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The AFB smear and culture test, also known as TB culture and sensitivity, is done to diagnose tuberculosis, identify mycobacterial infection, and to monitor how well treatment is going. The formal name of this test is acid-fast bacillus smear and culture and sensitivity.
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How this Test is Used
This test is used to determine if a patient has an active Mycobacterium tuberculosis infection, to tuberculosis-like symptoms associated with another medical condition. It is used to determine if tuberculosis is extrapulmonary (spread outside the lungs) or pulmonary (confined to lungs). This test will help health care providers determine which antimicrobial agents will be most effective in treating the infection.
Tuberculosis is an airborne infection, meaning that respiratory secretion droplets can go into the air when an infected person coughs, making this infection a public health risk. It is most common in confined places where several people are close together, such as in nursing homes, correctional facilities, and schools. This lab test can help to prevent TB infections, as well as minimize and track the spread of this infection in specific populations and assist in examining the treatment given is effective.
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When is it Ordered?
Several factors warrant an AFB smear and culture be ordered. These include:
- Patient has the symptoms of pulmonary tuberculosis, such as a lingering cough producing sputum or phlegm that could have blood streaks.
- The patient has been in close contact with someone who has tuberculosis, or another disease that puts someone at a high risk of infection, such as HIV/AIDS.
- The patient has a positive TB skin test along with an x-ray showing characteristic lung involvement.
- The patient is already being treated for TB and this test can help to check in and see how well treatment is working.
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Preparation and Procedure
The patient's sputum is most often tested because M. avium and M. tuberculosis most commonly infect the lungs. In most cases, three to five samples are collected early in the morning, in individual sterile cups on consecutive days. A bronchoscopy will be performed if the patient cannot produce sputum.
If extrapulmonary tuberculosis is suspected, other body tissues and fluids are typically affected, resulting in them being tested. For example, if it is suspected that the kidneys are affected, a urine sample will be collected, or if it is suspected that the joints are affected, joint fluid will be tested.
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What do the Results Mean?
If this test is positive, it most likely means the patient has a mycobacterial infection. If this test is still positive after the patient has been started on drug treatment, it can mean that the treatment is not effective and has to be changed. It also indicates the patient is most likely still contagious.
If the test is negative, the patient does not have any of the mycobacteria in their specimen, or an AFB infection. TB can still be present, but in another area of the body, requiring a different kind of sample be obtained for testing. If the test is negative several weeks after the patient began treatment, it means that the treatment is working and they are not contagious anymore.
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Lab Tests Online. (2007). AFB Smear and Culture. Retrieved on September 9, 2010 from Lab Tests Online: http://www.labtestsonline.org/understanding/analytes/afb_culture/test.html
Mount Nittany Medical Center. (2010). AFB Smear and Culture. Retrieved on September 9, 2010 from Mount Nittany Medical Center: http://www.mountnittany.org/wellness-library/healthsheets/documents?ID=171