What is an Acid-Fast Bacilli (AFB) test?
An Acid-Fast Bacilli (AFB) test looks for bacteria called acid-fast bacilli in a sample of your body fluid or tissue. These bacteria have a special cell wall that allows them to retain certain stains even after being treated with an acid-based solution.
The most important group of AFB is Mycobacterium, which includes Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB).
An AFB test can provide important information to help detect mycobacterial infections, particularly tuberculosis, and may also help identify other mycobacterial infections.
Other names: AFB test, acid-fast smear, AFB smear, acid-fast bacilli stain, mycobacterial smear
What is it used for?
An AFB test may be used to help:
Detect tuberculosis (TB)
AFB testing is commonly performed when tuberculosis is suspected, particularly when the infection may involve the lungs.
Symptoms that may lead to testing include:
Persistent cough
Coughing up blood or sputum
Fever
Night sweats
Unexplained weight loss
Fatigue
Chest pain
Detect other mycobacterial infections
Some species of Mycobacterium other than M. tuberculosis can cause infections, particularly in people with weakened immune systems or certain chronic conditions.
Monitor treatment for tuberculosis
AFB testing may sometimes be used during TB treatment to determine whether mycobacteria are still present in respiratory specimens.
Why do I need an AFB test?
You may need an AFB test if:
Your healthcare provider suspects tuberculosis.
You have symptoms that may suggest pulmonary TB.
You have been exposed to someone with infectious TB.
A chest X-ray or other imaging test shows findings that may be associated with TB.
Your provider suspects another mycobacterial infection.
You are being evaluated or monitored for a known mycobacterial infection.
What happens during an AFB test?
The type of sample depends on the suspected infection.
For suspected pulmonary tuberculosis, the test commonly uses a sputum sample.
You may be asked to produce sputum by coughing deeply from your lungs into a sterile container provided by the healthcare provider or laboratory.
Depending on the suspected site of infection, other specimens may be tested, such as:
Bronchial or respiratory specimens
Gastric aspirate
Urine
Body fluids
Tissue
Wound specimens
The laboratory prepares the specimen and uses a special acid-fast stain, such as Ziehl-Neelsen or an equivalent staining method, to look for acid-fast bacilli under a microscope.
Will I need to do anything to prepare for the test?
Preparation depends on the type of specimen being collected.
For a sputum AFB test, you may be instructed to:
Collect the specimen as directed by your healthcare provider or laboratory.
Rinse your mouth with water before collecting the sample.
Avoid contaminating the specimen with saliva when possible.
Follow any instructions regarding the timing and number of specimens required.
Your healthcare provider may request more than one sputum specimen because collecting multiple specimens can improve the likelihood of detecting mycobacteria.
Are there any risks to the test?
For a routine sputum sample, there are no significant medical risks.
If a more invasive specimen is required, such as a bronchoscopy or tissue biopsy, there may be additional risks associated with that procedure.
What do the results mean?
AFB results are generally reported as negative or positive, sometimes with an estimate of the number of organisms seen.
A positive AFB result
A positive AFB smear means that acid-fast bacilli were seen in the specimen.
This finding suggests a possible mycobacterial infection, but an AFB smear alone cannot always determine which species is present.
A positive result may be caused by:
Mycobacterium tuberculosis
Nontuberculous mycobacteria (NTM)
Other acid-fast organisms
Additional testing, such as mycobacterial culture and molecular tests (NAAT/PCR), may be needed to identify the organism and determine whether M. tuberculosis is present.
A negative AFB result
A negative AFB smear means that no acid-fast bacilli were seen in the specimen examined.
However, a negative result does not completely rule out tuberculosis or another mycobacterial infection. The number of bacteria may be too low to be detected by microscopy.
Culture and molecular testing may therefore still be needed when TB is strongly suspected.
Important note
An AFB smear is a rapid microscopic test, but it does not by itself confirm or exclude tuberculosis.
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