Interferon Gamma Release Assay for TB Testing

An interferon gamma release assay, commonly called an IGRA, is a blood test used to detect whether your immune system has encountered the bacterium that causes tuberculosis. It works by measuring how strongly your white blood cells release a signaling molecule called interferon-gamma when exposed to proteins unique to Mycobacterium tuberculosis. The test has become a mainstay of TB screening worldwide, largely because it sidesteps one of the oldest frustrations in TB diagnostics: false positives from the BCG vaccine. But the test has blind spots, quirks, and population-specific wrinkles that matter for anyone trying to interpret a result.

What Happens in the Lab

The basic idea behind an IGRA is straightforward. A blood sample is mixed with synthetic peptides that mimic proteins found in M. tuberculosis, specifically antigens called ESAT-6 and CFP-10, which are encoded by a genomic region present in the TB bacterium but absent from the BCG vaccine strain and nearly all environmental mycobacteria. If your T cells have previously been “trained” by a real TB infection, they recognize these peptides and release interferon-gamma in response. The lab measures how much interferon-gamma appears. Above a defined threshold, the result is positive.

Two controls run alongside the TB-specific tube. A “nil” tube contains no stimulant and captures your baseline interferon-gamma level; a “mitogen” tube uses a broad immune stimulant to confirm your immune cells are capable of responding at all. If the mitogen tube fails to trigger a response, the result is flagged as indeterminate rather than negative, because the test cannot confidently say your T cells had a fair chance to react.

The Two Commercial Platforms

Two IGRAs dominate clinical use globally. The QuantiFERON-TB Gold series (now in its Plus iteration) measures interferon-gamma concentration in plasma using an ELISA. The T-SPOT.TB counts individual interferon-gamma-producing cells as spots on a membrane. A large head-to-head comparison of both tests in TB patients found excellent agreement between them, with concordance over 90 percent and a kappa value of 0.82. T-SPOT.TB produced fewer indeterminate results, under 1 percent compared with about 4 percent for the QuantiFERON test, and its sensitivity and specificity were slightly higher, though the differences did not reach statistical significance.¹ In practice, most guidelines treat the two as interchangeable for screening purposes, though some clinicians prefer T-SPOT.TB for patients whose immune systems may be weakened, given its marginally lower indeterminate rate.

Why IGRAs Beat the Skin Test in BCG-Vaccinated People

The tuberculin skin test, or TST, has been used for over a century and remains cheap and widely available. Its Achilles’ heel is BCG vaccination. Because BCG shares many proteins with M. tuberculosis, people who received BCG as children can mount a skin reaction that looks identical to a true positive, even if they have never been infected with TB. This is a big deal: BCG is given at birth in much of the world.

IGRAs largely avoid this problem because the antigens they use, ESAT-6 and CFP-10, are not present in the BCG vaccine. Studies in BCG-vaccinated children have consistently shown this advantage. In one study of BCG-vaccinated children in eastern Egypt, IGRA demonstrated higher specificity for M. tuberculosis and fewer cross-reactions with BCG and non-tuberculous mycobacteria than the TST.² Another study, looking at BCG-vaccinated children and adolescents who were and were not exposed to a contagious TB case, found that all positive IGRA results occurred exclusively among those with known TB contact, while none of the unexposed, BCG-vaccinated children tested positive.³ For populations with high BCG coverage, this specificity advantage alone can justify choosing an IGRA over a TST.

The Biggest Limitation: Active Versus Latent

A positive IGRA tells you that your immune system has encountered TB. It does not tell you whether the bacteria are dormant or actively making you sick. This inability to distinguish active TB from latent TB infection is the single most important limitation of the test and one that surprises many patients who assume a blood test should deliver a definitive diagnosis.⁴ In children with negative sputum smears, where confirming active TB by culture is often difficult, the IGRA result and the magnitude of the immune response have both been shown unable to separate active disease from latent infection.⁵

This means clinicians still rely on chest imaging, microbiology, and the patient’s symptoms and exposure history alongside the IGRA result. A positive IGRA in someone with a cough and chest X-ray abnormalities points toward active TB. The same positive result in a healthy contact of a TB case, with a normal X-ray, points toward latent infection. The test contributes a piece of the puzzle, not the whole picture.

How Progression Risk Compares

One area where IGRAs pull ahead of the skin test is in predicting which infected people will eventually develop active TB. A meta-analysis of head-to-head comparisons found that the relative risk of progression to active disease was higher in IGRA-positive individuals than TST-positive individuals, with pooled estimates of roughly 5.4 for IGRA versus 3.0 for TST, although the difference did not quite reach statistical significance.⁶ A large UK prospective cohort study found that T-SPOT.TB-positive individuals progressed at a rate of about 13 per 1,000 person-years, somewhat higher than QuantiFERON’s rate of roughly 10 per 1,000 person-years, and both outperformed the traditional TST thresholds as predictors of progression.⁷

Still, the positive predictive value for progression remains modest in absolute terms. Across all studies using commercial IGRAs, pooled positive predictive value for progression was about 2.7 percent, rising to roughly 6.8 percent when only high-risk groups were considered.⁸ So even a positive IGRA means the vast majority of people will never develop active TB. The test is better at ruling out progression (its negative predictive value is high) than ruling it in.

Indeterminate Results and Pre-Analytical Headaches

An indeterminate IGRA result is not a gray zone between positive and negative. It means the test failed to run properly, usually because the mitogen control tube did not provoke enough interferon-gamma to prove the blood cells were working. In one analysis of indeterminate results, almost all of them, 97 percent, were caused by a low mitogen response rather than a high background nil value.⁹

One surprisingly common cause of indeterminate results is simply a delay in getting the blood tube into the incubator. A study testing different delay intervals found zero indeterminate results when incubation started immediately, compared to 10 percent with a six-hour delay and 17 percent with a twelve-hour delay. The culprit was a drop in mitogen responsiveness as cells sat at room temperature.¹⁰ This is a practical problem: if a clinic draws blood late in the day and the lab does not process it until the next morning, the risk of an indeterminate result climbs substantially. Clinicians ordering IGRAs need to coordinate timing with their lab, something not always mentioned at the point of care.

Immunosuppressed Patients

People on medications that suppress the immune system face a dual problem with TB screening: they are at higher risk of TB reactivation and simultaneously less able to produce the immune response the IGRA relies on. A systematic review and meta-analysis found that patients receiving immunosuppressive therapy were significantly less likely to test positive on an IGRA than patients not on these drugs, with the effect most pronounced in those receiving anti-TNF biologics.¹¹ The concern is that a negative result in this population could be falsely reassuring.

In people living with HIV, the picture is similarly complicated. A meta-analysis found pooled sensitivity of about 72 percent for T-SPOT.TB and 61 percent for QuantiFERON in HIV-positive individuals with active TB in lower-income countries. Neither IGRA was consistently more sensitive than the TST in direct comparisons, and the differences between the three tests were small.¹² Many guidelines now recommend using both an IGRA and a TST in immunocompromised patients, treating either positive result as grounds for further evaluation.

Children

A common question from parents is whether IGRAs are reliable in young children. This concern is not unfounded: young children have immature immune systems, and the TST has a well-documented track record of both false positives (from BCG) and false negatives (from immune immaturity) in this age group. A meta-analysis focused on children found pooled IGRA sensitivity of 85 percent and specificity of 94 percent, with T-SPOT.TB performing marginally better than QuantiFERON on sensitivity.¹³

Children under age two have historically been flagged as a tricky subgroup, and some older guidelines recommended TST over IGRAs for this group. More recent evidence has pushed back. One study pooling results from four cohorts of high-risk children under two found that none of 575 untreated children with a negative IGRA progressed to active TB, including 70 who had a positive TST but a negative IGRA.¹⁴ A separate meta-analysis of children under five found IGRA sensitivity of about 87 percent and specificity of 98 percent when compared with TST.¹⁵ The practical takeaway is that IGRAs are now considered an acceptable alternative to TST even in very young children, though some clinicians still use both tests in high-risk toddlers for extra safety.

Pregnancy

Pregnancy suppresses certain arms of the immune system as part of normal tolerance of the fetus, and this affects IGRAs in measurable ways. In a study of HIV-infected women, the interferon-gamma response to TB antigens was significantly lower during pregnancy than in the postpartum period, and the IGRA indeterminate rate was 16 percent during pregnancy versus zero percent postpartum.¹⁶ A cohort study in India confirmed this pattern more broadly, finding that significantly fewer women remained IGRA-positive at delivery compared to the antepartum period, with interferon-gamma levels at their lowest around the time of birth before rebounding postpartum.¹⁷

One reassuring finding is that IGRA percent positivity remained relatively stable between early pregnancy and delivery in one cohort, even as the magnitude of the interferon-gamma response dropped. It was postpartum that both IGRA and TST positivity rates jumped, likely reflecting immune reconstitution after delivery.¹⁸ For clinicians, this means a negative IGRA during pregnancy deserves more caution than the same result in a non-pregnant person. When screening is not urgent, some experts prefer to test after delivery.

Serial Testing in Healthcare Workers

Healthcare workers in hospitals and clinics are often screened for TB at regular intervals. This is where IGRAs get messy. The test was designed as a one-time diagnostic, not a surveillance tool, and serial use reveals an awkward amount of fluctuation around the positivity cutoff.

A study of Canadian healthcare workers with initially negative results found a QuantiFERON conversion rate of about 5 percent using the manufacturer’s standard threshold. When stricter thresholds were applied, that rate dropped to around 2 percent. More strikingly, among the 13 workers who had tested positive at baseline, 8 reverted to negative on retesting, a reversion rate of over 60 percent.¹⁹ A similar study in the United States found QuantiFERON conversion rates in the range of 7 to 12 percent depending on the threshold used, while reversions also occurred.²⁰ In a Nigerian cohort, IGRA reversion rates ran even higher, around 36 to 40 percent over follow-up.²¹

Much of this churn likely reflects biological variability near the test’s cutoff rather than true new infections and clearances. It puts occupational health programs in a bind: treat every conversion as a new infection and you will over-treat, but ignore them and you might miss someone who genuinely picked up TB. Many programs now repeat a positive IGRA or apply a higher threshold before committing a healthcare worker to a course of preventive therapy.

Does the Skin Test Interfere with Future IGRA Results?

A practical question that comes up in clinic is whether getting a TST can “boost” a subsequent IGRA result and create a false positive. The TST injects tuberculin into the skin, and in someone previously sensitized to TB antigens, this could theoretically ramp up circulating T-cell responses and push a borderline-negative IGRA into positive territory. Research on this has been reassuring for most patients: in individuals with negative TST results, having the TST performed did not significantly change subsequent IGRA results. However, in people who were already TST-positive, there was evidence that the skin test could boost interferon-gamma responses, potentially tipping an IGRA that might have been borderline.²² The practical rule of thumb most guidelines follow is to draw blood for the IGRA before placing the TST, or at least on the same day, rather than after the skin test has had days to percolate.

Cost-Effectiveness

IGRAs are more expensive per test than a TST. A skin test requires tuberculin solution, a syringe, and two clinic visits (one to place, one to read), while an IGRA requires a single blood draw but costly reagents and laboratory processing. Whether the IGRA saves money overall depends on how many TST false positives you avoid, since each false positive triggers chest X-rays, clinical evaluations, and sometimes unnecessary preventive treatment. A systematic review of cost-utility analyses found consistent evidence that IGRAs are cost-effective in high-income countries when used in high-risk populations, such as immigrants from TB-endemic regions and close contacts of active cases.²³ Modeling work on preimmigration screening for migrants to low-incidence countries also favored an IGRA-first strategy.²⁴ In low- and middle-income, high-burden settings, the economics are less clear, and many programs still rely on the TST or use a combination approach.

Beyond TB: Adapted IGRA Platforms

The IGRA principle of stimulating T cells with pathogen-specific peptides and measuring interferon-gamma can be repurposed for infections other than tuberculosis. One area of active development involves cytomegalovirus, or CMV, which is a concern after organ transplantation. Researchers have used a commercially available interferon-gamma release assay adapted for CMV peptides to monitor the recovery of CMV-specific immunity in kidney transplant recipients, aiming to individualize how long patients stay on antiviral prophylaxis.²⁵ The concept is the same: if a patient’s T cells can mount a strong interferon-gamma response to CMV peptides, they may no longer need the drug to keep the virus in check.

Next-Generation Assays and What They Might Solve

The newest QuantiFERON iteration, QFT-Plus, adds a second antigen tube designed to elicit both CD4 and CD8 T-cell responses. The original tubes primarily stimulated CD4 cells. The hope was that comparing the two tubes could flag recent infections, since CD8 T-cell responses appear to be more prominent early after TB exposure. One study found that the difference between the two tubes showed a significant association with recent TB exposure, particularly in patients with higher levels of contact.²⁶ However, when researchers compared responses in children with active TB versus latent infection, the two QFT-Plus tubes largely overlapped and could not reliably tell these groups apart.²⁷

That same study explored a different antigen, HBHA (heparin-binding hemagglutinin), and found that HBHA-induced interferon-gamma responses were significantly higher in children with latent infection than in those with active TB. If validated in larger cohorts, an HBHA-based IGRA could eventually help solve the fundamental limitation current IGRAs share: the inability to separate latent infection from disease. For now, though, these are research tools rather than clinic-ready tests, and the standard IGRA remains a detector of infection with no built-in staging capability.