QuantiFERON-TB Gold Test: How It Works & Results

The QuantiFERON-TB Gold test is a blood-based screening tool that detects tuberculosis infection by measuring how your immune cells respond to proteins unique to the TB bacterium. A single blood draw replaces the two-visit tuberculin skin test, and the result comes back as positive, negative, or indeterminate, usually within 24 to 48 hours. The test has become a standard tool for diagnosing latent TB infection, but the way it works, the situations that can trip it up, and the meaning of each result category are more nuanced than the simple positive-or-negative framing suggests.

What Happens Inside the Tubes

The test relies on a straightforward immunological principle. If you have been infected with Mycobacterium tuberculosis, your white blood cells will have learned to recognize certain TB-specific proteins. When those proteins are mixed with a fresh blood sample, the immune cells release a signaling molecule called interferon-gamma. The test measures how much interferon-gamma appears in the sample. A high level indicates prior exposure to the TB bacterium; a low level suggests the immune system has never encountered it.

The proteins used as bait are what make the test specific. The two main antigens, known as ESAT-6 and CFP-10, are encoded in a genomic region called RD1. That region is present in virulent strains of M. tuberculosis but is absent from the BCG vaccine strain used worldwide for TB immunization.¹ The RD1 region has been experimentally confirmed as important for the bacterium’s ability to cause disease, and its deletion from BCG is one of the key genetic changes that made the vaccine strain attenuated.² Because the test uses antigens absent from BCG, a prior BCG vaccination will not cause a false positive, which is one of its main practical advantages over the older skin test.

The Current Version and Its Two Antigen Tubes

The current generation of the test, called QuantiFERON-TB Gold Plus (QFT-Plus), comes with four blood collection tubes. One is a negative control (nil tube) that establishes a baseline. One is a positive control (mitogen tube) that checks whether your immune cells are capable of responding at all. The other two, labeled TB1 and TB2, contain the TB-specific peptides that do the actual diagnostic work.

TB1 contains peptides designed to stimulate CD4 T-cells, which are the immune cells traditionally measured in TB screening. TB2 adds a newer set of peptides intended to also activate CD8 T-cells.³ This dual-tube design was introduced because CD8 T-cells appear to play an important role in the immune response to active and recent TB infection.⁴ Whether the added CD8 stimulation meaningfully improves diagnostic accuracy in practice is still being evaluated, but the inclusion of both T-cell types represents the main technical upgrade from the earlier QuantiFERON-TB Gold In-Tube version.

Reading the Results

The laboratory subtracts the background interferon-gamma level measured in the nil tube from the levels in the TB1 and TB2 antigen tubes. If the difference meets or exceeds a threshold of 0.35 IU/mL in either antigen tube, the result is reported as positive. Below that cutoff, the result is negative, assuming the controls behaved properly.

A result is classified as indeterminate when the control tubes give unexpected readings. If the nil tube (the negative control) shows an interferon-gamma level at or above 8 IU/mL, it means there was too much background immune activation to interpret the antigen tubes. If the mitogen tube (the positive control) fails to reach 0.5 IU/mL, it means the immune cells did not respond even to a strong general stimulus, so a negative antigen result cannot be trusted.⁵ Indeterminate does not mean the person does or does not have TB. It means the test could not be interpreted, and it usually needs to be repeated or followed up with alternative testing.

Why Results Sometimes Come Back Indeterminate

Indeterminate results are more common in certain clinical populations. A study during the COVID-19 pandemic identified several risk factors: severe COVID-19 roughly quadrupled the odds of an indeterminate result, while immunosuppressive medications, severe lymphopenia, and anemia each increased the odds by around 70 to 90 percent.⁶ Being hospitalized for any reason, not just COVID, carried similarly elevated odds. The common thread is a weakened or overwhelmed immune system that cannot mount a detectable response to the test’s mitogen control.

There is also a less intuitive cause of indeterminate results. In rare cases, active disseminated TB itself can flood the nil tube with interferon-gamma, causing the background to exceed the threshold and rendering the test uninterpretable. Case reports have documented this in patients with miliary tuberculosis.⁷ The paradox is that the patient’s immune system is reacting intensely to TB, but that very intensity makes the test fail.

Laboratory handling errors are another common contributor. Underfilling or overfilling the collection tubes, mixing up tube positions, and delays in getting samples to the incubator can all push results into the indeterminate category.⁸ These preanalytical errors are addressed in more detail below.

How Specimen Handling Affects Accuracy

The QuantiFERON test is more sensitive to handling conditions than many routine blood tests. A major variable is how quickly the blood gets incubated after the draw. When researchers compared immediate incubation to delays of six and twelve hours, about one in five truly positive samples reverted to negative at both delay intervals.⁹ A separate study found that reducing the average incubation delay from about 10 hours to about 3 hours dropped the rate of indeterminate results from roughly 11 percent to under 3 percent.¹⁰

Other physical handling details matter too. Research on the older In-Tube version found that how vigorously the tubes are shaken after blood collection changes the interferon-gamma readings. Vigorous shaking roughly doubled the median response in the antigen tube compared to gentle shaking, which could push borderline-negative results over the positive threshold or vice versa.¹¹ Blood volume was also identified as a source of variability. The practical takeaway is that if you receive a borderline or unexpected result, the quality of the blood draw and how the sample was handled should be considered before assuming the result reflects your actual TB status.

The Advantage Over the Tuberculin Skin Test in BCG-Vaccinated People

The tuberculin skin test (TST), sometimes called the Mantoux test, has been the standard TB screening tool for over a century. It works by injecting a small amount of purified protein derivative from TB bacteria under the skin and checking for a reaction 48 to 72 hours later. The problem is that BCG vaccination, which is given to children in much of the world, can cause a false-positive TST because BCG shares many proteins with M. tuberculosis.

Because the QuantiFERON test uses antigens from the RD1 region absent in BCG, it avoids most of this cross-reactivity. A study comparing both tests in BCG-vaccinated adults found that the QuantiFERON assay was more specific for true TB infection, reducing the number of false positives.¹² In BCG-vaccinated children, the specificity gap was even more dramatic. One study found the skin test’s specificity was under 30 percent compared to the QuantiFERON result, meaning more than two out of three TST-positive children did not actually have TB infection.¹³ In cancer patients in a developing country, the overall agreement between the two tests was 86 percent, but the chance-adjusted agreement was only moderate, with BCG vaccination explaining much of the discordance.¹⁴

Performance in People With HIV

HIV infection poses a particular challenge for any test that depends on the immune system mounting a response. An earlier study of the QuantiFERON In-Tube version found that sensitivity in people with HIV and active TB was lower than in HIV-negative patients, roughly 65 percent compared to 81 percent. Much of that difference came from a higher rate of indeterminate results in the HIV-positive group (about 22 percent versus 9 percent). Once indeterminate results were excluded, sensitivity was comparable between the two groups, around 83 to 88 percent.¹⁵ CD4 count mattered: sensitivity was significantly lower in HIV-positive patients with CD4 counts below 300 compared to above 300.

With the newer QFT-Plus version, results appear more encouraging. Research on its accuracy in people living with HIV found that sensitivity was similar regardless of HIV status, and CD4 count did not significantly influence the distribution of interferon-gamma values.¹⁶ This may partly reflect the QFT-Plus design’s inclusion of CD8 T-cell stimulation, since CD4 cells are the ones most depleted by HIV. Still, clinicians working with severely immunocompromised patients treat a negative result with appropriate caution.

Testing in Children

The QuantiFERON test is sometimes viewed with skepticism in pediatric settings because young children’s immune systems are still developing and may produce weaker interferon-gamma responses. The data, though, is reasonably reassuring. In one study of children with suspected active TB, the QFT-Plus was positive in about 83 percent of confirmed cases, and the indeterminate rate among children screened for latent infection was low, around 2.5 percent.¹⁷

An earlier study using the older QuantiFERON-TB Gold found that overall sensitivity for active TB in children was about 88 percent, with specificity around 94 percent. Sensitivity improved with age: it was lower in toddlers and reached 100 percent in children older than four years. Among the children who received indeterminate results, a large majority turned out to have other active infections, bacterial, viral, or fungal, that were likely suppressing the mitogen response and causing the test to fail.¹⁸ For clinicians, this is a useful pattern: an indeterminate result in a sick child may itself be a signal that something else is going on.

What the Test Cannot Tell You

A positive QuantiFERON result means your immune system has been sensitized to TB bacteria. It does not tell you whether you have latent infection (the bacteria are dormant and you are not contagious) or active disease (the bacteria are multiplying and you may be symptomatic and infectious). Research has confirmed that interferon-gamma levels alone cannot reliably distinguish between these two states.¹⁹ If you test positive, your doctor will need chest imaging and possibly sputum cultures to determine whether the infection is active.

The test also does not work well as a way to monitor treatment. In health care workers who received a full nine-month course of isoniazid for latent TB, all continued to test positive on the QuantiFERON at the end of treatment, and the few retested three months later were still positive.²⁰ Even patients who completed treatment for active pulmonary TB and were clinically cured continued to test positive years and even decades afterward, with no significant decline in positivity rates over time.²¹ The memory T-cells that produce interferon-gamma persist long after the bacteria have been killed or controlled, so a positive result is essentially permanent. You should not expect it to turn negative after treatment, and a doctor who repeats the test to check whether treatment worked is using it for a purpose it was not designed for.

The Serial Testing Problem

When the QuantiFERON test is used for annual or periodic screening, as it is for many health care workers, the results become harder to interpret. The test was designed to give a snapshot at a single point in time, and when it is repeated, fluctuations around the 0.35 IU/mL cutoff create apparent conversions and reversions that may not reflect real changes in TB status.

In one U.S. study of over 2,300 health care workers tested a year apart, roughly 3 percent of initially negative workers converted to positive on repeat testing. But when a third test was performed on 41 of these converters, 90 percent reverted back to negative, suggesting that most of the conversions were not genuine new infections.²² The authors concluded that the test’s reproducibility at its cutoff was poor enough that it should not be used for serial screening of health care workers under the current interpretation criteria.

A separate large screening study found a 3 percent reversion rate among initially positive workers, with lower baseline interferon-gamma levels being the strongest predictor of reversion. Workers whose initial positive results were well above the threshold (0.70 IU/mL or higher) were far less likely to revert than those who were just barely positive.²³ Another study confirmed this pattern, finding that 70 percent of people whose QuantiFERON was positive but whose skin test was negative at baseline reverted on repeat testing, compared to only 7 percent of those who were concordantly positive on both tests.²⁴ Borderline positives in serial testing contexts should be treated with skepticism, especially in low-risk populations.

How Well a Positive Result Predicts Future TB Disease

Most people with latent TB never develop active disease. So one natural question is whether a positive QuantiFERON result carries meaningful predictive power for who will eventually get sick. The answer depends heavily on the population.

In a contact-tracing study, about 6 percent of QuantiFERON-positive contacts progressed to active TB during roughly two years of follow-up, compared to 0.4 percent of those who tested negative.²⁵ That difference was statistically significant, but it also means about 94 percent of people who tested positive did not develop TB within the follow-up period.

A large prospective study of immunocompromised individuals drew more nuanced conclusions. Among hundreds of people with chronic kidney disease, rheumatoid arthritis, or organ transplants who tested positive and did not receive preventive therapy, none developed active TB over thousands of person-years of follow-up. In contrast, among people living with HIV who tested positive and had active viral replication with low CD4 counts, progression occurred at a rate of about 4 per 100 person-years.²⁶ The test’s predictive value, in other words, is not a fixed number. It depends on what other risk factors are in play, and a positive result alone is not enough to determine who needs treatment.

How It Compares to the T-SPOT.TB Test

The QuantiFERON is not the only interferon-gamma release assay on the market. The T-SPOT.TB test measures the same basic signal, interferon-gamma production in response to ESAT-6 and CFP-10, but uses a different laboratory method. Instead of measuring the total amount of interferon-gamma in a blood sample, the T-SPOT counts individual immune cells that respond to the antigens.

Head-to-head comparisons suggest broadly similar performance. In one study of febrile patients being evaluated for active TB, the QuantiFERON-Plus had a sensitivity of about 92 percent, while the T-SPOT reached about 95 percent. The difference was not statistically significant. The T-SPOT had no indeterminate results in that study, compared to about 3 percent for the QuantiFERON.²⁷ The T-SPOT’s slightly lower indeterminate rate is often attributed to the fact that it separates and washes the immune cells before stimulation, which may reduce interference from background factors. The QuantiFERON test, on the other hand, is generally considered easier to process in high-volume labs because it does not require cell separation.

Cost Considerations

The QuantiFERON test costs more per draw than a tuberculin skin test, which requires only an inexpensive vial of purified protein derivative and a syringe. But the full cost picture is more complex than the sticker price of each test. The skin test requires two visits, one to place the injection and one to read it, and patients who do not return for the second visit produce no usable result. The QuantiFERON needs only a single visit. It also avoids false positives from BCG vaccination, which means fewer people are sent for unnecessary chest X-rays and follow-up evaluations.

A cost-effectiveness analysis from Colombia found that the QuantiFERON-Plus was cost-effective compared to the skin test for diagnosing latent TB in immunocompetent people, though the margin was sensitive to the local prevalence of latent TB and to each test’s sensitivity in that specific setting.²⁸ In populations where BCG vaccination is widespread and latent TB prevalence is moderate, the QuantiFERON’s higher specificity tends to save money downstream by reducing unnecessary treatment courses. In settings where BCG is not used and TB prevalence is very low, the economic case is less clear-cut.