Interferon-Gamma Release Assays (IGRAs) are tests that can help to diagnose whether or not a person may be infected by tuberculosis. There are two IGRA blood tests that have been approved and are commercially available today. The T-SPOT blood test and the QuantiFERON Gold blood test will both give medical providers about the possible presence of TB.
These blood tests work by measuring the immune reactions to the presence of tuberculosis. Most people who have been infected with TB will release the interferon-gammas when mixed with the antigens produced to combat the infection. For accurate results, the blood for the test must be mixed with antigens so that measurements can occur.
How Accurate Are the IGRA Blood Test Results?
The benefit of the IGRA blood test is that it only requires a single patient visit to the doctor. Unlike the under-the-skin TB tests, individuals simply have their blood taken and then are given the results, which are often available in 24 hours or less. The IGRA tests do not boost immune system responses with future tests like the skin tests do and prior vaccination against tuberculosis will not cause false results like other tests may.
Routine testing is not recommended.
What Do the Test Results Mean?
The issue with the IGRA blood test is that it cannot determine if a TB infection is active or passive. If someone has had TB in the past, then they may have a positive blood test result even though they are not currently infected. The blood test must be processed within 30 hours of collection, sometimes in as few as 8 hours, to make sure the white blood cells are still viable. Errors in transportation, collection, or testing can produce false positives.
Most people will have test results that are normal. For those that may have TB, either active or passive, then the antigens will produce results when mixed with the blood. The QFT blood test will measure the interferon-gamma concentrations to determine if the results are abnormal, while the total number of producing cells in the T-SPOT test will be counted.
Positive results do not necessarily mean that there is an active or passive TB infection. It simply increases the likelihood that an individual has been exposed to TB at some point in time.
Although there are advantages to the IGRA blood test, there are some specific disadvantages that may produce inaccurate results. Children younger than 5 will not receive accurate results whatsoever. If TB exposure was recent, then there is a possibility that the blood test may not react to the antigens after exposure. People who require serial TB testing or have a compromised immune system may also have inaccurate results.
What Is the Problem With IGRA Blood Tests?
The issue that medical providers face is that multiple negative IGRA blood test results cannot specifically exclude there being a TB infection. Even if every possible TB test that is available today were to come back as negative, there would still be a chance that tuberculosis would be present and active within the body. For this reason, only when there is a clinical possibility of TB should these blood tests be ordered.
Live virus vaccines may also affect the outcome of the blood test results. The complete interaction between vaccines and IRGA blood tests has never been studied, however, so outcomes from a recent vaccination may be mixed. The test results will be inaccurate for up to 30 days after someone receives a smallpox vaccination.
The advantage is that IRGA blood test results are not subjective. There is a defined set of criteria that guide medical providers to a result that is either positive or negative. Unlike the skin tests for TB, no physical examination of the test site is necessary to determine a positive result. This allows for more accuracy, fewer trips to the doctor, or the need to have test results redone if a 72 hour skin test window is missed.
The IGRA blood tests help to screen high risk populations for TB. If suspected, another test may be ordered or medication may be prescribed to treat the infection. The results may not be 100% accurate for all population demographics, but the benefits of this blood test certainly outweigh any risks that may exist.