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peabody88
@peabody88

Posts: 57
Joined: Sep 16, 2018

Any false positive TB blood test results out there?

Posted by @peabody88, Mon, Sep 17 11:31pm

I've been recently having some chronic health issues, but both my infectious disease specialist and I were shocked when I had my Quantiferon Gold TB blood test came back positive. When I originally asked him about false positives, he had said that the test was incredibly accurate and a false positive was unlikely; however, he recently said after doing more research he suspects it may in fact be a false positive. I'm set to have the test repeated soon, but I was just curious if anyone (or how many) people had received a false positive TB blood test before?

REPLY

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.

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