Acid-Fast Bacillus (AFB) Testing
If you have symptoms, such as a long lasting cough, weight loss, fever, chills, and weakness that your doctor thinks may be due to TB or another mycobacterial infection. If your doctor suspects that you have active TB or wants to monitor the effectiveness of TB treatment.
Usually, three separate spit (sputum) samples are collected early in the morning on different days. If you are unable to produce a sputum sample, a bronchoscope may be used to collect fluid during a procedure called a bronchoscopy. In children, stomach washings/aspirates may be collected. Depending on symptoms, urine, cerebral spinal fluid (CSF), other body fluids, small samples of tissue samples may be biopsied and used to help identify an infection.
The test looks for the presence of bacteria called acid-fast bacilli (AFB) which are rod shaped bacteria that can be seen and counted under the microscope in a specially stained sample on a glass slide, called an AFB smear. The most common AFB are of a type called mycobacteria.
Mycobacterium tuberculosis complex is the most common species of mycobacteria, and is the most infectious. Most AFB smears and cultures are collected because the doctor suspects that the patient has TB. Other few mycobacteria that have been identified cause infections in humans. They include:
- M. africanum, causes a disease similar to TB in certain parts of the world
- Mycobacteria avium-intracellulare complex (MAC), can cause a lung infection in immunosuppressed patients, such as the elderly and those with AIDS; this infection is not easily spread to other people, but can be difficult to treat because it is very resistant to antibiotics
- A few other mycobacteria, such as M. bovis, M.abscessus and M. chelonae can sometimes cause human infection if there are specific predisposing factors (i.e., respiratory disease and other conditions).
- All other mycobacteria other than M. tuberculosis are part of a wider group called non-tuberculous Mycobacteria (NTM).
Several smears from different samples should be checked for AFB since the number of bacilli may vary day to day. If acid-fast bacilli are present on any of the smears, a mycobacterial infection is likely. Since M. tuberculosis is the most common cause of respiratory infections with mycobacteria, a provisional diagnosis of TB can be made but other follow-up testing must be done to positively identify the acid-fast bacilli as either M. tuberculosis, or another mycobacteria species.
Body fluids such as urine or CSF are used for AFB cultures at the same time as the smears. The cultures are used to grow AFB in the laboratory. Since mycobacteria grow slowly, positive identification of the species that is/are present may take days to several weeks, while negative results (no mycobacterial growth) can take up to 6 to 8 weeks to confirm.
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 bacterial genetic code. 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.
Rapid testing using molecular amplification of bacterial DNA is becoming more common and some laboratories may offer a rapid PCR (polymerase chain reaction) test. Some of these tests can also provide information on the susceptibility of the bacteria if present (generally only one or two antibiotics, not the full panel) and only if the test is positive. Please note that a negative rapid test does not rule out a diagnosis and it does not replace the need to culture the sample. Multiple samples for monitoring treatment may be still required.
How is the sample collected for testing?
Since M. tuberculosis and M. aviummost frequently infect the lungs, sputum is the most commonly tested sample. Sputum is phlegm, thick mucous that is coughed up from the lungs. Usually, three to five early morning samples are collected (on consecutive days) in individual sterile cups.
If you are unable to produce sputum, your doctor may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows your doctor to look at, and collect samples from, your lungs by inserting a tube through your throat after giving a local anaesthetic.
Since young children will not be able to produce a sputum sample, stomach washings/aspirates may be collected. This involves putting a little salt solution (saline) into the stomach through a tube, and sucking the fluid back up the tube.
If your doctor thinks you may have TB outside of the lungs (which is fairly common in patients with AIDS), the body fluids and tissues which are most likely to be affected will be sampled and tested. For instance, if it is suspected that TB has infected your kidneys then one or more urine samples may be collected for testing. A needle may be used to collect fluid from your joints or from other body cavities, such as the lining around the heart (pericardium) or abdomen. Occasionally, your doctor may need to use a needle to collect a sample of cerebral spinal fluid (CSF) or perform a minor surgical procedure to obtain a tissue biopsy.
How is it used?
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 sort out 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 aerosol 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.
When is it requested?
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 or blood 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.
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 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.
Is there anything else I should know?
Can I be infected with TB and not be sick?
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.
What is the difference between MDR TB and XDR TB?
Both indicate resistant strains of M. tuberculosis that can be difficult to treat but XDR (eXtremely drug resistant) TB is resistant to more drug therapies. XDR TB is currently defined by the CDC (Centers for Disease Control and Prevention) in the US and WHO (World Health Organization) as M. tuberculosis that is resistant to isoniazid and rifampin, plus resistant to any fluoroquinolone and to at least one of three injectable 'second-line' drugs (amikacin, kanamycin, or capreomycin). The emergence of XDR TB is being closely watched by the world medical community and measures are being taken in hopes of limiting its spread.
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On This Site
Tests: TB Skin Test, Antibiotic Susceptibility Testing, Bacterial Wound Culture, Sputum Culture, Bacterial Body Fluid Testing, Adenosine Deaminase, Interferon Gamma Release Assay
Conditions: Tuberculosis, Nontuberculous Mycobacteria Infections, Meningitis and Encephalitis, Lung Diseases, HIV Infection and AIDS, Septic Arthritis, Wound and Skin Infections