Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.

share|improve this question
up vote 1 down vote accepted

The question asks PPD test in patients without M.tuberculosis. It does not say if the patient has previously had infection with M.tuberculosis. I think this question implies that the patient has never had before infection with M.tuberculosis. I am not sure how it can be in reality possible for adults, since M.tuberculosis spreads in air.

The reason for false positiveness can be that assays that use PPD use unspecific antigens, in contrast to IFN-gamma and second-generation assays.

The other reason is that once you get infected you always have some degree positive skin test reaction. So to say patient without M.tuberculosis is faulty in a way because I think most people get exposured to this bacteria at least little in life.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.