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Acid-fast bacteria, also known as acid-fast bacilli or simply AFB, are a group of bacteria sharing the characteristic of acid fastness. Acid fastness is a physical property that gives a bacterium the ability to resist decolorization by acids during staining procedures. This means that once the bacterium is stained, it cannot be decolorized using acids routinely used in the process. This important and unique feature of certain bacteria gives the ability to classify and detect them using relatively easy laboratory procedures such as microscopy.[1] Bacteria displaying acid fastness include: Genus Mycobacterium – M. leprae, M. tuberculosis, M. smegmatis, M. Avium complex, M. kansasii. Genus Nocardia – N. brasiliensis, N. cyriacigeorgica, N. farcinica, and N. nova. Acid fastness can also be attributed to other structures not classified as bacteria. These include: Bacterial endospores Head of sperm Cryptosporidium parvum Isospora belli Cyclospora cayetanensis Taenia saginata eggs Hydatid cysts Sarcocystis Nuclear inclusion bodies in lead poisoning Even though acid fastness can be attributed to many different bacteria, correlation with history makes it a fairly unique characteristic of M. tuberculosis in clinical practice.[2] This makes acid-fast staining sensitive and specific, provided clinical correlation is part of the equation. This writing will focus on the acid-fast bacteria M. tuberculosis. The diagnosis of M. tuberculosis using this characteristic is referred to as TB microscopy, acid-fast smear microscopy, and direct sputum near microscopy.[3] Even though the use of highly advanced molecular diagnostic tests has come into play, the value of this staining technique cannot be overstated, especially for low and middle-income countries.[4]

False-positive and false-negative results of acid-fast microscopy may have grave repercussions on the patients individually and also the society as a whole. False-positive results (i.e., a positive result for an actually negative patient) will result in unnecessary treatment with anti-tuberculosis drugs. These drugs can cause significant side effects ranging from mild elevation in liver enzymes to full-blown hepatic failure. Optic neuritis, color blindness, and peripheral neuropathy are just some of the other potential side effects of anti-tuberculosis medicines.[25][26] False-positive results can be a result of: Reusing old microscope slides AFB cross-contamination from one slide to another Presence of food particles Precipitation of stains Transfer of acid-fast bacilli through oil on the immersion lens On the other hand, false-negative results (i.e., the patient has tuberculosis but receives a negative report) will result in improper management of the patient leading to exacerbation of the disease and community spread. False-negative results can result from: Smears that are too thick Smears that are too thin Poor staining technique Incorrect heating of the slide Incomplete or inexperienced slide reading. Acid-fast bacilli microscopy poses a threat to the laboratory personnel as well. Pathologists working in the laboratory are at risk of acquiring tuberculosis infection if proper guidelines are not followed. Handling samples and smear processing must be done with extreme caution so that tuberculosis is not transmitted inadvertently. The Good Clinical Laboratory Practice standards require all labs handling M. tuberculosis to have: Standard operating procedures (SOPs), good practices, and accident management plans Controlled ventilation systems Adequate use of personal protective equipment Appropriate waste management procedures Lab safety procedures (including biohazards, fire, chemical, electrical, and physical safety). Furthermore, access to the lab must be restricted, and all samples must be stored appropriately in a Class II biosafety cabinet.