QuantiFERON Gold TB Blood Test
QuantiFERON Gold TB Blood Test FAQ
What is a QuantiFERON TB Test?
Why is this test important?
Who gets the QuantiFERON TB Blood Test?
What if I tested positive to a TB skin test previously?
What do my TB blood test results mean?
If my QuantiFERON Test is indeterminate, should I get another test?
Does every lab location perform this blood test?
What type of specimen will I submit?
Do I need to fast to take this test?
How long does it take to get test results?
Accesa Labs does not provide medical advice, diagnosis or treatment. All users should consult with a medical provider for specific health concerns.
More QuantiFERON Gold Test (IGRA Test) Information
The QuantiFERON Gold test, also known as a QuantiFERON Gold TB blood test, or IGRA test, is a blood test that helps with identifying people who might have been exposed to a tuberculosis bacteria known as Mycobacterium tuberculosis, or M. tuberculosis. The QuantiFERON TB test is one of a larger class of blood tests known as IGRA, or interferon-gamma release assay. This IGRA test is made by Qiagen and has become more accepted as an FDA-approved alternative to tuberculosis skin testing to help identify people with latent tuberculosis. Used in more than 130 countries, the QuantiFERON (QFT) test is also well tested with over 30 million tests performed since inception and 1000+ clinical and scientific studies supporting the technology.
The QuantiFERON TB blood test, and IGRA tests in general, work by measuring how reactive one's immune system is when exposed to components of the tuberculosis bacteria. The white blood cells in people who have been exposed to M. tuberculosis in the past will typically release a special chemical called interferon-gamma that can be measured during this blood test. The higher level of interferon-gamma suggests a prior exposure to tuberculosis when compared to the response from someone who has not been previously exposed. Technically speaking, the QuantiFERON TB test engages the body's cell-mediated immune response in reference to the mycobacterial proteins ESAT-6, CFP-10 and TB 7.7. These proteins are not typically part of BCG vaccines which is why QuantiFERON testing is beneficial for people who have received one (see below).
The QuantiFERON test does have some real advantages over the skin TB test from a convenience standpoint. For people not familiar with how the TB skin test (also known as a mantoux test or PPD test) works, it requires a small injection of a purified protein derivative related to tuberculosis and then a recheck 48-96 hours after the initial TB test placement. As a result, the TB test requires two separate office visits. In contrast, the QuantiFERON TB blood test only requires one visit for the necessary blood draw versus the two visits required by the TB skin test.
Additionally, the QuantiFERON Gold TB blood test is the preferred option in anyone who has had the BCG vaccine. The BCG (bacille Calmette-Guerin) vaccine is a tuberculosis vaccine that is given to many people who were born in foreign countries in which TB is prevalent. People who have had the BCG vaccine previously will almost always test positive on the TB skin test, whether or not they have actually been exposed to tuberculosis, because of the cross-reactivity between the vaccine and components of the PPD test. In contrast, QuantiFERON TB blood test results are not affected by prior BCG vaccination which makes it useful for anyone who has received the BCG immunization in the past and, in particular, people who have also tested positive on the TB skin test.
The QuantiFERON TB test has demonstrated improved accuracy in regards to test specificity relative to the skin test (reported >99% versus as low as 59% for the skin test). This increased QuantiFERON test accuracy is likely partially due to the fact that the blood testing results are not subject to reader interpretation error or bias. The TB skin test, on the other hand, requires the person interpreting the test to accurately measure the size of swelling, or induration, on the patient and even one millimeter can make the difference between one being labeled as positive or negative for latent tuberculosis.
Infrequently, people will need to get something called a two-step tuberculosis test for compliance reasons and the QuantiFERON Gold test is superior for these people. During a two-step TB skin test, some people may experience an inadvertent boosting of the second part of their two-step TB test because of the first one. In people who have this boosting response, the TB blood test is more useful as it eliminates any error from this occurring.
There are specific circumstances when the IGRA blood test is preferred over the TB skin test. As mentioned, the TB blood test is helpful for people who have received the BCG vaccination previously. People born outside of the United States typically fall into this demographic. The QuantiFERON Gold blood test is also helpful for healthcare workers who test positive on TB skin tests but have a history of negative chest x-rays. In these situations, the TB blood test can help avoid unnecessary annual radiation exposure from a TB x-ray. Generally speaking, the CDC now recommends that the IGRA test can be used as an alternative to the tuberculosis skin test for screening purposes.
There are some disadvantages to note with this blood test. The QuantiFERON TB blood test does require whole blood for testing which, for the most part, is not of any meaningful significance from the patient's perspective. Because it assesses the white blood cell response, the QuantiFERON test does need to be processed within a specific timeframe for the results to be valid. Given these requirements, this TB blood test also needs to be performed at specific lab locations although advancements in technology have permitted many labs nationwide to perform it. Additionally, the accuracy of this test is not well-known when performed on young children or pregnant women. Additionally, the accuracy of the TB blood test has not been well-researched on people who have impaired immune systems from conditions such as AIDS or who are on immunosuppressive treatments (e.g. steroids, chemotherapy). In these situations, it is possible that the immune system is too compromised to generate the response necessary to get an accurate QuantiFERON Gold result.
If you need to get the QuantiFERON Gold TB blood test near you, you likely have some nearby options available. Most lab locations that we work with are able to perform the test. To get this TB blood test, you will need to go to the lab and the lab staff will draw several vials of blood (typically a Nil tube, TB Antigen tube and a Mitogen tube) from you. After collection, the whole blood sample needs to be incubated for a period of time (no special isolation, washing or culturing required) and the results will be ready after that. If you click on the sample results link above, you can see what a QuantiFERON-TB Gold test report looks like. In this sample lab report, the QuantiFERON test results say negative which indicates a negative results. The numbers reported next to the Nil, Mitogen-Nil and TB-Nil rows indicated that the controls for the test (the additional vials of blood) were successfully performed as a comparison to the actual blood test. More specifically, the Nil vial checks for possible false positives, the mitogen control helps check for false negatives and the TB antigen tube helps with the patient's sample.
A negative QuantiFERON lab report, when accurate, is the best result. It indicates that the evidence of tuberculosis cannot be found in the blood specimen. A negative lab test is similar to a negative TB skin test and does not require a chest x-ray for confirmation or other diagnostic studies. And, if you are wondering how long your QuantiFERON TB blood test result is good for, the answer is it depends on who is asking for it. Broadly speaking, many organizations recommend annual TB blood testing for screening. That being said, every organization has its own recommended guidelines as far as how often one needs to be screened for tuberculosis. Typically, these guidelines have been created in consideration of the risk of possible TB exposure in the work or school environment. The best way to know for sure when you need to get another QuantiFERON TB test is to ask the person or organization requiring the test to see what their requirements are for retesting. If you are simply getting tested for your own reasons, then you will need to decide on your own comfort level for retesting. Someone who has a negative blood test result but is exposed to tuberculosis the very next day can have a positive result on a retest in a short period of time.
Why is routine tuberculosis screening performed and why it is so important? Tuberculosis testing is performed because, when tuberculosis is active, it is a highly contagious disease that is easily communicated through droplet exposure. If an infected person enters a community like a school or workplace with larger crowds of people, there is a much higher chance that other people will get tuberculosis from the infected individual. As a result, tuberculosis is thought of as a public health issue and active efforts are made to minimize the spread of tuberculosis when it does occur. The frequency of tuberculosis blood testing required for school or employment depends on the type of job and can range from every 6 months to annually or longer. And so, when thinking about how often one needs to get a tuberculosis screening test like the QuantiFERON blood test, it is important to ask the organization or person who is requiring the test. Tuberculosis testing conveys one's TB health status at a point in time and regular testing is performed to ensure that people in schools and workplaces are safe when possible exposures to tuberculosis may occur on an ongoing basis.
Although we are relatively insulated from the prevalence and implications of tuberculosis in the United States, the CDC has some startling statistics on the disease. According to them, one third of the world's population is infected with tuberculosis and there were almost two million deaths from TB worldwide in 2015. In the US, approximately three cases per 100,000 persons have occurred over the past several decades. These numbers have remained fairly stable and, fortunately, active TB surveillance programs exist at all levels of government in an effort to protect the public should a new case be discovered. In the US - and in most parts of the world - people who are at higher risk for getting TB are people with HIV, babies and children, and the elderly. Additionally, people with weakened immunes systems from chronic medical conditions, people who inject illegal drugs and people in correctional facilities may also be at a greater risk.
Because the QuantiFERON TB blood test is a relatively new test, it has only recently come of age within the traditional medical community. As such, facilities, organizations and companies that previously only accepted TB skin test results are now embracing the QuantiFERON test as a useful alternative for healthcare and educational workers. Often requested with immunity tests such as the Hepatitis B titer and MMR titer, the QuantiFERON Gold test will likely establish itself as the gold standard for tuberculosis screening in the years to come.
Helpful Results Overview
Here is how to interpret your QuantiFERON test results:
We hope you found this TB blood test lab review helpful.
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