How is it used?
AFB smears and cultures are used to determine whether you have an active
Mycobacterium tuberculosis infection, an infection due to another member of the Mycobacterium family, or
TB-like symptoms due to another cause. They are used to help determine whether the TB is confined to the lungs (pulmonary) or has spread to organs outside the lungs (extrapulmonary). They are ordered to identify
M. tuberculosis and determine the most effective antimicrobial agents to treat the infection.
M. tuberculosis may be resistant to one or more drugs commonly used to treat TB. If the bacteria are resistant to more than one or the primary drugs used for therapy, the organisms are called multi-drug resistant TB (MDR TB), and if the organisms are resistant to multiple first and second lines of therapy, they are called extensively drug-resistant tuberculosis (XDR TB). AFB cultures can be used to monitor the effectiveness of treatment and can help determine when a patient is no longer infectious.
Since TB is transmitted by airborne droplets from respiratory secretions, it is a public health risk. It can spread in confined populations, such as correctional facilities, nursing homes, and schools. Those who are very young, elderly, or have diseases and conditions such as AIDS that compromise their immune systems tend to be especially vulnerable. AFB smears and cultures can help track and minimize the spread of TB in these populations and help determine the effectiveness of treatment.
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When is it ordered?
AFB testing is ordered when:
- you have symptoms that suggest pulmonary TB, such as a lingering cough that produces phlegm or sputum that may have streaks of blood;
- you have a positive TB skin test and have characteristic lung involvement (as shown by X-ray);
- someone you are in close contact with, for example, a family member or co-worker, has been diagnosed with TB and you either have symptoms or you have a condition or disease that puts you at a much higher risk of contracting the disease, such as HIV/AIDS; (Those with AIDS are more likely than other affected patients to have extrapulmonary TB with few and vague symptoms.)
- you are being treated for TB; AFB smears and cultures are usually ordered at intervals, both when your doctor is evaluating the effectiveness of treatment and when he is attempting to determine whether or not you are still infectious.
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What does the test result mean?
Positive AFB smears indicate a probable mycobacterial infection. Positive AFB cultures identify the particular mycobacterium causing your symptoms and give your doctor information about how resistant it may be to treatment.
A positive AFB smear or culture several weeks after drug treatment has started may mean that your treatment regimen is not effective and needs to be changed. It also means that you are still likely to be infectious and can pass the mycobacteria to others through coughing or sneezing.
A negative culture means that you do not have an AFB infection or that the mycobacteria were not present in that particular specimen (which is why multiple samples are often collected). If you have
TB, the infection may be in another part of your body and a different type of body sample may need to be collected. A negative culture several weeks after treatment indicates that your TB is responding to drug treatment and that you are no longer infectious.
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Is there anything else I should know?
TB requires a lengthy course of multiple antibiotics to eradicate an active infection. Persons with inactive (latent) infections, although asymptomatic, may be treated with a single drug to reduce the risk of having an active infection in the future.
Several other testing methods, based on genetic components of mycobacteria, have been developed to help decrease the amount of time necessary to diagnose tuberculosis. These include genetic probes and molecular TB testing. They amplify/replicate pieces of the microorganismsÂ’ genetic code to detect mycobacteria in body samples in less than 24 hours and can narrow the identification to a complex of mycobacteria (a combination, of which M. tuberculosis is the most common). They are fairly sensitive and specific when they are paired with positive AFB smears; but when they are done on samples that are AFB negative by smear, they tend to be less accurate. These methods are approved for respiratory samples and must be confirmed with an AFB culture, but they do provide the doctor with a quick answer, allowing him to isolate potentially infectious patients and minimize the spread of the disease.
A faster lab method to culture Mycobacterium tuberculosis is in development. The new liquid culture method called Microscopic-Observation Drug-Susceptibility (MODS) assay takes only about 7 days to diagnose TB and finds the best antibiotic treatment at the same time. Since this method can recognize the presence of MDR TB much more quickly than conventional culture, it can help health care providers diagnose and treat the disease at an earlier stage and has the potential to help control the spread of infectious TB. The benefits and limitations of this test are still being evaluated. (See: In the News: Faster lab test for tuberculosis in development)
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