To help detect and identify infections caused by Mycobacterium tuberculosis, the cause of tuberculosis (TB), and other Mycobacterium species, which are known as acid-fast bacilli (AFB); to monitor the effectiveness of treatment
Acid-Fast Bacillus (AFB) Testing
When you have signs and symptoms of a lung infection, such as a chronic cough, weight loss, fever, chills, and weakness, that may be due to TB or a nontuberculous mycobacterial (NTM) infection; when you have a positive TB screening test and you are in a high-risk group for progressing to active disease; when you have a skin or other body site infection that may be due to mycobacteria; when you are undergoing treatment for TB
For suspected cases of tuberculosis lung infections, usually three sputum samples are collected early in the morning on different days. If the affected person is unable to produce sputum, a bronchoscope may be used to collect fluid during a procedure called a bronchoscopy. In children, gastric washings/aspirates may be collected. Depending on symptoms, urine, an aspirate from the site of suspected infection, cerebrospinal fluid (CSF), other body fluids, or biopsied tissue samples may be submitted for AFB smear and culture.
Most samples that are submitted for acid-fast bacilli (AFB) testing are collected because the health practitioner suspects that a person has tuberculosis (TB), a lung infection caused by Mycobacterium tuberculosis. Mycobacteria are called acid-fast bacilli because they are a group of rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure where the bacteria retain the color of the stain after an acid wash (acid-fast). AFB laboratory tests detect the bacteria in a person's sample and help identify an infection caused by AFB.
There are a several types of AFB that may be detected with this testing; however, the most common and medically important ones are members of the genus Mycobacterium. Mycobacterium tuberculosis is one of the most prevalent and infectious species of mycobacteria.
Since TB is transmitted through the air when an infected person sneezes, coughs, speaks, or sings, it is a public health risk. It can spread in confined populations, such as in the home and schools, correctional facilities, and nursing homes. Those who are very young, elderly, or have preexisting diseases and conditions, such as AIDS, that compromise their immune systems tend to be especially vulnerable. AFB testing can help track and minimize the spread of TB in these populations and help determine the effectiveness of treatment.
Another group of mycobacteria referred to as nontuberculous mycobacteria (NTM) can also cause infections. However, only a few of the more than 60 species of mycobacteria that have been identified cause infections in humans. Some examples include Mycobacterium avium-intracellulare complex (MAC), which can cause a lung infection in people with weakened immune systems, and Mycobacterium marinum, which can cause skin infections. (See the article on Nontuberculous Mycobacteria for more details on different types). In addition to TB, AFB testing can help identify infections caused by these nontuberculous mycobacteria.
AFB laboratory tests include:
- AFB smear — a microscopic examination of a person's specimen that is stained to detect acid-fast bacteria. This test can provide presumptive results within a few hours. It is a valuable tool in helping to make decisions about treatment while culture results are pending.
- Molecular tests for TB (nucleic acid amplification test, NAAT) — detect the genetic components of mycobacteria and are often done when the AFB smear is positive or TB is highly suspected. Like AFB smears, they can provide a presumptive diagnosis, which can aid in the decision of whether to begin treatment before culture results are available. Results of NAAT are typically available several hours after a sample is collected.
- AFB cultures to grow the bacteria are set up at the same time as the AFB smears. Though more sensitive than AFB smears, results of cultures may take days to several weeks.
- Susceptibility testing on the acid-fast bacteria grown in the cultures that are positive will determine the organism's susceptibility or resistance to drugs most commonly used to treat the infection.
How is the sample collected for testing?
Since M. tuberculosis and M. avium most frequently infect the lungs (pulmonary disease), sputum is the most commonly tested sample. Sputum is phlegm, thick mucus that is coughed up from the lungs. Usually, three early morning samples are collected on consecutive days in individual sterile cups to increase the likelihood of detecting the bacteria.
If a person is unable to produce sputum, a health practitioner may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows the health practitioner to look at and collect samples from the bronchi and bronchioles. Once a local anesthetic has been sprayed onto the patient's upper airway, the practitioner can insert a tube into the bronchi and smaller bronchioles and aspirate fluid samples for testing. Sometimes, the health practitioner 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 health practitioner suspects TB is present outside of the lungs (extrapulmonary), a condition fairly common in AIDS patients, he or she may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if the practitioner suspects TB has infected the kidneys. A needle may used to collect fluid from joints or from other body cavities, such as the pericardium or abdomen. Occasionally, the practitioner may collect a sample of cerebrospinal fluid (CSF) 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.
How is it used?
AFB testing may be used to detect several different types of acid-fast bacilli, but it is most commonly used to identify an active tuberculosis (TB) infection caused by the most medically important AFB, Mycobacterium tuberculosis.
Mycobacteria are called acid-fast bacilli because they are rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure in which the bacteria retain the color of the stain after an acid wash (acid-fast).
A few different tests may be used to help identify AFB as the cause of an infection:
- An AFB smear is used as a rapid test to detect mycobacteria that may be causing an infection such as tuberculosis. The sample is spread thinly onto a glass slide, treated with a special stain, and examined under a microscope for "acid-fast" bacteria. This is a relatively quick way to determine if an infection may be due to one of the mycobacteria, such as M. tuberculosis. AFB smears can provide presumptive results within a few hours and are valuable in helping to make decisions about treatment while culture results are pending. However, this rapid test is less sensitive than culture to diagnosis a mycobacterial infection.
- A molecular test for TB called nucleic acid amplification test (NAAT) may be done in conjunction with an AFB smear. NAAT detects the genetic components of mycobacteria by amplifying/replicating pieces of the microorganisms' genetic material. These tests can help decrease the amount of time necessary for a presumptive diagnosis of tuberculosis to less than 24 hours. The testing 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 performed on specimens where acid-fast bacteria were seen on the smear. When they are done on samples that are AFB-negative by smear, they tend to be less sensitive. The test methods are approved for respiratory samples but must be confirmed with an AFB culture. They provide the health practitioner with a quick answer, allowing him or her to isolate potentially infectious people and minimize the spread of the disease. Guidelines from the Centers for Disease Control and Prevention recommend that people with signs and symptoms of TB have at least one sample tested using nucleic acid amplification in conjunction with AFB smear and culture.
- AFB cultures are used to diagnose active M. tuberculosis infections, infections due to nontuberculous mycobacteria, or to determine whether TB-like symptoms are due to another cause. They are used to help determine whether the TB is confined to the lungs (pulmonary disease) or has spread to organs outside the lungs (extrapulmonary disease). AFB cultures can also be used to monitor the effectiveness of treatment and can help determine when a person is no longer infectious. Though this test is more sensitive than an AFB smear, it takes longer for results to become available. Mycobacteria grow more slowly than other types of bacteria so positive identification may take days to several weeks, while negative results (no mycobacterial growth) can take up to 6 to 8 weeks to confirm.
- Susceptibility testing is usually ordered in conjunction with an AFB culture to determine the most effective antibiotic to treat the mycobacterial infection. M. tuberculosis may be resistant to one or more drugs commonly used to treat TB. In addition to routine susceptibility testing, there are now some molecular tests available that can identify the genes in bacteria that confer resistance to the most commonly prescribed drugs.
When is it ordered?
AFB testing is ordered when:
- Someone has symptoms that suggest pulmonary TB or other mycobacterial lung infection, such as:
- A person has symptoms associated with a TB or other mycobacterial infection located outside of the lungs (extrapulmonary); the symptoms vary depending on the area of the body that is affected. Some examples include back pain and paralysis (spinal TB), weakness due to anemia (TB in the bone marrow), altered mental state, headache, and coma (TB meningitis), joint pain or abdominal pain.
- A TB screening test is positive and the person is at increased risk for active disease and/or characteristic signs are seen in an X-ray of the lung.
- Someone has been in close contact with a person who has been diagnosed with TB and the exposed person either has symptoms or has a condition or disease that puts him or her at a much higher risk of contracting the disease, such as HIV/AIDS. (Those with AIDS are more likely than other affected people to have extrapulmonary TB with a few, vague symptoms.)
- An individual is being treated for TB; AFB testing is usually ordered at intervals, both for evaluating the effectiveness of treatment and for determining whether or not a person is still infectious.
- An individual has a chronic skin infection that does not respond to the usual antibiotics given for a bacterial infection; NTM may be the cause of the infection since they do not respond to the same antibiotics used to treat a staphylococcal or streptococcal infection.
What does the test result mean?
AFB Smear and NAAT
A negative AFB smear may mean that no infection is present, that symptoms are caused by something other than mycobacteria, or that the mycobacteria were not present in sufficient numbers to be seen under the microscope. Usually three samples are collected to increase the probability that the organisms will be detected. Nevertheless, if AFB smears are negative and there is still a strong suspicion of a mycobacterial infection, then additional samples may be collected and tested on different days. A smear negative sample may still grow mycobacteria since the culture media allows low numbers of bacteria that cannot be seen in a microscopic examination to multiply and be detected.
Positive AFB smears indicate a probable mycobacterial infection. However, a culture must be performed to confirm a diagnosis and identify the species of mycobacteria present.
For people with signs and symptoms of an active TB infection, AFB smear results are considered together with results from NAAT for TB, as recommended by the Centers for Disease Control and Prevention. Though definitive diagnosis requires results from a culture, results from the smear and NAAT may be helpful in deciding what to do. For example, if there is a presumptive diagnosis of TB based on rapid test results, most health practitioners would treat.
Interpretation of smear and NAAT results are summarized in the following table. Again, all results must be confirmed by results from culture.
AFB smear result NAAT result for TB Interpretation Positive Positive Presumptive diagnosis for TB Negative Positive NAAT is more sensitive than smear so this may occur in people with true disease; may test additional samples using NAAT. If more than one sample is positive by NAAT, this is a presumptive diagnosis for TB. Positive Negative Questionable results for TB; an inhibitor may be present in the specimen or the AFB seen on the smear are not M. tuberculosis. A test for the inhibitor may be performed. Negative Negative Symptoms probably not due to active mycobacterial infection.
Positive AFB cultures identify the particular mycobacterium causing symptoms, and susceptibility testing on the identified organism gives the health practitioner 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 the treatment regimen is not effective and needs to be changed. It also means that the person is still likely to be infectious and can pass the mycobacteria to others through coughing or sneezing.
A negative culture means that the person tested does not have an active AFB infection or that mycobacteria were not present in that particular sample (which is why multiple samples are often collected) or were present in numbers too low to be detected. Cultures are held for six to eight weeks before being reported as negative. The person tested may have a latent infection that caused a TB screening test to be positive but does not have active TB.
If someone has a TB infection in another part of the body, a different type of sample may need to be collected and tested to identify the infection.
A negative culture several weeks after treatment indicates that the TB infection is responding to drug treatment and that the person is no longer infectious.
Susceptibility testing results will list the antibiotics that will likely be most effective in treating the infection. Isoniazid and rifampin are two 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 multidrug-resistant TB (MDR-TB), and if the organisms are resistant to multiple drugs approved for first and second lines of therapy, they are called extensively drug-resistant tuberculosis (XDR-TB).
Is there anything else I should know?
TB requires a lengthy course of multiple antibiotics to eradicate an active infection. People 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.
A faster lab method to culture Mycobacterium tuberculosis has been developed. Culturing the sample in a liquid broth-based medium allows the organisms to be detected sooner. Some of the broth cultures require an automated instrument to detect the presence of the mycobacteria, while other methods can be read manually. A liquid culture method, called Microscopic-Observation Drug-Susceptibility (MODS) assay, takes only about 7 days to diagnose TB and detects bacterial resistance to antibiotics at the same time. Since this method can recognize the presence of multidrug-resistant TB (MDR-TB) much more quickly than conventional culture, it can help health practitioners 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 non-automated test are still being evaluated in resource-limited countries with high prevalence of TB.
In December 2010, the World Health Organization recommended use of a fully automated, cartridge-based nucleic amplification assay that can simultaneously detect TB and rifampicin resistance directly from sputum in less than two hours. It was approved for marketing in the U.S. in July 2013 and this technology has been recently adopted by many laboratories. However, this NAAT test does not replace AFB cultures. All samples submitted for AFB testing should be cultured to ensure that any mycobacteria that are present are available for further testing, according to the Centers for Disease Control and Prevention.
Can I have a tuberculosis (TB) infection and not be sick?
Yes. There are many people in the United States and worldwide who have a latent form of TB infection. They have been exposed to the bacteria, but their body's immune system has confined it to a localized area in their lungs, in an inactive form. People with latent TB infections are not sick and they are not infectious, but the bacteria are still there and still 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.
Those who do have active TB may not feel ill at first. Early symptoms may be subtle and, if the TB is extrapulmonary (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 multidrug-resistant TB (MDR-TB) and extensively-resistant TB (XDR-TB)?
Both indicate strains of Mycobacteria tuberculosis that can be difficult to treat, but XDR-TB is resistant to more drug therapies. MDR-TB is resistant to the two most powerful drugs, isoniazid and rifampin. XDR-TB is currently defined by the Centers for Disease Control and Prevention and the 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.
Why is the doctor asking me to take my TB medication in the presence of a nurse?
The practice of taking TB medications in the presence of a health practitioner is known as direct observed therapy (DOT). DOT ensures that people are taking their medications and continuing their therapy for the required length of time. Unlike other bacterial infections that can be cured in 7-10 days, TB must be treated with two or more drugs for several months. People tend to forget to take their medication when they are feeling better. Since TB medications must be taken for many months, the risk of non-compliance is high. Having a health practitioner administer the medications weekly increases the likelihood that the entire regimen will be completed and decreases the likelihood that someone will relapse with a more resistant strain of TB.
Besides TB, what other types of mycobacteria can be identified with AFB testing?
Examples of other mycobacteria that can cause infections and are detected using AFB tests include:
- Mycobacterium avium-intracellulare complex (MAC)—can cause a lung infection in people with weakened immune systems, such as those with AIDS; this infection is not contagious but it can be difficult to treat as it tends to be highly resistant to antibiotics.
- Mycobacterial species, such as Mycobacterium marinum, grow in water, such as fish tanks, and can cause skin infections.
- Mycobacterium fortuitum and Mycobacterium chelonae, and other rapidly growing mycobacteria, cause skin and wound infections following cosmetic surgery, prosthetic device implantation, and visits to nail salons.
- A few mycobacteria, such as Mycobacterium bovis, can sometimes be transferred from animals to humans.
See the article on Nontuberculous Mycobacteria for more examples and details.
Nocardia species are not a type of mycobacteria but can be detected using some AFB laboratory tests. Nocardia can cause infections of the lungs, brain, or skin.
On This Site
Tests: TB Screening Tests; Bacterial Wound Culture; Bacterial Sputum Culture; Susceptibility Testing; Body Fluid Analysis; Adenosine Deaminase
Conditions: Tuberculosis, Nontuberculous Mycobacteria, Meningitis, Lung Diseases, Human Immunodeficiency Virus, Septic Arthritis, Wound and Skin Infections
Elsewhere On The Web
Centers for Disease Control and Prevention: Tuberculosis
CDC: Traveler's Health Yellowbook - Tuberculosis
World Health Organization: Tuberculosis
American Lung Association: Tuberculosis
MedlinePlus Medical Encyclopedia: Mycobacterial Infections
National Jewish Medical and Research Center: Nontuberculous Mycobacteria (NTM)