What Can Cause a False Positive TB Blood Test?

TB blood tests, known as interferon-gamma release assays (IGRAs), can return false positive results for several reasons, including cross-reactivity with certain environmental bacteria, results that land in an unstable borderline zone near the test’s cutoff threshold, contamination or defects in the blood-collection tubes themselves, and underlying autoimmune conditions that interfere with the immune signal the test measures. While IGRAs are more specific than the older tuberculin skin test, they are far from infallible, and the causes of false positives range from the biological to the purely mechanical.

How the TB Blood Test Works

IGRAs detect TB infection indirectly. Instead of looking for the bacterium itself, they measure your immune system’s reaction to proteins found on Mycobacterium tuberculosis. A blood sample is mixed with synthetic versions of TB-specific antigens (called ESAT-6 and CFP-10), and if your white blood cells release interferon-gamma in response, the test reads as positive. The idea is that only someone whose immune system has previously encountered TB would mount that specific response. The QuantiFERON-TB Gold Plus (QFT-Plus) is the most widely used version; it reports a numerical result in international units per milliliter (IU/mL), with 0.35 IU/mL as the standard cutoff for a positive result. A second platform, T-SPOT.TB, counts individual interferon-producing cells instead of measuring the total interferon concentration, but the principle is the same.

The problem with any test built on immune recognition is that the immune system is not always as precise as the test designers assumed. Several situations can trick the assay into reporting a positive when there is no actual TB infection present.

Cross-Reactivity with Non-Tuberculous Mycobacteria

The antigens used in IGRAs were chosen because they are absent from the BCG vaccine strain and from most environmental mycobacteria. That selection works well for the majority of common soil and water mycobacteria, but a handful of species carry their own versions of ESAT-6 and CFP-10 that are similar enough to fool the test. Mycobacterium kansasii is the best-documented offender. Research on the T-cell-level cross-reactivity between M. kansasii and M. bovis (a close relative of M. tuberculosis) found that nearly all the tested peptide regions were fully cross-reactive, meaning the immune system could not distinguish between the two species at those sites.¹

Beyond M. kansasii, clinical reports have identified positive IGRA results in patients infected with M. szulgai, M. gordonae, M. flavescens, and M. marinum.² These are environmental organisms that most people never encounter in a clinically significant way, but certain occupations and hobbies raise the odds. M. marinum, for instance, lives in aquarium water and can infect people who handle fish tanks with open cuts on their hands. M. kansasii is common in tap water in parts of the southern and central United States. If you test positive on an IGRA and have no obvious TB risk factors, your doctor may consider whether exposure to one of these species could explain the result.

The Borderline Zone

The 0.35 IU/mL cutoff used by the QuantiFERON test is a line drawn through a cloud of biological noise. Results that land just above it are unreliable in a way that results well above it are not. A study that followed up on initial borderline results (defined as 0.35 to 0.99 IU/mL) found that more than half of the subjects were clearly negative, below 0.20 IU/mL, when retested within four weeks.³ The researchers interpreted this as strong evidence that the initial positive was a false alarm driven by normal test variability rather than any real immunological event.

A separate study in a low-burden setting confirmed the pattern: when people with initial borderline results were retested, roughly a third came back convincingly negative, and none of them developed active TB within two years.⁴ Longitudinal follow-up data from an intermediate-burden setting showed the same instability, with borderline results fluctuating over time in ways that made it hard to distinguish true infection from noise.⁵

What this means practically is that a result of, say, 0.42 IU/mL carries far less certainty than a result of 3.0 IU/mL. Some clinicians and labs now report a “borderline” or “indeterminate” zone rather than issuing a clean positive, and retesting is often recommended before committing to treatment for latent TB.

Lab Errors and Tube Contamination

Some false positives have nothing to do with your biology and everything to do with the equipment. The QuantiFERON test uses specially coated blood-collection tubes: one contains TB antigens, one is a negative control (the “nil” tube), and one is a positive control. If anything goes wrong with any of those tubes, the math falls apart.

One well-documented incident involved a sudden spike in positive rates at a U.S. academic institution, jumping from about 10% to 31%. An investigation traced the problem to a specific lot of antigen tubes. When blood samples were retested using tubes from a different lot, the inflated positive rate disappeared.⁶ Manufacturing defects in commercial diagnostic kits are uncommon, but when they happen they can affect hundreds of patients before anyone notices the pattern.

A subtler problem involves endotoxin contamination in the blood-collection tubes. Endotoxins are fragments of bacterial cell walls that can trigger a broad immune response, including interferon-gamma release, even without any TB antigens present. A study examining this issue found that nearly a third of clinical samples showed elevated background signals in the nil control tube, and endotoxin contamination in the collection tubes was identified as a significant contributing factor.⁷ When the nil tube reads high, the software subtracts a larger background value from the antigen tube, which normally would make a positive harder to achieve. But contamination can also elevate the antigen tube signal in unpredictable ways, and some configurations of elevated readings lead to false positives or indeterminate results that get repeated and eventually called positive.

High interferon-gamma concentrations in the nil tube have also been flagged as a separate source of indeterminate results, with researchers investigating methodological improvements to reduce this problem.⁸ Pre-analytical handling matters too. If blood sits too long before incubation, if the tubes are not shaken properly to coat the blood with antigens, or if incubation temperature drifts, results can be unreliable in either direction.

Autoimmune Conditions and Spontaneous Interferon Production

This is one of the less widely appreciated causes of false positive IGRAs. In seropositive rheumatoid arthritis (RA), the immune system is already in overdrive, and some RA patients spontaneously produce interferon-gamma at levels high enough to push the test past its positive threshold without any TB exposure at all. A study of seropositive RA patients found that their IGRA interferon levels often exceeded 35 IU and that the results correlated with the level of anti-cyclic citrullinated protein (anti-CCP) antibodies, a hallmark of the disease.⁹ After treatment of the RA, the IGRA results alternated between positive and negative, further suggesting the test was reacting to the autoimmune process rather than a TB infection.

The mechanism appears to involve immunoglobulin binding protein (BiP), a molecule found on the surface of cells in inflamed joints. BiP can trigger T-cell activation and cytokine release, including interferon-gamma, in a pattern that the IGRA misinterprets as a TB-specific response. One clinical report documented a false positive QuantiFERON result in a patient with systemic lupus erythematosus (SLE), where BiP is found in the blood of up to 30% of patients. That result reverted to negative after treatment of the lupus with an interferon-blocking drug.¹⁰

If you have RA, lupus, or another autoimmune condition and receive a positive IGRA result, it is worth raising the possibility of a false positive with your doctor, especially if you have no history of TB exposure or travel to high-burden countries. The researchers who identified the RA connection were blunt in noting that the IGRA, originally designed for use in HIV-positive patients, is not ideal for seropositive RA patients.⁹

BCG Vaccination and Why It Matters Less for Blood Tests

The BCG vaccine is the single most common cause of false positive results on the tuberculin skin test (TST), and this distinction trips up a lot of people who assume the same applies to blood tests. A meta-analysis of the skin test found that BCG-vaccinated people were roughly twice as likely to have a positive TST compared to unvaccinated people, though the effect faded after about 15 years.¹¹ That was, in fact, one of the main reasons IGRAs were developed: the antigens they use (ESAT-6 and CFP-10) are absent from the BCG strain, so the vaccine should not trigger a positive IGRA.

In practice, BCG vaccination is not considered a significant cause of false positive IGRAs. If you were vaccinated with BCG as a child and your blood test comes back positive, the BCG itself is very unlikely to be responsible. The skin test is a different story, and clinicians in countries where BCG is routine (most of the world outside the United States) often prefer IGRAs precisely because they avoid the BCG confound.

Live-Virus Vaccines and Timing

It has long been standard practice to avoid giving the TB skin test within four to six weeks of a live-virus vaccine (such as MMR or varicella), based on the concern that the temporary immune suppression from the vaccine might cause a false negative. Less attention has been paid to whether live vaccines affect IGRA results. A large study of children undergoing U.S. immigration medical examinations found a surprising pattern: children tested during the “critical interval” shortly after receiving live vaccines actually had higher positive rates on both IGRA and TST compared to children tested before or after that window. The odds of a positive IGRA were roughly 27% higher during the critical interval compared to after it.¹²

The finding challenges the conventional assumption that live vaccines only suppress test results. One interpretation is that the transient immune activation triggered by the vaccine boosts background interferon-gamma release enough to push some borderline results over the positive threshold. This is a relatively new finding, and clinical guidelines have not yet been updated to reflect it, but it is worth being aware of if you or your child recently received a live vaccine and then tested positive.

Pregnancy and Altered Immune Function

Pregnancy reshapes the immune system in ways that can affect TB testing in both directions. The body dials down certain immune responses to tolerate the fetus, which alters interferon-gamma production, the exact molecule IGRAs measure. A study of women in a high-burden setting found that pregnancy reduced the rate of detecting latent TB by both TST and IGRA, suggesting it pushes results toward false negatives rather than false positives.¹³

That said, pregnancy-related immune changes are not uniform. Some pregnant women experience heightened inflammatory responses in certain pathways, and the interplay between these shifts and IGRA antigens is not fully understood. The practical takeaway is that TB test results obtained during pregnancy should be interpreted with extra caution. A positive result in a pregnant woman with no TB risk factors may warrant retesting after delivery, while a negative result in a woman with genuine exposure risk should not be taken as fully reassuring either.

Immunosuppressive Medications

People taking immunosuppressive drugs, including glucocorticoids, biologics, and conventional immunosuppressants, have a more complicated testing picture. A comparison of QFT-Plus and T-SPOT.TB in febrile patients found that the use of glucocorticoids, immunosuppressive agents, or biologics was associated with lower odds of testing positive on QFT-Plus.¹⁴ In other words, these drugs tend to suppress the immune response the test relies on, making false negatives the bigger concern for immunosuppressed patients.

But the relationship is not always straightforward. Some biologics target specific cytokine pathways and can paradoxically alter the balance of interferon-gamma in ways that are hard to predict. And as discussed earlier, autoimmune patients who are between treatments or on lower doses may still be producing enough spontaneous interferon to generate a false positive. The direction of the error depends on how active the underlying disease is and how effectively the medication is suppressing it.

What to Do After a Borderline or Unexpected Positive

If you receive a positive IGRA result and your risk profile does not fit, the most practical step is to ask your clinician about repeat testing. Given the instability of borderline results, a second draw even a few weeks later can resolve the question. Some modeling work has suggested that requiring a confirmatory test at the standard 0.35 IU/mL cutoff can substantially reduce the false-positive rate over time, although it comes at the cost of missing some true infections. Raising the cutoff threshold for a “conversion” (from negative to positive) to 1.0 IU/mL is another strategy that has been modeled, though it too involves a trade-off with sensitivity.

In practice, clinicians often combine the IGRA result with other information: your history of TB exposure, travel to or residence in high-burden countries, chest X-ray findings, and symptoms. A positive IGRA in someone who grew up in a TB-endemic region and has an abnormal chest film means something very different from a positive IGRA in someone with rheumatoid arthritis, no risk factors, and a clear X-ray. The blood test is one data point, not a verdict.

The Skin Test Comparison

Many people searching for information about false positive TB blood tests are actually trying to figure out whether the blood test or the skin test is more trustworthy. The two tests have different vulnerability profiles. The TST is heavily affected by BCG vaccination and by prior exposure to environmental mycobacteria, and its reading depends on a human measuring an area of skin swelling, which introduces some subjectivity. The IGRA sidesteps the BCG problem and removes reader subjectivity, but it introduces the borderline-zone instability, lab-handling sensitivity, and autoimmune interference described above.

In a study of Indian children, the QFT had a specificity of about 90% for WHO-defined TB, which was comparable to the TST’s 89%.¹⁵ Neither test is dramatically more accurate than the other in absolute terms. The advantage of the IGRA is that its false positives come from different, and generally rarer, sources than the skin test’s false positives, which makes it the better choice in BCG-vaccinated populations. In settings where BCG is not used and environmental mycobacterial exposure is low, the skin test remains a reasonable alternative.

One area where the skin test has a quiet advantage is cost and infrastructure. An IGRA requires a certified lab, temperature-controlled shipping, and timely processing. The TST requires a needle, a vial of purified protein derivative, and a trained reader two to three days later. In resource-limited settings, the practical constraints around IGRA handling can themselves become a source of error, since delays in processing or breaks in the cold chain degrade the sample in ways that do not apply to the skin test.

When Testing Populations Changes the Math

False positive rates are not just a function of the test itself; they also depend on who you are testing. In a population where TB is common, a positive result is much more likely to represent a true infection, simply because there are more true infections to find. In a population where TB is rare, the same test with the same specificity will produce a higher proportion of false positives among all positives returned, because most people being tested are genuinely uninfected. Modeling has shown that even with reasonably specific screening tools, the positive predictive value in low-prevalence populations can drop below 50%.¹⁶

This is why mass screening of low-risk groups generates so many headaches. A hospital employee wellness program in a city with very little TB, for example, will flag people who probably do not have latent TB, and each of those people then faces the anxiety and potential treatment side effects that come with a positive result. Healthcare systems in low-prevalence countries have increasingly moved toward targeted testing, screening only people with genuine risk factors, precisely to reduce this problem. If you are being tested because of a routine workplace requirement rather than because of an actual exposure, and you get a result just above the cutoff, the statistical odds are tilted more toward a false positive than they would be for someone tested after close contact with an active TB case.