Acid Fast Bacillus Test

AFB Tests

Acid-Fast Bacillus Test AFB tests are performed on sputum or faeces to detect the presence of acid fast bacilli. How AFB test is done & what do the results mean

Acid-Fast Bacillus Test is commonly used to identify the presence of Mycobacterium tuberculosis in samples of sputum, urine, body fluids, and tissue. AFB results can be used to diagnose tuberculosis and provide treatment guidance.

What is the Acid-fast bacillus test?

Acid-fast bacillus (AFB) is a bacterium that causes tuberculosis and other infections. Tuberculosis, or TB, is a serious bacterial infection that primarily affects the lungs. It can also have an impact on other parts of the body, such as the brain, spine, and kidneys. Coughing or sneezing spreads tuberculosis from person to person.

Tuberculosis can be dormant or active. If you have dormant 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 experience symptoms and may infect others.

Acid-Fast Bacillus are usually ordered for people who have active TB symptoms. The tests are designed to detect the presence of AFB bacteria in your sputum. Sputum is thick mucus secreted by the lungs and coughed up. It differs from spit and saliva.

This test can also be known as: TB culture and sensitivity test, mycobacterial smear and society, AFB smear and civilisation, TB NAAT.

Types of AFB Tests

There are two types of AFB Tests:

  • AFB Smear

Your sample is “smeared” on a glass slide and examined under a microscope in this test. It may provide results in 1-2 days. These findings may indicate possible or likely infection, but they do not provide a definitive diagnosis.

  • AFB Culture

In the lab, your sample is placed in a special environment to encourage the growth of bacteria. An acid-fast bacilli culture can confirm a TB or other infection diagnosis. However, growing enough bacteria to detect an infection takes 6–8 weeks.

What are they used for?

Acid fast bacillus test

AFB tests are most commonly used to diagnose active tuberculosis (TB). They may also be used to assist in the diagnosis of other types of AFB infections. These are some examples:

  • Leprosy, a once-feared disease that affects the nerves, eyes, and skin, but it is now rare and easily treatable. Skin frequently becomes red and flaky, with loss of sensation.
  • Infection similar to tuberculosis primarily affects people with HIV/AIDS and others with compromised immune systems.
  • AFB tests can also be used to screen people who have already been diagnosed with tuberculosis. The tests can determine whether the treatment is effective and whether the infection can still spread to others.

When acid fast bacillus test is needed?

If you have active TB symptoms, you may require an AFB test. These are some indications:

  • Cough that lasts at least three weeks.
  • Coughing up blood and/or sputum is a common symptom of a blood or sputum infection.
  • Chest ache.
  • Fever.
  • Fatigue
  • Sweating at night.
  • Unknown cause of weight loss.

Other than the lungs, active tuberculosis can cause symptoms in other parts of the body. The symptoms differ depending on the part of the body that is affected. As a result, if you have any of the following symptoms, you may require testing:

  • Backache.
  • Bloody urine.
  • Headache.
  • Weakness in the joints.

In case you have certain risk factors, you may also need an acid-fast bacillus test. You may be at a higher risk of contracting tuberculosis if you:

  • Have had close contact with someone who has been diagnosed with tuberculosis.
  • Have HIV or another disease that causes your immune system to deteriorate.
  • Live or work in an area with a high rate of tuberculosis infection. Homeless shelters, nursing homes, and prisons are examples of such facilities.

How is the sample collected for the test?


A sputum sample will be required by your doctor for both an AFB smear and an AFB culture. The two tests are usually performed simultaneously. To obtain sputum samples:

  1. Cough deeply and spit into a sterile container. This should be done for two or three days in a row. This ensures that your sample contains enough bacteria for testing.
  2. If you are having difficulty coughing up enough sputum, your provider may instruct you to inhale a sterile saline (salt) mist to help you cough more deeply.

If you are still unable to cough up enough sputum, your provider may perform a bronchoscopy. Bronchoscopy allows the doctor to examine and collect samples from the bronchi and bronchioles. Following the application of a local anesthetic to the patient’s upper airway, a thin, lighted tube will be inserted into your airways through your mouth or nose.

A suction or a small brush can be used to collect the sample. A small amount of saline is sometimes introduced through the tubing and into the bronchi and then aspirated to collect a bronchial washing.

Due to the inability of young children to produce a sputum sample, gastric washings/aspirates may be collected. This requires inserting a tube into the stomach, followed by fluid aspiration.

If doctors suspect TB is present outside of the lungs (extrapulmonary TB), they may test the body fluids and tissues that are most likely affected. If the practitioner suspects tuberculosis has infected the kidneys, for example, one or more urine samples may be collected. Your doctor will use a needle to collect fluid from your joints or other body cavities such as the abdomen or pericardium. To obtain a tissue biopsy, the practitioner may occasionally collect a sample of cerebrospinal fluid (CSF) or perform a minor surgical procedure.

Are there any preparations for AFB Tests?

There are no special preparations required for an AFB smear or culture. But you may need to rinse your mouth with water prior to collecting the sputum sample.

Is there any serious risk in the test?

There is no danger in coughing into a container to obtain a sputum sample. If you have a bronchoscopy, your throat may feel sore afterward. There is also a slight risk of infection and bleeding at the site of the sample collection.

What do the findings mean?


If your AFB smear or culture results were negative, you most likely do not have active tuberculosis. However, it is possible that there were insufficient bacteria in the sample for your doctor to make a diagnosis.

If your AFB smear was positive, you most likely have tuberculosis or another infection, but an AFB culture is required to confirm the diagnosis. Since refinement results can take several weeks, the doctor may treat your infection in the meantime.

If your AFB culture came back positive, you have active tuberculosis or another type of AFB infection. The culture can tell you what kind of infection you have. Your provider may order a “susceptibility test” on your sample after you have been diagnosed. A susceptibility test is used to determine which antibiotic will be the most effective.

Is there anything else I should be aware of concerning these tests?

To cure an active infection of tuberculosis, a lengthy course of multiple antibiotics is required. People who suffer from inactive (latent) infections, but are not asymptomatic, may be treated with a single drug to reduce the risk of developing an active infection in the future.

Scientists have developed a new faster way to sophistication Mycobacterium. The organisms can be detected sooner if the sample is cultured in a liquid broth-based medium. To detect the presence of mycobacteria in some broth cultures, an automated instrument is required, whereas others can be read manually.

A liquid culture method known as the Microscopic-Observation Drug-Susceptibility (MODS) assay takes only about 7 days to diagnose TB while also detecting bacterial resistance to antibiotics. Because this method detects the presence of multidrug-resistant tuberculosis (MDR-TB) much faster than conventional culture, it can assist health practitioners in diagnosing and treating the disease at an earlier stage, and it has the potential to help control the spread of infectious TB. The advantages and disadvantages of this non-automated test are still being studied in resource-constrained countries with high TB prevalence.

Other new methods are being used in some laboratories to quickly and accurately identify M. TB once the mycobacteria are growing in culture.

What other types of mycobacteria can AFB testing detect?

Other mycobacteria that can cause infections and are detected by AFB tests include the following:

  • Mycobacterium avium-intracellulare complex (MAC):

can cause a lung infection in people with compromised immune systems, such as those with AIDS; this infection is not contagious, but it can be difficult to treat due to antibiotic resistance.

  • Mycobacterial species

    • such as Mycobacterium marinum, thrive in water and can cause skin infections.
    • Mycobacterium fortuitum, abscessus, chelonae, and other rapidly growing mycobacteria cause skin and wound infections after cosmetic surgery, prosthetic device implantation, and nail salon visits, among other things.
    • Some bacteria, such as bovis can be passed from animals to humans.
  • Nocardia

Although Nocardia species are not mycobacteria, they can be detected using some AFB laboratory tests. Nocardia infections can occur in the lungs, brain, or skin.

If you have any concerns about your Acid fast bacillus results, speak with your doctor.

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