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What To Know About IGRA TB Tests
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 World Health Organization (WHO), TB is the 13th-leading cause of death globally and the second-leading cause of death by an infectious disease worldwide after COVID-19. In 2018, roughly 23% of the world's population had a TB infection, and evidence suggests it claims 1.5 million lives each year.
While a majority of cases occur in Asia and Africa, the Centers for Disease Control and Prevention (CDC) state that up to 13 million people in the United States are living with a latent TB infection, and doctors diagnosed 7,174 cases in the U.S. In 2020.
A doctor can either use a blood or skin test to screen for TB. A healthcare provider will select which test is more suitable by considering factors such as the reason for testing, test availability, and cost.
In this article, we discuss what the TB blood test is, how it works, and who may require the 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 latent TB, or an active TB disease. Medical history, physical examination, chest X-rays, and further laboratory testing are necessary to determine whether a person has TB disease.
The CDC recommend that certain people should test for TB infection because they are at higher risk. Those at higher risk for infection by TB bacteria include:
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:
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.
There are two kinds of TB-related infections: inactive or latent TB infection and active or TB disease. Not everyone with a TB infection becomes sick.
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 2 years of infection. Without treatment, evidence suggests that 5–10% of people with a TB infection will develop TB disease.
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 depends on whether the person has latent TB infection or TB disease. Treating latent TB can significantly reduce the risk of it progressing to TB disease and help control the spread of bacteria.
In the U.S., the CDC recommends several regimens to treat the infection, using one or a combination of the following drugs:
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 resistant to these medications.
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 Bacille Calmett-Guérin vaccine. Countries with high TB rates use the vaccine, but health experts do not recommend it for use in the U.S. Except for those at a higher risk, such as healthcare workers and children.
Pregnant people and immunocompromised individuals should not take the vaccine. People with the vaccine may receive a false-positive TB skin test, but it does not affect the results of a TB blood test.
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.
Even When You Don't Have TB Symptoms, This Century-old Test Can Still Detect The Disease.
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New Blood Test Can Help Doctors Diagnose Tuberculosis And Monitor Treatment
Researchers at Tulane University School of Medicine have developed a new highly sensitive blood test for tuberculosis (TB) that screens for DNA fragments of the Mycobacterium tuberculosis bacteria that causes the deadly disease.
The test could give doctors a new tool to both quickly identify TB and then gauge whether drug treatments are effective by monitoring levels of DNA from the pathogen circulating through the bloodstream, according to a new study published in the journal The Lancet Microbe.
Tuberculosis is now the second most deadly infectious disease in the world, behind only COVID-19. In 2020, an estimated 10 million people contracted TB and 1.5 million people died from it, according to the World Health Organization.
Most TB tests rely on screening sputum, a thick type of mucus from the lungs. But collecting sputum from patients suspected of having TB can be difficult, especially for children. TB can also be harder to diagnose in immunocompromised HIV patients and others where the infection migrates outside of the lungs into other areas of the body. In these extrapulmonary cases, patients can have little bacteria in the sputum, which leads to false negatives using current testing methods, said lead study author Tony Hu, PhD, Weatherhead Presidential Chair in Biotechnology Innovation at Tulane University.
"This assay may be a game-changer for TB diagnoses that not only provides accurate diagnosis results but also has the potential to predict disease progression and monitor treatment," Hu said. "This will help doctors rapidly intervene in treatment and reduce the risk of death, especially for children living with HIV."
The study evaluated a CRISPR-based assay that screened for cell-free DNA from live Mycobacterium tuberculosis bacilli. The screening target is released into the bloodstream and cleared quite rapidly, providing a real-time snapshot of active infection.
Researchers tested preserved blood samples from 73 adults and children with presumptive TB and their asymptomatic household contacts in Eswatini, Africa.
The test identified adult TB with 96.4% sensitivity and 94.1% specificity and pediatric TB with 83.3% sensitivity and 95.5% specificity. (Sensitivity refers to how well a test can diagnose a positive case, while specificity is a measure of a test's accurately determining a negative case.)
Researchers also tested 153 blood samples from a cohort of hospitalized children in Kenya. These were HIV-positive patients who were at high risk for TB and presented with at least one symptom of the disease. The new test picked up all 13 confirmed TB cases and almost 85% of unconfirmed cases, which were cases that were diagnosed due to clinical symptoms and not existing gold standard testing methods.
The CRISPR-based test uses a small blood sample and can deliver results within two hours.
"We are particularly excited that the level of Mycobacterium tuberculosis cell-free DNA in HIV-infected children began to decline within a month of treatment, and most of the children's blood was cleared of the bacteria DNA fragments after treatment, which means that CRISPR-TB has the potential to monitor treatment and will give physicians the ability to better treat worldwide TB infections," Hu said.
The researchers have since adapted the assay to a rapid test platform that can deliver results in 30 minutes without any special equipment. Results would be viewable on a paper strip like a rapid COVID-19 test.
"A highly accurate, rapid blood test that could be used anywhere would benefit millions of people living in resource-limited areas with a high TB burden," Hu said.
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