What Do My TB1 and TB2 Test Results Mean?

TB1 and TB2 are the two antigen tubes in the QuantiFERON-TB Gold Plus (QFT-Plus) blood test, and each one measures a slightly different arm of your immune response to the bacteria that cause tuberculosis. TB1 primarily captures the response of one type of immune cell (CD4 T-cells), while TB2 captures that response plus the activity of a second type (CD8 T-cells). When your results come back, they report the interferon-gamma levels from each tube separately, and how those two numbers relate to each other tells a more detailed story than either one alone. Understanding the interplay between them can help clarify whether a positive result is robust, whether a borderline result deserves a retest, and even offer hints about the nature of the infection.

What TB1 and TB2 Tubes Actually Measure

The QFT-Plus test draws your blood into four separate tubes. Two of them, the nil (negative control) and mitogen (positive control), exist to make sure the test itself is working properly. The other two are the ones that matter for your TB result: TB1 and TB2. Both tubes contain synthetic peptides that mimic proteins found in Mycobacterium tuberculosis. If your immune system has encountered TB bacteria before, your T-cells will recognize those peptides and release a signaling molecule called interferon-gamma. The lab measures how much interferon-gamma ends up in each tube.

The TB1 tube contains peptides designed to activate CD4 T-cells, which are a major part of the immune response to TB infection. The TB2 tube contains all of those same peptides plus additional shorter peptides specifically designed to also stimulate CD8 T-cells.¹ This is the key upgrade over the older QuantiFERON-TB Gold In-Tube (QFT-GIT) test, which only had one antigen tube. The rationale is straightforward: CD8 T-cells play an important role in the body’s defense against TB, and by measuring their contribution separately, the test can capture infections that might be missed by looking at CD4 responses alone.²

Research using cloned T-cells has shown that the design works as intended. CD4 T-cell clones respond to both TB1 and TB2, but CD8 T-cell clones respond overwhelmingly to the TB2 tube, with a greater than 140-fold difference compared to their response in TB1.¹ So if your TB2 value is noticeably higher than your TB1 value, that gap likely reflects a strong CD8 T-cell response.

How a Positive, Negative, or Indeterminate Result Is Determined

The lab subtracts the nil tube’s interferon-gamma value from each antigen tube’s value, giving you a “TB1 minus nil” and a “TB2 minus nil” number. If either one is 0.35 IU/mL or above (and at least 25% of the nil value), the overall result is reported as positive. If both are below 0.35 IU/mL, the result is negative, assuming the nil and mitogen controls behaved normally. In other words, you only need one tube to cross the threshold for the whole test to count as positive.

An indeterminate result happens when the control tubes misbehave. Either the nil tube is too high (meaning there was too much background immune activity) or the mitogen tube is too low (meaning your immune cells did not respond to anything, which suggests the test conditions were off or your immune system is suppressed). Multiple factors raise the risk of an indeterminate result. A study during the COVID-19 pandemic found that severe COVID, immunosuppressive medications, severe drops in lymphocyte counts, anemia, and non-COVID hospitalization were all independently linked to indeterminate QFT-Plus results.³ If you get an indeterminate result, it does not mean you have or do not have TB. It means the test could not give an answer and typically needs to be repeated, ideally after addressing whatever was suppressing the immune response.

When Both Tubes Agree

The majority of the time, TB1 and TB2 will give the same verdict. In a head-to-head comparison study, the two tubes agreed about 97% of the time, and in positive cases, the median interferon-gamma levels were similar (around 2.4 IU/mL for TB1 and 2.8 IU/mL for TB2).⁴ When both tubes are clearly positive, the result is straightforward: your immune system recognizes TB antigens, which means you have been infected at some point. When both are clearly negative, you can be reasonably confident you have not.

Across a large meta-analysis of studies using QFT-Plus in people with confirmed active TB, the pooled sensitivity was about 94%, meaning the test correctly identified roughly 94 out of 100 people who truly had active disease. The specificity in healthy individuals was around 96%.⁵ These numbers compare well with older-generation interferon-gamma release assays and with the T-SPOT.TB test, with a systematic review finding no statistically significant difference in sensitivity or specificity between QFT-Plus and those alternatives.⁶

Discordant Results and What They Suggest

Things get more interesting when TB1 and TB2 disagree. The two main patterns are TB1-negative/TB2-positive and TB1-positive/TB2-negative. Each can tell a different story.

A result where TB2 is positive but TB1 is negative suggests that CD8 T-cells are doing most of the heavy lifting. Some research has linked this “TB2-only” pattern to latent TB infection specifically, with one study concluding that a response limited to the TB2 tube was associated with latent rather than active disease.⁷ There is also some evidence suggesting that the CD8 T-cell response may be stronger early after exposure, meaning a TB2-dominant result could reflect recent infection.⁸ That said, the evidence here is preliminary. Researchers have been careful to note that current studies have not tracked individual patients from the moment of exposure long enough to confirm this timing theory in humans.

The opposite pattern, where TB1 is positive but TB2 is negative, is rarer. In a study of people living with HIV, among those who tested positive overall, the vast majority were positive on both tubes; only about 2% were positive on TB1 alone.⁹ This pattern does not carry the same tentative clinical interpretation as the TB2-only pattern, and in practice it is often treated as a positive result like any other. It simply indicates a CD4-driven response without a measurable CD8 contribution.

A prospective study comparing QFT-Plus to the older QFT-GIT found that the difference between TB2 and TB1 values (TB2 minus TB1) tended to be higher in people with active TB compared to those with latent infection.¹⁰ This is a tantalizing clue, but no guideline currently recommends using the TB2-minus-TB1 gap to distinguish latent from active TB in individual patients. The overlap between groups is too large, and the test was not designed for that purpose.

The Borderline Zone

Some of the trickiest results are those that land near the 0.35 IU/mL cutoff. A value of 0.36 IU/mL is technically positive, while 0.34 IU/mL is technically negative, even though the biological difference is negligible. Researchers have started paying serious attention to this “borderline zone,” typically defined as roughly 0.2 to 0.7 IU/mL.

A large retrospective study tracking 770 people with initial borderline results found that TB1 and TB2 values in this zone showed only weak agreement with each other. TB2 was the more variable tube during follow-up, with about a third of borderline TB2 results changing category on the first retest.¹¹ People who had borderline values in both TB1 and TB2 showed the highest rates of results flipping between positive and negative on subsequent tests. In contrast, if at least one tube had a clearly low-negative result (below 0.2 IU/mL), the overall result tended to stay stable.

Another study followed 64 patients with initial borderline results through repeat testing. Among those whose first borderline result fell on the positive side, nearly three-quarters reverted to negative or borderline-negative on retesting. Among those whose first borderline result fell on the negative side, only about 17% converted to positive or borderline-positive.¹² The practical takeaway: if your result is borderline, especially borderline-positive, the number may not stick. Repeat testing is warranted, and looking at both TB1 and TB2 values (rather than just the overall positive/negative call) helps gauge how much confidence to place in the result.

Why Your Result Might Be Wrong Before the Lab Even Runs It

The QFT-Plus test is sensitive to how the blood sample is handled before it reaches the lab. Studies have shown that the amount of blood drawn into each tube, how vigorously the tube is shaken, and how long it sits before incubation all affect the result. In one controlled experiment, filling tubes with slightly less blood than recommended (0.8 mL instead of 1.0 mL) actually increased the interferon-gamma signal, while overfilling to 1.2 mL nearly doubled the false-negative rate in people known to have latent TB.¹³ Vigorous shaking of the tubes also boosted the interferon-gamma readings compared to gentle shaking.

This matters because in real-world clinical settings, blood draws are not always performed by people who are familiar with the specific quirks of this test. If your result seems inconsistent with your clinical picture, pre-analytical handling is one of the first things worth questioning. Guidelines stress the importance of drawing the correct blood volume, shaking tubes adequately to dissolve the dried antigens coating the walls, storing tubes at the right temperature, and getting them into the incubator promptly.¹⁴

If You Have Been Vaccinated with BCG

One of the main advantages of the QFT-Plus test over the older tuberculin skin test (TST) is that it is far less likely to give a false positive due to the BCG vaccine. The BCG vaccine is used routinely in many countries to protect against severe childhood TB, and it is well known to cause positive skin test results years or even decades later. The QFT-Plus test uses peptides from proteins (ESAT-6 and CFP-10) that are present in TB bacteria but absent from the BCG vaccine strain, so the vaccine should not trigger a positive result on this blood test.

Research confirms this distinction in practice. A study of healthcare workers found that among those with positive skin tests and a history of BCG vaccination, more than half were negative on the blood-based QuantiFERON test, suggesting their skin test positivity was from the vaccine rather than true infection.¹⁵ If you were vaccinated with BCG and your QFT-Plus TB1 and TB2 are both positive, the result is almost certainly reflecting real TB exposure, not the vaccine. This is one of the clearest situations where the blood test provides a more reliable answer than the skin test.

Results in People with Weakened Immune Systems

If your immune system is suppressed, by HIV, organ transplant drugs, chemotherapy, or conditions like rheumatoid arthritis treated with immunosuppressive medications, the QFT-Plus test becomes less reliable. The test depends on T-cells responding to TB antigens, and if those T-cells are depleted or suppressed, the response may be too weak to register even if TB bacteria are present.

A large prospective study tracking immunocompromised individuals found that the test’s sensitivity for active TB dropped to about 70% in this group, compared to roughly 81% in people with healthy immune systems. Specificity also declined somewhat.¹⁶ The same study had a striking finding about what happens after a positive result in different immunocompromised groups. Among people with conditions like chronic kidney failure or rheumatoid arthritis who tested positive but did not receive preventive therapy, none developed active TB over years of follow-up. But among people living with HIV who had positive results and did not receive preventive treatment, those with uncontrolled viral replication and low CD4 counts developed active TB at a rate of about 4 per 100 person-years.

In a study of HIV-positive patients in China, the QFT-Plus detected TB infection at a similar rate to the older QFT-GIT, with about a quarter of patients testing positive. Among those positive, nearly all were positive on both TB1 and TB2, though a small fraction (about 5%) were positive on TB2 only.⁹ The bottom line for immunocompromised individuals: a positive result should be taken seriously, but a negative result cannot fully rule out TB infection. Your doctor may supplement with other testing or imaging.

Pediatric Testing Has Its Own Quirks

In children and adolescents, the QFT-Plus test performs reasonably well but does not appear to be a leap forward over older versions. A multicenter study of 158 children with confirmed TB disease found an overall sensitivity of about 83%.¹⁷ This is useful but means roughly one in six children with active TB will have a falsely negative result.

A separate study in children found 100% agreement between TB1 and TB2 results, suggesting that the two tubes do not tend to diverge in younger patients the way they occasionally do in adults.¹⁸ Importantly, that same study found that the TB1 and TB2 values could not distinguish active TB from latent infection in children. This means clinicians still rely on symptoms, imaging, and clinical context to make that call in pediatric patients, even with the additional information from the second tube.

Serial Testing and the Problem of Reversions

If you are tested repeatedly over time, particularly in workplace screening programs for healthcare workers, you may see your result flip. A large study of nearly 1,900 healthcare workers found that about 3% experienced reversion (going from positive to negative) and about 2% experienced conversion (going from negative to positive) over the study period.¹⁹ The biggest predictor of whether a positive result would revert was the initial interferon-gamma level. People whose baseline positive result was below about 0.74 IU/mL were far more likely to revert to negative, while those with higher values tended to remain persistently positive.

In a high-burden setting in Nigeria, reversion rates were considerably higher, approaching 40% over two years among healthcare workers who initially tested positive.²⁰ This variability underscores that a single positive QFT-Plus result, especially one near the cutoff, is not necessarily permanent. If you are in a serial testing program and your result flips, your doctor will look at the magnitude of your TB1 and TB2 values, your exposure history, and your risk profile before deciding whether to treat.

Your Results Will Not Track Treatment

One of the most common misconceptions about TB blood tests is that they should turn negative after you complete treatment for latent TB infection. They usually do not. Multiple studies have demonstrated this clearly. In one study, the vast majority of people who were positive before starting a nine-month course of isoniazid remained positive afterward, with the researchers stating unequivocally that the test should not be used to assess whether treatment worked.²¹ Another study found that roughly 85% of patients were still positive at both three months and fifteen months after completing preventive therapy, with average interferon-gamma levels essentially unchanged.²²

The newer QFT-Plus test does not appear to solve this problem. A direct comparison found that interferon-gamma levels measured by QFT-Plus showed the same lack of meaningful change after latent TB treatment as the older QFT-GIT.²³ The reason is that the test detects immune memory, and your immune system does not forget that it encountered TB just because the bacteria have been killed. If your doctor tells you to complete your medication course even though your test is still positive, that is exactly the right approach. A persistently positive result after treatment does not mean the treatment failed.

How QFT-Plus Compares to the Skin Test in Cost and Convenience

Beyond accuracy, practical considerations shape which test you actually receive. The tuberculin skin test requires two visits: one to inject a small amount of protein under your skin, and another 48 to 72 hours later to have a healthcare worker measure the resulting bump. The QFT-Plus test requires a single blood draw, with results typically available in one to three days. For people who are unlikely to return for a skin test reading, the blood test is often the better choice by default.

On cost, the blood test is more expensive per individual than the skin test. However, a cost-effectiveness analysis from Colombia found that when you factor in the costs of false-positive skin test results leading to unnecessary treatment, chest X-rays, and follow-up visits, QFT-Plus was cost-effective for diagnosing latent TB in immunocompetent adults.²⁴ The specificity advantage of the blood test over the skin test, estimated at roughly 12 percentage points higher in some analyses, drives much of this benefit by preventing unnecessary downstream costs.⁶ The calculus shifts depending on the country, the population being screened, and how common BCG vaccination is among those being tested.