What Is the QuantiFERON TB Test and How Does It Work?

The QuantiFERON-TB test is a blood-based diagnostic that detects tuberculosis infection by measuring the immune system’s response to proteins specific to the TB bacterium. Unlike the older tuberculin skin test, it requires a single blood draw, delivers results in about 24 hours, and avoids false positives from prior BCG vaccination. Since its introduction, the test has gone through several generations, with the current version (QuantiFERON-TB Gold Plus, or QFT-Plus) adding the ability to stimulate a broader range of immune cells. The test is widely used for screening latent TB infection, but its strengths and blind spots are more nuanced than many people realize.

How the Test Works

The QuantiFERON test relies on a straightforward immunological principle. When someone has been infected with Mycobacterium tuberculosis, their immune system develops T cells that recognize specific TB proteins. In the lab, a blood sample is mixed with synthetic versions of two proteins found in the TB bacterium, called ESAT-6 and CFP-10. If the person’s T cells have seen TB before, they react by releasing a signaling molecule called interferon-gamma (IFN-γ). The test measures how much IFN-γ appears in the sample. A result above 0.35 IU/mL in the antigen tube, after subtracting the background level, is reported as positive.

The blood is drawn into specially designed tubes. One tube contains the TB-specific antigens. A “nil” tube has no stimulant and serves as a baseline to check for background noise. A “mitogen” tube contains a substance that should activate any healthy immune cells, acting as a positive control. If the mitogen tube fails to produce a response, the test cannot confirm that the immune system was capable of responding at all, and the result is reported as “indeterminate” rather than positive or negative.

The BCG Advantage

One of the biggest practical reasons clinicians prefer the QuantiFERON test is that it sidesteps the problem of BCG vaccination. The BCG vaccine, given at birth or in childhood in much of the world, uses a live strain of Mycobacterium bovis to protect against severe TB in children. Because BCG shares many proteins with the TB bacterium, the tuberculin skin test often comes back positive in BCG-vaccinated people who were never actually infected with TB. This creates a headache for clinicians trying to decide who genuinely needs treatment.

The QuantiFERON test uses ESAT-6 and CFP-10, proteins that are present in M. tuberculosis but absent from the BCG vaccine strain. That means BCG vaccination should not trigger a false positive on the QuantiFERON test.¹ A study of healthcare workers who had all been BCG-vaccinated found that among those with a skin test result of 15 mm or larger, more than half were QuantiFERON-negative, suggesting many of those skin test positives were driven by BCG rather than real TB infection.² In populations with high BCG coverage, this difference matters enormously. It can mean the difference between treating a real infection and putting someone through months of unnecessary medication.

How It Compares to the Tuberculin Skin Test

The tuberculin skin test (TST) has been around for over a century and remains the most commonly used TB screening tool worldwide, largely because it is cheap and requires no laboratory equipment. But the QuantiFERON test outperforms it in several specific scenarios. Among newly arrived asylum seekers, one study found that half of those who were TST-positive tested negative on the QuantiFERON, while agreement between the two tests was higher in people who had never been vaccinated with BCG.³ When researchers used both blood-based tests as a reference standard, the skin test’s sensitivity was only about 72% at a 10 mm cutoff and dropped to about 40% at a stricter 15 mm cutoff.⁴

The QuantiFERON test also appears more reliable in people with advanced liver disease. A study of patients awaiting liver transplantation showed that the skin test was significantly less likely to come back positive in those with the most severe liver dysfunction, while the QuantiFERON result was not affected by how sick the liver was.⁵ This makes intuitive sense: the skin test depends on a functioning immune response in the skin itself, which can be impaired by malnutrition, low protein levels, and the immune suppression that often accompanies organ failure. The blood test bypasses the skin altogether.

From QFT-GIT to QFT-Plus

The current generation of the test, QFT-Plus, replaced the older QFT-Gold In-Tube (QFT-GIT) version around 2016. The upgrade added a second antigen tube called TB2. The original antigen tube (now called TB1) uses longer protein fragments that mainly activate CD4 T cells. The new TB2 tube adds shorter peptide fragments designed to also stimulate CD8 T cells, a different branch of the immune system that becomes active during more intense or recent TB exposure.⁶

Research has shown that the TB2-specific CD8 response is more strongly associated with active TB disease than with latent infection.⁷ This sparked hope that the QFT-Plus might be able to distinguish active TB from latent infection. In practice, though, that hope has not panned out for routine clinical use. A study directly assessing this question concluded that the QFT-Plus does not reliably discriminate between active and latent TB.⁸ The test tells you someone has been infected; it cannot tell you whether they are currently sick.

Where the QFT-Plus does show a practical edge over the older version is in immunocompromised patients. In people receiving long-term immunosuppressant therapy, the QFT-Plus detected more cases of latent TB than the QFT-GIT did, likely because the added CD8 stimulation picks up immune responses that still function even when CD4 cells are suppressed.⁹

When the Test Returns “Indeterminate”

An indeterminate result is neither positive nor negative. It means the test cannot be interpreted, usually because the mitogen control tube failed to produce a strong enough immune response. This does not mean the person has or does not have TB; it means the blood sample did not perform as expected.

Several factors raise the odds of getting an indeterminate result. The strongest predictors are conditions that suppress immune function. A study during the COVID-19 pandemic found that severe COVID-19 carried roughly four times the odds of an indeterminate result, and being hospitalized for non-COVID reasons carried a similar risk. Pharmacological immunosuppression, severe lymphopenia, and anemia were also independently associated with indeterminate outcomes.¹⁰

Pre-analytical handling matters too. One study found that low albumin levels in the patient’s blood were the single strongest predictor of an indeterminate result, with about seven times the odds compared to patients with normal albumin. But a surprising contributor was who drew the blood: samples collected by staff other than trained phlebotomists were three times more likely to produce indeterminate results, probably due to improper tube handling or mixing.¹¹ And delays in getting the blood tubes into the incubator cause real problems. Compared to immediate incubation, even a six-hour delay led to about one in five positive samples reverting to negative.¹² If you have a QuantiFERON test done at a clinic that ships samples to a distant lab, the timing of that transport can quietly affect accuracy.

The Test in People Living with HIV

HIV was an early concern for interferon-gamma release assays because the virus specifically attacks CD4 T cells, the very cells the test depends on to produce a response. In theory, a person with a very low CD4 count might not mount enough of an immune reaction for the test to detect, leading to a false negative. Some older studies supported this worry, particularly for the previous-generation assay.

More recent data on the QFT-Plus are somewhat reassuring. In a study of nearly 700 people living with HIV in a low TB-burden setting, only about 1% of tests came back indeterminate, and there was no association between indeterminate results and CD4 or CD8 cell counts.¹³ The addition of the TB2 tube in the QFT-Plus, which stimulates CD8 cells alongside CD4 cells, may help maintain sensitivity even when CD4 counts are depleted. Still, interpreting results in people with severe immunosuppression requires clinical judgment, and a negative result does not rule out TB infection in someone at high risk.

Using the Test in Children

Diagnosing TB in children is notoriously difficult. Children often cannot produce sputum for culture, their chest X-rays can look atypical, and the skin test is unreliable in very young children whose immune systems are still maturing. The QuantiFERON test offers a more objective readout, but its performance in children is not identical to adults.

The sensitivity of the QFT-Plus for active TB in children ranges from roughly 55% to 87% depending on the study, with no clear difference between children under five and those five and older.¹⁴ One study of nearly 200 children found that about 83% of those with confirmed active TB had a positive QFT-Plus, and the rate of indeterminate results in children screened for latent infection was low, around 2.5%.¹⁵ That said, a sensitivity ceiling of 87% means roughly one in eight children with active TB could be missed. The test is useful, but no clinician should use a negative QuantiFERON result alone to rule out TB in a child with suggestive symptoms.

Extrapulmonary TB and Where the Test Struggles

TB does not always confine itself to the lungs. It can affect lymph nodes, bones, the brain, the lining of the heart, the gut, and many other organs. Diagnosing these extrapulmonary forms is challenging because tissue sampling is often invasive and cultures from these sites are frequently negative. The QuantiFERON test can help fill the gap, but its performance varies substantially by site of disease.

One study found an overall false-negative rate of about 29% in patients with extrapulmonary TB. The rate was lowest for lymph node and gastrointestinal TB and substantially worse for central nervous system TB, where two-thirds of possible cases were QuantiFERON-negative.¹⁶ Similarly, a study focusing on cervical lymph node TB found relatively good sensitivity of about 86%, but skeletal TB sensitivity dropped to just 45%.¹⁷ A separate study comparing the QuantiFERON to the skin test in extrapulmonary cases found the blood test was consistently more sensitive, with an overall sensitivity of 86% versus 57% for the skin test.¹⁸ So the QuantiFERON is better than the skin test for extrapulmonary TB, but it still misses a meaningful fraction of cases, and it should never be the sole basis for ruling the diagnosis in or out.

Serial Testing and the Conversion Problem

Healthcare workers, prison staff, and others with ongoing TB exposure are sometimes tested repeatedly over time. This is where the QuantiFERON test reveals a frustrating quirk: results can bounce around the cutoff point without any new TB exposure. A study of Canadian healthcare workers found a conversion rate of about 5% among those with a previously negative result, yet none of the conversions were linked to any identifiable TB exposure, and 62% of those who had been positive on the first test reverted to negative on the second.¹⁹

A study of U.S. healthcare workers showed a similar pattern: QuantiFERON conversion rates ranged from about 7.5% to 11.6% depending on the definition used, which was comparable to skin test conversion rates. Among those who were positive at baseline but whose skin test had been negative (discordant results), 70% reverted to negative on retesting.²⁰ German healthcare workers showed conversion rates between about 2.5% and 6% depending on how strictly “conversion” was defined, and using a wider “uncertainty zone” around the cutoff value helped reduce these unstable fluctuations.²¹

The practical lesson here is that a single borderline QuantiFERON result in a repeatedly tested, low-risk person should be interpreted cautiously. Many apparent conversions near the 0.35 IU/mL cutoff reflect biological variability in immune response rather than new infection. Some guidelines recommend using a higher threshold or a “borderline zone” when interpreting serial results in low-risk populations to reduce unnecessary treatment triggered by these fluctuations.

Why It Cannot Monitor Treatment

A common question from patients and clinicians alike is whether the QuantiFERON test can track whether TB treatment is working. The short answer is no. A systematic review of studies examining this question found no consistent pattern in how results changed during treatment. In most studies, the majority of patients remained QuantiFERON-positive even after completing a full course of therapy.²² One study reported that about 62% of patients were still positive when they finished treatment, and reversions mostly happened in people whose baseline interferon-gamma levels were close to the cutoff to begin with.²³ While average interferon-gamma levels tend to drop somewhat during treatment, the variation between individual patients is so large that the test provides no reliable signal for any single person.

QuantiFERON During Pregnancy

Pregnancy suppresses certain parts of the immune system, which raises the question of whether TB testing remains accurate. The evidence actually suggests the QuantiFERON test holds up better during pregnancy than the skin test does. In a study of HIV-infected women in a high-burden setting, significantly more women tested positive on the QuantiFERON than on the skin test during pregnancy, and this gap persisted postpartum as well.²⁴

Another study tracking women across pregnancy stages found that the QuantiFERON percent positivity remained more stable between the antepartum period and delivery compared to the skin test, whose positivity rate fluctuated more. Interestingly, median interferon-gamma concentrations did dip at the time of delivery but rebounded postpartum.²⁵ For HIV-positive pregnant women specifically, interferon-gamma responses tend to be lower overall, which has led some researchers to suggest that a lower cutoff value might be worth considering for this group.²⁶ No guideline has formally adopted a lower threshold yet, but it is an active area of discussion.

False Positives from Nontuberculous Mycobacteria

Although the QuantiFERON test is designed to respond only to proteins from the TB complex and not to BCG or most environmental mycobacteria, rare exceptions exist. A case report documented a QuantiFERON conversion caused by Mycobacterium gordonae, a water-dwelling mycobacterium that occasionally colonizes plumbing and medical devices. The patient had no risk factors for TB, and the false positive was attributed to the environmental organism.²⁷ This appears to be uncommon, but it is worth knowing that “not affected by BCG” does not mean “immune to all false positives.” In low-risk populations where the pre-test probability of real TB infection is very small, even a rare false-positive rate can produce confusing results.

QuantiFERON vs. T-SPOT.TB

The QuantiFERON is not the only blood-based TB test. The T-SPOT.TB assay uses a different laboratory technique (counting individual T cells that respond to the same ESAT-6 and CFP-10 proteins rather than measuring the total interferon-gamma released). In a head-to-head comparison among febrile patients being evaluated for active TB, the T-SPOT.TB had a sensitivity of about 95% versus 92% for the QFT-Plus, a difference that was not statistically significant. The T-SPOT.TB produced no indeterminate results in that study, compared to about 3% for the QFT-Plus.²⁸ In practice, both tests perform similarly for most clinical scenarios. The QuantiFERON is easier to automate and process in high-volume labs, which has made it the more common choice in many health systems. The T-SPOT.TB may have a slight edge in severely immunosuppressed patients because of its lower indeterminate rate, though the QFT-Plus narrowed that gap compared to the older QFT-GIT.

Cost-Effectiveness Depends on the Setting

Whether the QuantiFERON test is worth the extra cost over the skin test depends heavily on the local context. A cost-effectiveness analysis in the Malaysian migrant screening context found that the QuantiFERON was both more effective and less costly than the skin test over a lifetime horizon, largely because the skin test’s higher false-negative rate meant more missed cases that later progressed to active disease.²⁹ A Colombian analysis in children also found the QuantiFERON cost-effective compared to the skin test.³⁰

But the picture flips in other settings. A Brazilian analysis found the skin test was the most cost-effective strategy for contact screening, with the QuantiFERON alone costing substantially more per averted case of active TB.³¹ The key variables are the cost of the test itself (the skin test is dramatically cheaper per unit), the prevalence of BCG vaccination in the population (high BCG coverage means more false positives with the skin test and more unnecessary treatment costs), and the local TB burden (in high-burden areas, the skin test’s lower specificity matters more because the downstream cost of misdiagnosis adds up).

Emerging Skin Tests That Use the Same Antigens

An interesting development bridges the gap between the convenience of a skin-based test and the specificity of the QuantiFERON. New skin tests use the same ESAT-6 and CFP-10 proteins instead of tuberculin, aiming to match the QuantiFERON’s specificity while eliminating the need for a lab. A study among jail detainees in China found strong agreement between the ESAT-6/CFP-10 skin test and the QFT-GIT, with a specificity of 97% for the skin-based version, though its sensitivity was lower at about 67%.³² These tests are still in varying stages of regulatory approval around the world, but they represent a potential future where blood draws are not the only way to get a BCG-independent TB screen. For settings with limited laboratory infrastructure, that could be a meaningful advance.