Any false positive TB blood test results out there?

Update to all those glued to this riveting discussion -- I'm now further befuddled. My repeat Quantiferon Gold came back negative, which isn't CRAZY surprising, but what seems odd to me is the huge (at least it seems so to me) shift in the value just a month apart:

8/24/18 - Positive* at 1.06 iU/mL; NIL was 0.01 (IU/mL); TB-NIL was 1.07 (IU/mL); Mitogen-NIL was >10 (IU/mL).
9/19/18 - Negative* at 0.13 iU/mL; NIL was 0.01 (IU/mL); TB-NIL was 0.14 (IU/mL); Mitogen-NIL was >10 (IU/mL).

From the preliminary research I've done, you can revert from positive to negative or convert from negative to positive in a month, particularly when it's a low positive (I was just slightly above the low positive maximum value of 1 on first testing), but it's usually just a small change -- like 0.01-0.05 points. My chest X-ray and CT scan were both normal, and while I did initially have a slight mucus-producing cough, it's since disappeared and my only "potential" TB symptoms would be unexplained fever (but it's low-grade) and night sweats (but it's not like sheet-drenching; in fact, it's almost more feeling hot/flushed and emitting body odor from the armpits although the rest of the body remains dry) or possibly a genitourinary form of extrapulmonary TB manifesting with urinary frequency, so I am by no means convinced I do (or do not) have TB. My questions are thus:

1. IS that amount of variation within a month normal, or does it more suggest some sort of disruptive influence (see question 2) or mishandling/testing by the lab?
2. What factors can affect the Quantiferon Gold result (specific medications, autoimmune diseases, abnormal levels of something else, etc.)? Are there any other diseases/conditions that are cross-reactive that the initial positive might have been reacting to?
3. What is the potential significance (if any) to the fact that the first (positive) test was conducted two weeks after treatment with Doxycycline antibiotic, and five days after treatment with two doses of Diflucan for a suspected vaginal yeast infection (culture later negative), but the second (negative) test was conducted two weeks after starting/while still taking another round of Doxycycline?
4. Again, anybody else had a false positive? Other than the fact that it was negative, which is more in line with our expectations since I don't travel much, is there any other reason to think the second test is more accurate than the first?

I've never had a positive TB skin or blood test previously, including my most recent tests in 2015 (skin) and 2011 (skin AND blood). Love to hear if anyone else has any insights/similar experience.

*For a test to be considered positive, the TB antigen tube value minus the Nil tube value must be ≥0.35 IU/mL (FDA cleared breakpoint). Low positive values (0.35–1.0 IU/mL) may not repeat as positive in some patient populations; some authorities recommend that the patient be retested to confirm low positive tests. My calculated value of 1.06 (TB antigen tube value minus the Nil tube value) can show a 52% reversion rate.

I have had blurry vision in left eye and Gold quantiferon test came positive they are no other symptoms. Chest x-ray, CT scan, blood work all normal . Eye infection is recovered almost 70-80% after steroid drops prescribed by Retina specialist . Can Gold quantiferon test be false positive ? How soon can I repeat test?

Hello @aavhad and welcome to Mayo Clinic Connect. I see you are looking for some information on false positive. I did a search to see if I could provide any helpful information you may wish to consider and came across the information below.

It is a bit of a heavy read given the research nature of the article, so I pulled out an excerpt that I thought may be helpful for you.

From article:
In realistic settings of low risk of TB infection, even with high cutoffs and confirmation, the post-test probability of a positive QFT test on serial testing may still only approach 60%, while annual testing with a single QFT using the recommended diagnostic cutoff may cause over 25% of individuals to have a false-positive test over a span of ten years.

Serial testing for latent tuberculosis using QuantiFERON-TB Gold In-Tube: A Markov model:
https://www.nature.com/articles/srep30781
Does this resonate with you?

Can I have a false positive tb test (subcutaneus tuberculin) related to getting a Monkeypox vaccine (Jynneos)? I was vaccinated two days ago, must I wait a period of time to get the tb skin test, or there is no direct interference with the monkeypox vaccine in relation to false positive or negative results?

These two infectious agents are completely different - TB is a mycobacteria, monkeypox is a virus. Based on that alone, there should be no risk of a false positive after the vaccination. If by chance you should get a positive result from the TB Mantoux test, this can be confirmed or refuted by the more accurate Quantiferon TB Gold test, which requires a vial of blood be drawn.

If you have more questions, you could ask your health provider.
Sue

Thank you very much.

Have you received any more info regarding this? I am very confused since the same thing happened to me recently. My first test had a TB1-Nil of 0.59, TB2-Nil of 0.02, and Mitogen-Nil of 9.93. I got a chest x-ray afterwards, which came out fine, and I had no symptoms, so I retook the test at a different lab, and my results were negative: TB1-Nil of 0.07, TB2-Nil of 0.09, Nil value of 0.01, and Mitogen value of >10. As you can see, this is a big variation. Also, my last test from a year ago was negative. Upon researching the test, I do see that the lab is supposed to receive the sample within 14 hours of collection. My first test results show that the lab did not receive my sample for over 24 hours. I wonder if this is what affected my results. Also, the first lab took only 1 vial of blood, and the second lab took 4 vials (which upon reading is the correct way). I want to make sure that I am truly negative and not missing anything. I am also curious to see if there are other conditions that can possibly cause a false positive so I can get checked out for those too.

I have had the gold plus test three times, twice in 2021 and once in 2022 and they all came back positive but my chest ex ray is clean. Where do I go from here? Should I request further testing and if so what? Should I be on medication or is it okay to just not worry about it? Thanks

My daughter, brother and I always test positive for TB to to repeated exposures. The clear chest x-rays indicates no treatment is required.
This can be tricky, because some doctors believe the gold test is definitive. My ID doc and my daughter's (in 2 different health systems) disagree.
Do you have a known exposure? My daughter was a jail nurse, but my brother and I have no idea where we got this. Unless you are symptomatic, I would say you can relax.
Sue

Positive test is due to exposure, not necessary a person to have anysymptoms or anything on chest xray. Reconended to get treated for exposure which prevents likehood of getting active tb