2 TB skin tests. Bumps measured 15cm. RN stated I possibly have LTBI & ordered QuantiFERON Gold in-tube test. Could anything cause a false positive?

Not at 15cm, but... The bump is more likely 15mm, because 15cm is 6 inches and HUGE. A 15mm bump on a screening test is big but may still be a false positive (a screening test means the patient has NO symptoms of tuberculosis). If a patient has contacted somebody with TB, or has current symptoms, she likely has a true positive unless her bump is due to past BCG vaccine. The Quantiferon-TB blood test is very accurate.
Yes. . I think the measurement is mm, not cm. 15 cm would be hugely positive. There are some things that will give a false positive ppd. If you received the BCG vaccine, that will usually cause a false positive. There are other mycobacterium infections that are not TB that could give a false positive result. The Quantiferon Gold is a very sensitive test for TB. .