A TB blood test is a diagnostic test to detect the presence of Mycobacterium tuberculosis. This is the bacteria that causes tuberculosis (TB). These tests use a sample of blood to check for an immune reaction to the bacteria.
TB is an infectious condition that usually affects the lungs but can damage other organs in the body. According to the
While a majority of cases occur in Asia and Africa, the
A doctor can
In this article, we discuss what the TB blood test is, how it works, and who may require the test.
A TB blood test, also called interferon-gamma release assay (IGRA), is a test that detects if a person has a Mycobacterium tuberculosis infection. This is the bacteria that causes TB. Currently, two IGRA tests have Food and Drug Administration (FDA) approval and are available in the U.S.: the QuantiFERON-TB Gold Plus test and the T-SPOT.TB test.
While these blood tests are useful for screening, they can only tell if the person has an infection but cannot determine if it is inactive, also called
- people who have had contact with someone who has TB disease
- Individuals from a country where TB disease is more common
- those who live or work in high-risk settings, such as correctional facilities, nursing homes, and homeless shelters
- healthcare workers
- infants, children, and teens in close contact with adults who are at increased risk for latent TB infection or TB disease
Many people with a latent TB infection may not progress to TB disease. However, certain individuals may be at a higher risk of developing TB disease, including:
- people with HIV
- those who contracted a TB infection in the last 2 years
- babies and young children
- individuals who inject illegal drugs
- immunocompromised people
- older adults
- people who did not receive the correct treatment for TB in the past
Typically, a doctor will not recommend a TB test for people with a low risk of infection with TB bacteria.
If a person suspects a possible exposure or shows symptoms of TB disease, they can speak with a doctor who can order a TB test. A healthcare professional can perform the test in a clinic, doctor’s office, or urgent care center. Some local pharmacies and community health centers may also provide TB tests.
To perform the test, a health practitioner will draw blood samples into a special tube using a needle and deliver them to a laboratory. The test measures interferon-gamma (IFN-g), a protein produced by a person’s T cells. The healthcare professional mixes a sample of the person’s blood with deactivated antigens of the TB bacteria.
The laboratory places the mixture in an environment that promotes immune activity. Then, the laboratory will measure the amount of IFN-g, which reflects the amount of immune system activity. Higher levels of IFN-g indicate a greater likelihood of previous exposure to Mycobacterium tuberculosis.
If a person receives a negative test result, it means their blood did not react to the TB antigens in the test. This indicates that it is unlikely that they have a TB infection. However, it is possible for people with very advanced cases of TB to have a negative result. This is because later stages of TB disease can suppress the immune reaction, causing a false negative.
A positive result can indicate that a person has either latent TB or TB disease. Latent TB refers to an inactive TB infection, meaning a person is not contagious and does not experience symptoms but can develop TB in the future. TB disease is when the bacteria are active, and a person is contagious. A doctor will order more tests to confirm the diagnosis and whether a person has an active or inactive TB infection.
With latent TB, a person’s immune system can fight the infection, preventing the TB bacteria from growing. In these people, the bacteria can remain inactive for a lifetime without causing disease. However, in other cases, especially among those at a higher risk, the bacteria may become active later and cause TB disease.
People with latent TB are more likely to progress to active infection within the first
TB disease occurs when the immune system is unable to stop the bacteria from growing and multiplying. When active, the bacteria causes people to experience symptoms such as fever, pain in the chest, and coughing up blood or sputum. People with TB disease can transmit TB to others.
Many people with latent TB never develop TB disease. However, some people may develop TB disease weeks after exposure to the bacteria. Other people may become ill years later when their immune system weakens for another reason. This is why it is vital to identify and treat the condition while it is still latent.
Treatment for TB
In the U.S., the
For latent TB infection, a doctor may recommend a short course regimen, such as 4 months of daily rifampin or 3 months of once-weekly isoniazid and rifapentine, since they are less toxic to the liver and have higher completion rates than longer courses.
People with TB disease, or active TB infection, need to take anti-TB drugs for 6–9 months. They must take them as prescribed and finish the course of medication. Otherwise, the disease may return or cause the bacteria to become
There are currently 10 drugs with FDA approval for treating TB disease. However, four first-line medications make up the core drugs doctors use in treatment regimens:
There are additional considerations for TB treatment in specific people, such as those with HIV, pregnant people, and children. Moreover, treatment for drug-resistant and multi-drug-resistant TB is complex and requires close consultation with a disease expert.
A vaccine is available for TB, called the
Pregnant people and immunocompromised individuals
TB is a potentially life threatening infectious disease. A TB blood test is an essential tool for screening for the presence of TB bacteria in a person’s body. However, doctors need to run further tests to determine if a person has latent or active TB.
Several treatment regimens are available for people with TB, which vary depending on what type of TB a person has and other considerations such as pregnancy, HIV co-infection, and drug resistance.