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...
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 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.
Rapid testing
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.
