AKA IGRAs
Interferon-gamma release assay
QuantiFERON-TB gold in-tube assay
Tuberculosis assay
InBrief
by Jay A. Zaslow, MD, MPH and
The QuantiFERON-TB gold assay is a blood test used to detect latent tuberculosis (TB) infection (LTBI).
It is the most commonly used interferon-gamma release assay (IGRA) used to diagnose infection with Mycobacterium tuberculosis. In patients infected with M tuberculosis, white blood cells release interferon-gamma when exposed to M tuberculosis antigens. Results can be available to the patient within 24 hours.
The QuantiFERON-TB gold blood test has a sensitivity of 80%-90% and a specificity of 95% for LTBI. It is not designed to diagnose active tuberculosis, which requires microbiologic testing (ie, sputum acid-fast bacilli and culture).
Advantages of IGRAs
One practical advantage of IGRAs over the TB skin test (TST) is that the patient does not need to return for a second visit for test interpretation. Unlike the TST, the IGRA is not affected by Bacillus Calmette-Guerin (BCG) vaccination status. The PPD has a specificity as low as 60% in individuals from countries where BCG vaccination is used. Algorithms are available to confirm a positive PPD using the QuantiFERON-TB gold test in low-risk individuals suspected of having a false-positive PPD (eg, those having BCG vaccination, serial testing, or cross-reactivity from other nontuberculous mycobacterium strains).
Disadvantages and Limitations of IGRAs
Blood samples must be processed within 8-30 hours after collection while white blood cells are still viable.
Errors in collecting or transporting blood specimens or in running and interpreting the assay can decrease the accuracy of IGRAs.
There is limited data on the use of IGRAs to predict who will progress to TB disease in the future.
There is limited data on the use of IGRAs for:
Children younger than 5 years of age;
Persons recently exposed to M. tuberculosis;
Immunocompromised persons; and
Serial testing.
Tests may be expensive.
Pearl to Know
As with TST, live virus vaccines might affect IGRA test results. However, the effect of live virus vaccination on IGRAs has not been studied. Until additional information is available, IGRA testing in the context of live virus vaccine administration should be done as follows:
Either on the same day as vaccination with live-virus vaccine or 4-6 weeks after the administration of the live-virus vaccine
At least one month after smallpox vaccination
References:
CDC: MMWR 12/16/05 54(RR15) 49-55
IGRAs– Blood Tests for TB Infection Fact Sheet. Interferon-Gamma Release Assays (IGRAs) – Blood Tests for TB Infection. Division of Tuberculosis Elimination, National Center for HIV, Viral Hepatitis, STD, and TB Prevention, Centers for Disease Control and Prevention. Page last reviewed: May 4, 2016.
STD, and TB Prevention, Centers for Disease Control and Prevention. Page last reviewed: October 19, 2021
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