Acid-Fast Bacillus (AFB) Tests

Why this test may be ordered

• Symptoms of active TB: cough lasting >3 weeks, coughing up blood or sputum, chest pain, chills, fever, night sweats, weight loss, fatigue.
• Higher risk: close contact with TB patient, HIV or weakened immune system, living/working in high-TB settings, substance use, silicosis, diabetes, severe kidney disease, organ transplant, head/neck cancer.

Preparation and collection

There are no special preparations needed for AFB testing.

For sputum: breathe deeply, cough, and spit into a sterile container. If difficulty, inhale saline mist or undergo bronchoscopy. For leprosy suspicion, a skin or nerve biopsy may be taken.

No risk for sputum sample by coughing. Bronchoscopy may cause sore throat. Skin/nerve biopsy has small risk of infection and bleeding at the site.

Understanding results

Results from AFB tests should be interpreted by a healthcare provider in the context of your symptoms, risk factors, and other tests. No single test result is diagnostic by itself. The performing laboratory and your clinician will determine what your results mean. Below are common patterns, but always discuss with your provider.

• Negative (AFB smear): No active TB infection is likely, but there may not have been enough bacteria in the sample to detect. Limitation: A negative smear does not fully rule out TB, especially in early or low-burden infections. Next step: Your provider may repeat the test or proceed to culture if symptoms persist.
• Positive (AFB smear): You may have a mycobacterial infection, but the smear cannot distinguish between TB and other types of AFB bacteria. Limitation: Smear alone cannot confirm the specific type of mycobacterium; a culture is needed. Next step: An AFB culture is required for confirmation. Your provider may start treatment while waiting for culture results.
• Positive (AFB culture): Active TB or another specific AFB infection is identified. The culture determines the exact bacteria. Limitation: Culture results take 6-8 weeks to become available. Next step: A susceptibility test may be ordered to determine the most effective antibiotic. Treatment typically lasts 6-12 months.
• Positive (Molecular / NAAT test): You may have TB or another kind of AFB infection; the test can also detect rifampicin resistance. Limitation: NAAT may not identify all types of mycobacteria and may have false positives in low-prevalence settings. Next step: Your provider will consider both smear and NAAT results to decide on treatment while awaiting culture results.

• Negative results do not completely rule out infection, especially if the sample had few bacteria.
• AFB smear cannot confirm the specific type of mycobacterium; culture is required.
• AFB culture takes 6-8 weeks, delaying final diagnosis.
• Molecular tests may not detect all mycobacterial species.

What to do next

If you are diagnosed with TB, you will need to take antibiotics as prescribed, usually for 6-12 months. It is crucial to complete the full course even if you feel better to prevent relapse. A susceptibility test may guide the choice of antibiotics. Talk to your provider about any follow-up testing needed to monitor treatment.

• What are the symptoms that would require an AFB test?
• How is the sputum sample collected if I have trouble coughing?
• What does a positive AFB smear mean while waiting for culture results?
• How long does it take to get results from each AFB test?
• Is it possible to have a false negative or false positive result?

Evidence source

U.S. National Library of Medicine: Acid-Fast Bacillus (AFB) Tests