AFB Smear and Culture
If you have symptoms, such as a chronic cough, weight loss, fever, chills, and weakness, that may be due to TB or another mycobacterial infection; if your doctor suspects that you have active TB; if your doctor wants to monitor the effectiveness of TB treatment
Usually, three sputum samples are collected early in the morning on different days. If the patient is unable to produce sputum samples, 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, cerebrospinal fluid (CSF), other body fluids, or biopsied tissue samples may be submitted for culture.
Acid-fast bacilli (AFB) are rod-shaped bacteria that can be seen and counted under the microscope on a specially stained sample on a glass slide, called an AFB smear. The most common acid-fast bacilli are members of the genus Mycobacterium.
Mycobacterium tuberculosis is the most prevalent species of mycobacteria and the most infectious. Most samples that are submitted for AFB smears and cultures are collected because the doctor suspects that the patient has TB. Only a few of the more than 60 species of 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 contagious but can be difficult to treat as it tends to be highly resistant to antibiotics
- Other mycobacterial species, such as Mycobacterium marinum, grow in water, such as fish tanks, and can cause skin infections, while other rapidly growing mycobacteria can infect wounds and prosthetic devices.
- A few mycobacteria, such as M. bovis, can sometimes be transferred from animal to human.
Several AFB smears from different samples should be screened 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 presumptive 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.
Patient samples are processed for AFB cultures at the same time as the smears. The cultures are used to grow acid-fast bacilli in the laboratory. Body fluid or tissue samples are decontaminated to remove the normal respiratory bacteria, digested to emulsify the mucus, and concentrated to increase the number of bacteria before being introduced into a nutrient environment and incubated. 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.
Como a amostra é obtida para o exame?
Since M. tuberculosis and M. avium most frequently infects the lung (pulmonary), sputum is the most commonly tested sample. Sputum is phlegm, thick mucus 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 bronchi and bronchioles. These branching tubes connect the trachea in your throat to your lungs, providing a pathway for air to enter your lungs. Once a local anaesthetic has been sprayed onto the trachea, your doctor can insert a tube into your bronchi and smaller bronchioles and aspirate 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 your doctor suspects extrapulmonary TB (outside of the lungs, fairly common in AIDS patients), he may test the body fluids and tissues most likely affected. For instance, you may collect one or more urine samples if he suspects TB has infected your kidneys. A needle may used to collect fluid from your joints or from other body cavities, such as the pericardium or abdomen. Occasionally, your doctor may need to use a needle to collect a sample of cerebrospinal fluid (CSF) or perform a minor surgical procedure to obtain a tissue biopsy.
NOTA: Se exames médicos em você ou em alguém importante para você o deixam ansioso ou constrangido, ou se você tem dificuldade de lidar com eles, leia um ou mais dos seguintes artigos: Lidando com dor, desconforto ou ansiedade durante o exame, Conselhos sobre exames de sangue, Conselhos para ajudar crianças durante exames médicos, e Conselhos para ajudar idosos durante exames médicos.
Outro artigo, Siga essa amostra, fornece uma visão da coleta e do processamento de uma amostra de sangue e de uma amostra de cultura da garganta.
Como o exame é usado?
AFB smears and cultures are used to determine whether you have an active Mycobacterium tuberculosis infection, an infection due to another member of the Mycobacterium family, or TB-like symptoms due to another cause. They are used to help determine whether the TB is confined to the lungs (pulmonary) or has spread to organs outside the lungs (extrapulmonary). They are ordered to identify M. tuberculosis and determine the most effective antimicrobial agents 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 primary drugs used for therapy, the organisms are called multi-drug resistant TB (MDR TB), and if the organisms 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 determine when a patient is no longer infectious.
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 such as AIDS 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.
Quando o exame é pedido?
AFB testing is ordered when:
- you have symptoms that suggest pulmonary TB, such as a lingering cough that produces phlegm or sputum that may have 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 extrapulmonary TB with few and vague symptoms.)
- you are being treated for TB; AFB smears and cultures are usually ordered at intervals, both when your doctor is evaluating the effectiveness of treatment and when he is attempting to determine whether or not you are still infectious.
O que significa o resultado do exame?
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 regimen 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 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.
Há mais alguma coisa que eu devo saber?
TB requires a lengthy course of multiple antibiotics to eradicate 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. They amplify/replicate pieces of the microorganisms genetic code 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 sensitive and specific when they are paired with positive AFB smears; but 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 they do provide the doctor with a quick answer, allowing him to isolate potentially infectious patients and minimize the spread of the disease.
A faster lab method to culture Mycobacterium tuberculosis is in development. The new liquid culture method called Microscopic-Observation Drug-Susceptibility (MODS) assay takes only about 7 days to diagnose TB and finds the best antibiotic treatment at the same time. Since this method can recognize the presence of MDR TB much more quickly than conventional culture, it can help health care 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 test are still being evaluated.
Can I be infected with TB and not be sick?
Yes. There are about 10 to 15 million people in the United States, and many more 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 few of their cells, 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 MDR TB and XDR TB?
Both indicate strains of M. tuberculosis that can be difficult to treat, but XDR TB is resistant to more drug therapies. XDR TB is currently defined by the CDC (Centers for Disease Control and Prevention) 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.
Why is the doctor asking me to take my TB medication in the presence of a nurse?