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Acid-Fast Bacillus (AFB) Tests
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Acid-Fast Bacillus (AFB) Tests

Acid-fast bacillus (AFB) tests are used to detect mycobacterial infections, most commonly tuberculosis (TB), by identifying bacteria in sputum or other samples.

Overview

Acid-fast bacillus (AFB) is a type of bacteria that causes tuberculosis and other mycobacterial infections, such as leprosy (Hansen's disease). TB is a serious bacterial infection caused by Mycobacterium tuberculosis. It primarily affects the lungs but can impact other areas, including the brain, spine, and kidneys. TB can be latent (bacteria present but no symptoms and not contagious) or active (symptoms present and potentially contagious).

Why AFB Tests Are Used

AFB tests are primarily ordered for individuals who have had a positive TB screening test, abnormal chest x-rays, or symptoms of active TB. They are also used to monitor patients already diagnosed with TB to determine if treatment is working and if the patient remains contagious.

Types of AFB Tests

  • AFB smear: Sputum is smeared on a glass slide and examined under a microscope. Results are available in 1-2 days but cannot provide a definitive diagnosis.
  • AFB culture: The sample is grown in a lab to confirm a diagnosis. This process takes 6-8 weeks.
  • Molecular test (NAAT): Uses nucleic acid amplification to detect TB bacteria and check for resistance to rifampicin. Results are typically available in less than two hours.
  • Susceptibility test: Often ordered with a culture to identify the most effective antibiotic for the specific infection.

Symptoms of Active TB

Symptoms depend on where the bacteria are growing. Pulmonary (lung) TB symptoms include:

  • Cough lasting three weeks or more
  • Coughing up blood or sputum
  • Chest pain

Other symptoms may include fever, chills, fatigue, night sweats, unexplained weight loss, back pain, blood in urine, joint pain, or abdominal pain.

Testing Procedure

Testing requires a sputum sample, which is thick mucus coughed up from the lungs. If a patient cannot produce enough sputum, a provider may use a sterile saline mist or perform a bronchoscopy. For leprosy, a skin or nerve biopsy may be performed.

Understanding Results

  • Negative: Likely no active TB, though it could mean the sample lacked enough bacteria for detection.
  • Positive (Smear/NAAT): Suggests TB or another mycobacterial infection; culture is required for confirmation.
  • Positive (Culture): Confirms active TB or another AFB infection.

If untreated, TB can be deadly. Treatment typically involves antibiotics taken for six to twelve months to ensure the infection is cured and to prevent recurrence.

Frequently Asked Questions

No, a negative AFB test does not completely rule out tuberculosis. The sensitivity of the smear test is lower than that of the culture, meaning it is possible to have TB with a negative smear, especially if the bacterial load is low. If you have persistent symptoms like a chronic cough, night sweats, or weight loss, your provider may repeat the tests, order additional imaging, or consider a bronchoscopy to obtain a deeper sample. Always follow up with your doctor if symptoms persist despite negative initial tests.
positive AFB smear indicates that acid-fast bacteria are visible under a microscope, suggesting an active infection, but it cannot distinguish between Mycobacterium tuberculosis and other non-tuberculous mycobacteria. A positive AFB culture confirms the specific type of bacteria present and is considered the gold standard for diagnosis. Doctors order both because the smear provides rapid preliminary results to start treatment if needed, while the culture ensures accurate identification and allows for susceptibility testing to determine which antibiotics will work best.
Molecular tests (NAAT) usually provide results in less than two hours, AFB smears take one to two days, and AFB cultures can take six to eight weeks. While waiting for culture results, if you have a positive smear or strong clinical suspicion, your healthcare provider may start treatment to prevent transmission and reduce symptoms. You should continue to follow isolation precautions, such as wearing a mask and staying in a well-ventilated room, until your provider confirms you are no longer contagious.
You generally do not need to fast before providing a sputum sample. However, it is recommended to rinse your mouth with water before coughing up the sample to reduce the chance of saliva contaminating the sputum. Avoid eating or drinking immediately before the test if it makes you feel nauseous, as this could interfere with your ability to produce a deep lung sample. Your healthcare provider will give you specific instructions on how to collect the sample, which often involves deep coughing from the lungs rather than just spitting saliva.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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