unexplained weight loss

• Night sweats and fever

• Chest pain or difficulty breathing

• Swelling or fluid accumulation (in extrapulmonary TB)

Abnormal Results

• Positive AFB stain:

  • Indicates the presence of acid-fast organisms, most commonly TB bacteria

  • Confirms active or latent infection depending on clinical context

• Negative AFB stain:

  • Does not completely rule out TB (organisms may be few in number)

  • May require repeat tests or more sensitive methods (e.g., culture, PCR)

Risks

• No risks from the stain itself

• Risks depend on the sample collection method:

  • Sputum: Non-invasive, only coughing required

  • Bronchoscopy or tissue biopsy: Mild discomfort, rare complications (bleeding, infection)