Acid Fast Bacillus test (AFB), Types and benefits
Acid Fast Bacillus test (AFB)
Acid Fast Bacillus test (AFB),TB test, or a sputum test. What Is an Acid-Fast Bacillus Test? How does it work? What are the benefits of using it?
Table of Contents
Acid Fast Bacillus test (AFB) is a diagnostic test for tuberculosis (TB) and infections caused by other Mycobacterium species, also known as acid-fast bacilli (AFB), in people at risk of developing mycobacterial infections; and to monitor treatment effectiveness.
What is Acid-fast bacillus test (AFB)?
Acid-fast bacillus bacteria is responsible for tuberculosis and other infections. Tuberculosis (TB) is a fatal bacterial infection of the lungs. It could also have an effect on other organs such as the kidneys, spine, and brain. Tuberculosis spreads easily through coughing or sneezing.
Tuberculosis can be latent or active. If you have latent tuberculosis, you will have TB bacteria in your body but will not feel sick and will not be able to spread the disease to others. If you have active tuberculosis, you will develop symptoms and may infect others.
AFB tests are also known as acid-fast smear and culture test, are usually ordered for people who have active TB symptoms, to detect the presence of AFB bacteria, and to test if the patient is suffering from a bacterial infection.
In this test, a staining dye is added to a bacterial culture, which is then washed in an acids solution. Following this, it is discovered that while some bacterial cells retain their colour, others lose it partially or completely. The test thus distinguished bacteria based on their ability to retain dye, also known as “acid fast stain (AFS)”.
Brief Description of AFB test
| Other names | AFB smear and culture, Sputum Test, mycobacterial culture, TB culture and sensitivity, TB NAAT, Acid-Fast Bacillus Smear and Culture and Sensitivity, Mycobacterium tuberculosis Nucleic Acid Amplification Test. |
| purpose | AFB tests are typically used to detect active tuberculosis (TB) infection. They may also be used to aid in the diagnosis of other types of AFB infections. |
| preparation | No special preparation required, except for rinsing the mouth with water before taking a sputum sample |
| Specimen |
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| Criteria for Rejection |
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| Normal range | Nil |
| Methodology | Semi-automated radiometric detection and/or standard culture media |
Types of Acid Fast Bacillus test (AFB)
AFB tests are classified into two types:
- AFB Smear: The sample is “smeared” on a microscopic slide and examined under a microscope after being stained with a special dye. lab can produce results in as little as 1-2 days. These findings may indicate possible or probable infection, but they will not provide a definitive diagnosis.
- AFB culture: The sample is taken to a laboratory and placed in a special environment to stimulate bacterial growth in this test. AFB culture can be used to confirm a diagnosis of tuberculosis or another infection. However, growing enough microbes to detect an infection takes 6-8 weeks.
Sputum Smears vs. Cultures
| Feature | Smears | Cultures |
| Basis of procedure | Using a microscope to look for acid-fast bacilli on a slide | Growth and identity of tubercle bacilli or other pathogenic organisms on culture media in incubator |
| Significance of a negative result |
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| Significance of a positive result |
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| Reporting Time | 1 day | 4 days to 12 weeks (based on the sample used and how rapidly the microbe grows) |
| Equipment needed |
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When do you need AFB test?
If you have active tuberculosis symptoms, you may need an AFB test. These are some examples:
- Cough that lasts at least three weeks
- spitting up blood or sputum
- Fatigue
- Chestache
- Sweating at night
- Fever
- Unknown cause of weight loss
Active tuberculosis could also cause symptoms in parts of the body other than lungs. The symptoms differ depending on the part of the body that is affected. You may require testing if you have:
- Backache
- Blood in urine.
- Headache
- Weakness in the joints
If you have certain risk factors, you may also require testing. You have a higher chance of developing tuberculosis if you:
- Have a positive IGRA blood test or Tuberculin skin test (TST) and are in a high-risk group for developing active TB
- Have had close contact with someone who has been diagnosed with TB
- Have HIV or another disease that causes your immune system to worsen?
- Live or work in an area where tuberculosis is prevalent.
What happens during AFB testing?
Your doctor will need a phlegm sample for both an AFB smear and an AFB culture. The two tests are typically performed concurrently. To obtain sputum samples:
You will be requested to cough gravely and spit into a sterile container. You’ll need to do this for three days in a row. This ensures that your sample contains an adequate number of bacteria for testing.
If you are having difficulty coughing up enough sputum, your provider may ask you to breathe in a sterile saline (salt) mist to help you cough more deeply.
If you still can’t cough up enough sputum, your doctor may perform a bronchoscopy. You will be given pain medication before proceeding with this procedure. Then, a thin, lighted tube will be inserted through your mouth or nose and into your airways. The sample can be collected using suction or a small brush.
In case of Children gastric washings/aspirates can be collected. Urine, an aspirate from the suspected infection site, cerebrospinal fluid (CSF), other body fluids, or biopsied tissue samples may be collected for AFB testing, depending on the signs.
Is the test risky?
There is no danger in coughing into a container to obtain a sputum sample. If you have a bronchoscopy, your throat may be sore afterward. There is also a slight risk of infection and bleeding at the site of sample collection.
Results Interpretations
If your AFB smear or culture results were negative, you most likely do not have active TB. However, it is possible that there were not enough bacteria in the sample for your doctor to make a diagnosis.
If your Acid-fast bacilli smear was positive, you most probably have tb or some other infection, but an AFB culture is essential to make the final diagnosis. Because culture results can take weeks, your doctor may recommend to treat your infection until then.
If your AFB culture was positive, you have active tuberculosis or some other type of AFB infection. The culture can identify the type of infection you have. Your doctor may order a “sensitivity test” on your sample after you have been diagnosed. A sensitivity test helps determine which antibiotic will be most effective in treating the patient.
As mentioned, culture results typically take 2 to 6 weeks. If your doctor suspects you have pulmonary tuberculosis, you may be treated during this time.
Sputum coughed up from deep inside your lungs is usually subjected to more rapid but less conclusive tests first. These assist your healthcare provider in deciding whether to isolate and treat you prior to the availability of culture results.