At a glance
Also known as
TB culture and sensitivity; Mycobacterial smear and culture
Why get tested?
To help identify a mycobacterial infection; to diagnose tuberculosis (TB); to monitor the effectiveness of treatment
When to get tested?
When your doctor suspects TB because you have symptoms of a lung infection that may be due to TB or another mycobacteria – symptoms may include chronic cough, weight loss, fever, chills, and weakness; when you are likely to be a of TB although you do not have symptoms; when you have had a positive result from a screening test and are in a high-risk group for progressing to active disease; when you are undergoing treatment for TB - to monitor the effectiveness of the treatment.
When your doctor suspects another mycobacterial infection such as when you have a skin or other body site infection that is not responding to routine antibiotics. These may be caused by Nontuberculous Mycobacteria (NTM) and examples are infections of the lungs, lymph glands, skin, soft tissues and bones and disseminated infection throughout the body. To date at least thirty species of mycobacteria that do not cause tuberculosis or leprosy have been identified.
For suspected cases of mycobacterial lung infections, three samples are collected early in the morning on different days. If the affected person is unable to produce , a bronchoscope may be used to collect fluid during a procedure called a bronchoscopy. In children, gastric washings/ may be collected. Depending on symptoms, urine, an aspirate from the site of suspected infection, , other body fluids, or samples may be submitted for AFB smear and culture.
Test preparation needed?
No test preparation is needed.
What is being tested?
Acid-fast bacilli (AFB) are rod shaped they can be seen and counted under the microscope when smeared on a slide and treated with an "acid-fast" staining procedure that differs from the routine stain. The most common and medically important acid-fast bacilli are members of the genus Mycobacterium.
Mycobacterium tuberculosis is one of the most prevalent and infectious species of mycobacteria. Most samples that are submitted for AFB smears and cultures are collected because the doctor suspects that someone has a lung infection caused by M. tuberculosis (TB). Another group of bacteria referred to as non-tuberculous mycobacteria (NTM), can also cause infections. These organisms are common in the environment (including water and soil) however only a few of them cause infections in humans. They include:
- Mycobacteria avium-intracellulare complex (MAC) can cause a lung infection or a disseminated infection in patients, such as the elderly and those with AIDS; this infection is not contagious, but it can be difficult to treat as it tends to be highly resistant to antibiotics.
- Cervical lymphadenitis can be seen in young children as a swollen lymph gland in the neck. It is most commonly caused by MAC, but also by M. tuberculosis or other NTM. Surgical excision is often required for treatment.
- Rapid growing mycobacterium (RGM) (M. abscessus, M. chelonae, M. fortuitum) may cause lung or non-pulmonary disease such as wound infection of prosthetic device infection.
- Mycobacterium ulcerans causes sporadic cases of non-healing ulcers (some names include Bairnsdale ulcer, Buruli ulcer, Daintree ulcer) that often require surgical treatment.
- Mycobacterium marinum grows in water, such as fish tanks, and can cause skin infections.
- Mycobacterium leprae causes leprosy.
- Some mycobacteria, such as Mycobacterium bovis, can sometimes be transferred from animal to human.
A definitive diagnosis requires the mycobacteria to be cultured. Mycobacteria grow more slowly than other types of bacteria so 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.
An AFB smear, which can provide presumptive results in a few hours, is a valuable tool in helping to make decisions about treatment while culture results are pending. Patient samples are processed for AFB cultures at the same time as the smears.
Typically, several AFB smears from different samples are screened for AFB since the number of bacilli may vary from sample to sample and day to day. If acid-fast bacilli are present on any of the smears, a mycobacterial infection is likely. A presumptive diagnosis of TB can be made if a patient has risk factors for disease, but other follow-up testing must be done to positively identify the acid-fast bacilli as either M. tuberculosis or another mycobacteria species.
Tests that may be done in addition to an AFB smear and culture include:
- Molecular tests for TB that detect the genetic components of mycobacteria have been developed to help decrease the amount of time necessary for a presumptive diagnose of tuberculosis. These include genetic probes and molecular TB testing. They /replicate pieces of the microorganisms' genetic material 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 and when they are paired with positive AFB smears; 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 a positive result will be available more rapidly allowing the potentially infectious patient to be isolated to minimise the spread of the disease.
- Antibiotic susceptibility testing is complex and may take a long-time to return a result. It is generally performed on isolates of Mycobacterium tuberculosis. This testing is occasionally performed on other species of Mycobacteria, however there is no evidence that the results can predict the outcome of therapy with the drug tested, except in very specific cases.
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 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.
How is the sample collected for testing?
Since M. tuberculosis and M. avium most frequently infect the lungs (pulmonary disease), 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 a person is unable to produce sputum, the doctor may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows the doctor to look at and collect samples from the bronchi and bronchioles. Once a local anaesthetic has been sprayed onto the airway, the doctor can insert a tube into the bronchi and smaller bronchioles and fluid samples for testing. Sometimes, they will introduce a small amount of saline through the tubing and into the bronchi and then aspirate it to collect a bronchial washing.
Since young children cannot produce a sputum sample, gastric washings/aspirates may be collected. This involves introducing saline into the stomach through a tube, followed by fluid aspiration.
If the doctor suspects TB is present outside of the lungs (extrapulmonary), they may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if they suspect TB has infected the kidneys. A needle may be used to collect fluid from or from other body cavities, such as the or abdomen. Occasionally, the doctor may need to collect a sample of or perform a minor surgical procedure to obtain a tissue biopsy.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.