What is an Acid-Fast Bacillus (AFB) smear?
Acid-fast bacteria are a group of infectious bacteria that are responsible for causing tuberculosis and mycobacterial infections like leprosy. AFB are resistant to discoloration due to acid staining as they contain high mycolic acid content which makes them acid-fast, this property is also beneficial in their identification under a microscope. They cause an infection by invading the host and releasing specific substances that damage the host tissues and cells leading to the onset of various diseases.
This test is vital in the diagnosis of active tuberculosis (TB), leprosy, mycobacterial infections, skin and soft tissue infections, and nocardia. Early detection is essential to control the spread and management of disease.
How is the Test Performed?
AFB smear test is performed on body fluids typically sputum, bronchoalveolar lavage, tissue biopsy samples, and pleural fluid.
Preparation
- No dietary restrictions are needed for AFB smear.
- Informing your healthcare provider is crucial for true results.
Collection
- Sputum sample is normally collected in the morning as the bacterial load is highest at that time.
- The patient is instructed to rinse their mouth with water once to remove possible contaminants like food particles or others.
Laboratory Analysis
- The sample is stained using different standard staining methods which makes the acid-fast bacteria visible.
- The sample that was stained is then observed under the microscope.
- The presence of acid-fast bacteria is referred to as a positive result and it indicates the presence of active TB or ongoing mycobacterial infection.
What are symptoms that prompt this test?
AFB smear test is suggested for patients who exhibit symptoms of a recent infection or an active TB. Below some of the symptoms are mentioned that prompt this test.;
Pulmonary TB symptoms
- Cough lasting for more than 3 weeks
- Shortness of breath
- Chills
- Fever
- Chest pain and discomfort
- Night sweats
- Wheezing
- Appetite loss
TB might also affect other parts of the body and cause several other symptoms including constipation, headache, dark or cloudy urine, etc.
Extrapulmonary TB symptoms
- Backache caused by spinal TB
- Pain and swelling in the abdomen
- Joint pain
- Headaches and confusion
High-risk groups include individuals with HIV, immunocompromised individuals, people having close contact with active TB patients, and healthcare workers.
What are the Normal Ranges for This Test?
AFB smear test provides qualitative results rather than quantitative value;
Negative Results:
When no bacteria are detected it suggests a negative result but this does not rule out the fact that TB is not active as bacterial load might be low at that time. Other tests will be recommended like AFB culture or PCR to confirm the presence or absence of acid-fast bacteria.
Positive Results:
Acid-fast bacteria are detected in the sputum suggesting that an active TB or mycobacterial infection is present in an individual. Further testing is needed to know the type of bacteria and assess drug resistance.
** A single negative test does not confirm the absence of TB or other mycobacteria and multiple tests are required for confirmation. **
What Happens if Test Results Show Abnormal Levels?
When the AFB smear test comes back positive the healthcare provider may start further testing and treatment;
Treatment Protocols
- Antibiotics for 6 months
- Preventative therapy
Further Tests
- AFB culture to confirm the presence of Mycobacterium tuberculosis
- Chest X-ray
- Blood Tests
