Acid-Fast Bacillus (AFB) Testing

AFB smears and cultures are used to check whether you have an active Mycobacterium tuberculosis (TB) infection, an infection due to another member of the Mycobacterium family, or TB-like symptoms due to another cause. They are used to help sortout whether the TB is only in the lungs or has spread to organs outside the lungs. They are used to identify M. tuberculosis, and work out the most effective antimicrobial medicine 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 common drugs used for therapy, the organisms are called 'multi-drug resistant' TB (MDR TB),  and if the bacteria 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 find out when a patient is no longer infectious.

Since TB is spread by droplets in the air from coughing and sneezing, 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  can damage their immune systems tend to be especially likely to become infected. AFB smears and cultures can help track and reduce the spread of TB in these groups of susceptible people and can help work out the effectiveness of treatment.

AFB testing is requested when:

• You have symptoms that suggest pulmonary TB, such as a lingering cough that produces phlegm or sputum that may contain 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 TB outside of the lungs and have few or vague symptoms)
• You are being treated for TB.  AFB smears and cultures are then usually requested at intervals, both when your doctor is working out the effectiveness of treatment and when seeing whether or not you are still infectious.

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 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 AFB smear or 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.

TB requires a lengthy course of multiple antibiotics to treat 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. In less than 24 hours these tests can amplify/replicate pieces of the mocrorganisms' genetic code to detect mycobacteria in body samples and can narrow the identification of the mycobacteria (a combination, of which M. tuberculosis is the most common). These methods are suitable 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 reduce the spread of the disease.

A faster laboratory method of culturing Mycobacterium tuberculosis is available in some laboratories.  The new liquid culture method called Microscopic-Observation Drug-Susceptibility (MODS) assaytakes only about 7 days to diagnose TB and finds the best antibiotic treatment at the same time it. This new method can help healthcare 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 new test are still being evaluated.

Yes. There is a hidden (latent) form of TB infection. Some people have been exposed to the bacteria but their body's immune system has reduced it to a few of their cells and in an inactive form. People with latent TB infections are not sick and they are not infectious, but the bacteria are still there and alive.  If those with latent infections are tested, most would have a positive TB skin test. The majority of people with latent TB infection, about 90%, will never progress to active tuberculosis disease. However, if the patient's immune system is compromised, there is a higher risk of progression to active disease.

Those who do have active TB may not feel ill at first. Early symptoms may be subtle and, if the TB is outside of the lungs in organs such as the kidney and bone, the tuberculosis may be fairly advanced by the time it causes noticeable symptoms.