Definition

Acid-fast bacillus (AFB) refers to a collection of bacteria that possess the distinctive attribute of acid fastness, which is responsible for the development of tuberculosis and various other illnesses. Acid fastness is a characteristic of bacteria that enables them to withstand the removal of pigment by acids during staining procedures. 

Tuberculosis, also referred to as TB, is a severe bacterial infection mostly impacting the respiratory system. Additionally, it has the potential to impact other organs, such as the brain, spine, and kidneys. TB is transmitted via respiratory droplets expelled during coughing or sneezing.

AFB tests are typically recommended for individuals exhibiting symptoms of active tuberculosis. The tests aim to detect the existence of AFB bacteria inside the sputum, a viscous mucus that is expelled from the lungs during coughing. It is not the same as saliva or spit. The AFB tests can be classified into two primary categories, they are AFB smear and AFB culture.

We also have an article on tuberculosis which you can read here: TB - Definition, Risk Factors and Management.

Indication

AFB tests are primarily employed for the diagnosis of an active tuberculosis (TB) infection caused by acid-fast bacilli (AFB), which are other species of Mycobacterium. 

This testing is also recommended in cases where: 

• An individual has been exposed to someone with TB and has symptoms or medical conditions that increase their susceptibility to TB, such as HIV
• They are suffering from a persistent skin illness that is resistant to antibiotics commonly prescribed for treating bacterial infections
• AFB testing is typically conducted periodically when a patient is receiving TB therapy in order to assess treatment efficacy and determine whether the patient remains infectious.

Contraindication

There are no individuals for whom it is advised against to do this test. Everyone is eligible to undergo AFB smear or culture.

Preparations Prior to Test

The test can be conducted at home, at a hospital, clinic, or laboratory. There is no need to prepare for this test, except for rinsing the mouth with water and brushing their teeth before taking the sputum sample. It should be noted that patients shouldn't use antiseptic mouthwash when brushing their teeth.

Test Procedures

A sample of your sputum will be required for an AFB smear as well as an AFB culture. To obtain sputum samples: 

• In instances where pulmonary TB infections are suspected, it is customary to get three sputum samples during the early morning hours on separate occasions. This ensures that your sputum contains a sufficient amount of microorganisms for testing. Patients will be instructed to engage in a deep cough and expel sputum into a sterile cup.
• If you experience difficulty coughing sufficient sputum, your healthcare practitioner may instruct you to inhale a sterile saline (NaCl) mist, which can help you cough out more sputum.
• Your doctor might recommend a bronchoscopy if you are still unable to cough up enough sputum. During this procedure, you will initially get analgesic medication to alleviate any pain. A small, illuminated tube will then be inserted into your airways via your mouth or nose. The samples can be obtained using suction or by employing a tiny brush.

Healthcare professionals may examine other bodily fluids and tissues that are most likely to be impacted if they have a suspicion that TB is present outside of the lungs. AFB testing may involve the collection of several samples other than sputum, such as urine, cerebrospinal fluid (CSF), other body fluids, or biopsied tissue samples, depending on the presence of symptoms.

Making a smear is the first step in the microscopic examination of sputum for acid-fast bacilli. Before beginning the staining process, the smeared slides should be well heated. Specific staining or dye can be used to visualize acid-fast structures under a microscope.

Normal and Abnormal Values

The WHO-IUATLD (World Health Organization and the International Union Against Tuberculosis and Lung Diseases) have established the following guidelines for reporting acid-fast microscopy results.

A negative AFB smear may indicate that there is no infection, that the symptoms are caused by something other than mycobacteria, or that the mycobacteria were not present in sufficient numbers to be visible under the microscope. Positive AFB smears suggest a possible mycobacterial infection.

Results and Suggestions (Follow-Up Tests)

A negative AFB smear may indicate the absence of an infection. Typically, three samples are obtained to improve the likelihood of the bacteria being discovered. On other days, however, additional samples may be collected and analyzed if AFB smears remain negative despite a strong suspicion of a TB infection. 

Positive AFB smear results suggest that you may have a TB infection. To confirm a diagnosis and determine the specific species of mycobacteria that are present, a culture must be carried out. Susceptibility testing on the identified bacteria provides the physician with information regarding the potential resistance of the mycobacterium causing the symptoms with usual prescribed antibiotics. Meanwhile, positive AFB cultures identify the specific bacteria responsible for the symptoms.

Consult To The Right Doctor

A long course of multiple antibiotics is necessary to eradicate an active TB infection. Untreated TB can result in catastrophe. However, the majority of TB cases are curable with antibiotics used as prescribed by doctors. To understand the significance of your test results, speak with your physician or pulmonologist.

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