In this case TST simply unreliable.
The only way you have to diagnose a relapse is AFB, culture as gold standard, then you can have CXR, flogosis indexes and symptoms.
Even if not routinely used in most affected country, molecular technics are also relevant to diagnose relapse in TB .
TST does not help here.More over If a person vaccinated with BCG/person with latent infection appears positive for TST. In relapsed cases culture or molecular method is the best method to diagnose
Don't rely too much on molecular technics.
PCR on smear or bronchoaspiration may be positive in some healed patients even many years after treatment and without relapse. Culture remain gold standard.
The reason I asked this question is the public health regulations of certain developed countries where you need to produce a TST for clearance and people from countries with higher prevalence of TB or Past History of TB or even at times this BCG makes it a kind of hurdle, second is if its a remote history of say 20 years ago what will be the impact ?
Here in Italy to obtain LTBI diagnosis on vaccinated people, we directly perform IGRA tests. But QFT or T-spot (IGRAs) are unuseful to diagnose a relapse. Even if you assume that IGRAs response glides gradually down in the years after a TB, what can you say if you see an healed patient who showed a prevoious value of 1.5 IU/ml and now has a 3.3 IU/ml? On the basis of these data alone can you say that this patient has a relapse? Certainly not. To say it is a relapse you need to perform smear examination and culture, CXR, flogosis indexes, clinical story etc. TB relapse diagnosis sometimes is a difficult affair
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