I got this question which I interpret as
Why PPD test in patients without M. tuberculosis often have slightly positive PPD reaction i.e., a lifelong slightly positive skin test reaction?
because slightly positive -phrase does not make so much sense to me. I first thought it is about false positiveness, but it does not make sense.
I am reading Murray Microbiology book. It says there
[An alternative to the PPD skin test,] is in vitro IFN-gamma release assays. The tests use immunoassays to measure IFN-gamma produced by sensitised T cells stimulated by M.tuberculosis antigens. If an individual was previously infected with M.tuberculosis, exposure of sensitised T cells present in whole blood to M.tuberculosis-specific antigens results in IFN-gamma production.