What Is Tuberculosis?
Tuberculosis (TB) is a condition caused by infection with a bacteria called Mycobacterium tuberculosis. The bacteria is transmitted between people through the air when an infected person talks, sneezes, or coughs, spreading droplets that can be inhaled by others.
TB infections are more easily contracted in crowded environments like prisons, hospitals, and homeless shelters. The risk of infection is increased in people with poor nutrition and weakened immune systems from other health conditions.
Tuberculosis develops in three stages: primary infection, latent infection, and active disease:
- Primary infection: A primary infection can occur when a person is first exposed to bacteria that cause tuberculosis. For most people, the immune system defends against this initial infection and there are no noticeable symptoms. It is uncommon for a person to be contagious at this stage.
- Latent infection: The initial immune reaction to a primary infection does not kill all tuberculosis bacteria. Instead, some bacteria remain in the body in a dormant state. During this time, a person is not contagious and does not experience any symptoms. It is estimated that as many as 25% of people worldwide have a latent TB infection (LTBI). For more than 90% of people with infection, the bacteria never reactivates.
- Active disease: In active TB, also called TB disease, the bacterial infection can no longer be controlled by the immune system. People with active TB have symptoms and are able to spread the infection to others. Although active TB can occur right after a primary infection, it is more common for it to develop when a latent infection reactives weeks, months, years, or even decades after a person was initially exposed to TB. Most TB reactivation happens within two years after primary infection.
Most people who are exposed to tuberculosis never develop active disease. It is not known for certain why some people quickly develop active TB after primary infection or experience the reactivation of a latent infection. However, the risks of developing active TB are higher in people with weakened immune systems. People at higher risk of active TB include:
- Older adults
- Infants and very young children
- People with HIV
- People taking medications that suppress the immune system, including many chemotherapies for cancer
- People with diabetes
- People who use tobacco
- People with chronic kidney problems
- People who are malnourished or have recently experienced major weight loss
Mycobacterium tuberculosis can grow in different areas of the body. Tuberculosis typically affects the lungs in a form of the disease known as pulmonary TB. Less often, Mycobacterium tuberculosis spreads to other areas of the body, which is referred to as disseminated TB.
Pulmonary TB can cause symptoms like cough and difficulty breathing as well as fever, excessive sweating at night, and fatigue.
Disseminated TB can spread to organs such as the kidneys, lymph nodes, brain, bones, joints, and other organs and areas of the body. Symptoms of disseminated TB can vary based on the part of the body affected and typically include general symptoms like fever, fatigue, and reduced appetite.
If not properly diagnosed and treated, TB can cause serious and life-threatening complications.
The Role of Tuberculosis Tests
There are many kinds of tuberculosis tests that can be used for several different purposes. In general, tuberculosis tests are performed for screening, diagnosis, treatment planning, and monitoring.
Screening is testing that looks for a health problem in people who do not have any symptoms. For tuberculosis, screening tests can look for a latent tuberculosis infection. Some tests can check whether a person’s immune system has ever reacted to the bacteria that causes tuberculosis. Detecting latent TB enables treatment that can reduce the chances of developing active TB.
Diagnosis is testing to determine whether a person has a specific condition. Diagnostic tests for tuberculosis are used to see if a person has an active TB infection that requires treatment. The diagnostic process can also include evaluating the severity of the infection and determining if there is disseminated disease.
Treatment planning is how doctors decide the best way to treat tuberculosis in a specific patient. There are strains of tuberculosis bacteria that have become resistant to antibiotic medications. Testing allows the laboratory to see whether a patient’s TB infection is drug-resistant and test which medicines are most likely to be effective.
Monitoring is how health care providers track a patient’s condition over time. For tuberculosis, monitoring tests can be used to see if treatment is helping to fight the infection. Tests may also be used to check for side effects from medications used to treat TB.
Another role of tuberculosis testing is to support public health programs. Because tuberculosis is a contagious disease, most national, state, and local health departments have protocols for reporting and tracking cases of TB. These programs are designed to reduce the spread of TB in the population, decrease the prevalence of drug-resistant TB, and identify people at high risk of TB who may benefit from testing.
Who should get testing?
Tuberculosis testing can be appropriate in many situations. The type of test and when it should be performed depends on the purpose of testing.
People who do not have any symptoms of TB may have screening tests to detect a latent TB infection. This type of testing is most frequently recommended in people who have a known or likely exposure to TB, including:
- People who have been in close contact with someone who is known to have active TB
- Health care workers or others who work in settings where active TB infections are more common
- People who were born or have spent significant time in countries where TB is prevalent
Screening tests may also be recommended in people who are at greater risk of reactivation of a latent TB infection, including:
- People with HIV or other diseases that impair the immune system
- People taking medications that suppress the immune system
- Older adults
- People with chronic health conditions like diabetes, kidney disease, and certain cancers
Tests to diagnose or rule out active infection for TB are frequently performed if screening tests find a latent TB infection. Diagnostic testing may also be recommended in people that have symptoms that could be explained by active TB. Potential symptoms of active tuberculosis include:
- Coughing that has lasted for three or more weeks and often brings up mucus
- Chest pain
- Difficulty breathing
- Swollen lymph nodes
- Significant sweating at night
- Unexpected weight loss
- A specific area of the body showing signs of disseminated infection
After being initially diagnosed with tuberculosis, a patient may have additional tests in order to understand the extent of the disease and to identify the type of treatment that is most likely to work against the strain of tuberculosis bacteria causing the infection.
People who have been diagnosed with tuberculosis and have received treatment usually need ongoing testing as a way of monitoring their health condition, including whether treatment has been effective against the infection.
Getting test results
When your test results become available depends on the type of testing that is being performed. It is common for test results to come back within a few business days, but some types of tests can take a few weeks before results are ready.
Sometimes multiple tests may need to be conducted either at the same time or in a specific sequence, and your doctor may wait until all the results are available before reviewing them with you. In most cases, your doctor will explain what the results mean during an in-person appointment or over the phone. Test reports may also be available through an online health portal or by mail.
The results of different tuberculosis tests can help determine if you have a latent infection or active disease that needs treatment. Testing can also play a role in determining the type of treatment you should have and in assessing how well that treatment is working.
Types of Tuberculosis Tests
There are many different types of tuberculosis tests. Each test provides information that can be valuable in certain situations. For example, the tests prescribed for screening for a latent TB infection are distinct from tests that diagnose active TB. Testing is tailored to each individual’s situation, and the doctor providing care can explain the role of each test.
The following sections provide more detail about several of the tests that are most commonly used for tuberculosis screening, diagnosis, treatment planning, and monitoring.
Health review and physical exam
A first step in evaluating people for a possible TB infection is a combination of a health review and physical exam.
During a health review, also known as a health history, the doctor can ask about any symptoms you have, possible exposure to TB, and any coexisting health issues that may affect your risk of tuberculosis.
During a physical exam, the doctor can check for disrupted lung function, swollen lymph nodes, or other signs of infection that could be tied to tuberculosis.
Your health care provider can use information from these assessments to determine whether additional tests are needed.
Tuberculosis skin test
A tuberculosis skin test determines whether your immune system has reacted to the bacteria that causes tuberculosis in the past. It is commonly used to screen for latent TB infection and may provide useful information when trying to diagnose active TB.
To take the test, a small shot with inactivated tuberculosis bacteria, called a pure protein derivative (PPD), is injected under the skin. Normally this injection is done around the forearm. Two to three days later, you return to the doctor to see if there has been an immune reaction in the skin.
While a TB skin test can tell if you have come in contact with tuberculosis, it cannot determine whether an infection is latent or active. In addition, a skin test may not detect TB exposure until up to 10 weeks after a primary infection.
In order to improve the accuracy of skin testing, you may be required to have two consecutive negative TB skin tests. People with ongoing exposure to TB may have periodic TB skin testing. In contrast, once a person has had one TB skin test with a positive result, they never need to have the test again because it will always be positive.
IGRA blood test
Like the TB skin test, an interferon-gamma release assay (IGRA) blood test is frequently used to screen for latent TB infection. An IGRA test can determine if you have been exposed to tuberculosis before, but it cannot distinguish between an active and past infection.
For an IGRA test, a blood sample is usually taken from a vein in the arm. A laboratory then exposes the blood to synthetic tuberculosis bacteria proteins and checks for substances that the immune system produces if a person has ever been infected by TB.
Bacteria detection tests
Bacteria detection tests use different laboratory methods to check for the presence of Mycobacterium tuberculosis in a sample taken from the body. These tests are central to the diagnosis of tuberculosis and also enable drug susceptibility testing for planning treatment.
Methods of identifying bacteria in a sample include microscopic analysis, culture, and molecular tests. In many cases, more than one of these methods is used on the same sample.
Microscopic analysis is looking at a test sample with a microscope to see what type of cells are present. Chemicals, called stains, are applied to the sample to help recognize certain types of bacteria. For tuberculosis, a stain called the acid-fast stain is commonly used to better identify the Mycobacterium tuberculosis bacteria. While microscopic analysis can be performed quickly, it is less precise than other methods. As a result, results usually must be confirmed by other bacterial detection tests.
Culture is a type of testing that uses a sample from the body to grow bacteria in a dish in a laboratory. The bacteria that grow in the dish provide evidence about the type of bacteria in the body. Culture tests may take several weeks to provide results since the bacteria need time to grow, but the test is considered highly accurate. If tuberculosis bacteria are found, the lab can conduct further tests to see which types of drugs kill the bacteria most effectively.
Molecular tests look for traces of the genetic material of Mycobacterium tuberculosis in a sample. This type of testing can provide results within a few days, making it useful for the initial diagnosis of TB. Molecular tests can also provide information about the genetic makeup of the strain of bacteria in a person’s body, which can help guide treatment.
Many different kinds of tissues or fluids from the body can be tested to look for signs of the bacteria that causes TB. The kind of sample needed and the way it is obtained depends on where the doctors believe an active TB infection is affecting the body. These samples can include:
- Sputum, a thick mucus produced in the lungs
- Tissue from lymph nodes
- Blood or urine
- Skin cells
- Fluid from the spine, known as cerebrospinal fluid (CSF)
- Bone or fluid from inside the joints
- Fluid from inside the abdomen or around the heart or outside the lungs
- Tissue from the liver or other parts of the digestive system
Urine antigen test
People with HIV may have weakened immune systems and are at an increased risk of developing active TB disease. Studies have found that some test methods, such as microscopic analysis, can fail to detect tuberculosis in people with HIV and delay the start of treatment.
In people with HIV, another type of bacterial detection test, known as a urine antigen test, may be used. This test looks for a substance called lipoarabinomannan (LAM), which makes up part of the cell structure of the tuberculosis bacteria. A urine LAM test can be conducted rapidly but needs confirmation with culture or molecular tests.
Tests that provide a picture of the inside of the body are frequently used in the diagnosis of tuberculosis. For patients with symptoms of TB or who have tested positive for a latent TB infection, imaging tests of the lungs can reveal signs of active TB that help determine whether further testing is necessary.
A frequently used imaging test is an x-ray of the chest. Less often, doctors may recommend a CT scan of the chest, which involves multiple x-ray images taken at different angles that are then combined by a computer. Imaging tests of other parts of the body and/or other types of imaging tests, such as PET scans and MRIs, are only used in more specific and limited circumstances.
After a person is diagnosed with tuberculosis, doctors may recommend several tests to check for other health conditions that are more common in people with TB. This often involves a blood test for HIV infection and may include tests for hepatitis B and hepatitis C.
Doctors may prescribe other tests to assess your overall health, and these tests may be performed over time to help monitor for side effects from medications. These tests can include evaluations of liver function, kidney health, and vision.
Getting Tested for Tuberculosis
Tuberculosis testing is usually ordered by a health care provider. Testing may be conducted in a doctor’s office, health clinic, or hospital. People with active TB are often contagious, so special precautions may be needed to perform a test without exposing others to the bacteria.
Anyone who has symptoms of TB, has a known or suspected exposure to TB, or has a compromised immune system can talk with their doctor about what type of testing is appropriate and where that testing can be performed.
Tuberculosis testing is not currently available with an at-home kit. Tests for both latent and active TB infections require careful administration and interpretation by a health professional in a medical setting.
Anyone interested in TB testing should talk with their doctor about what kind of testing may be appropriate for their situation.
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