Of those subjects who were positive TST and negative IFN-γ and whose blood samples successfully completed the prolonged stimulation test (where they assessed the IFN-γ samples after 9 days of incubation) 8 remained negative and 2 were positive.
The 8 TST positives that remained IFN-γ negative indicate that something is not quite right with the TST. The researchers considered this option
we have to consider as potential explanation the fact that we cannot confirm the accuracy of the previous positive TST results performed in other institutions, because the tests were not done under study conditions, and maybe some of them were erroneously considered as positive (the milimetres of induration of the previous positive TST were not recorded).
Interestingly the subjects appeared not too impressed by the TST
we asked some HCWs about the possibility of repeating the TST and they all refuse.Normally TST is not conducted under laboratory conditions and therefore the degree of accuracy of the test cannot be measured or confirmed - it is not an accurate scientific procedure and the accuracy of most if not all currently available TST data is debatable.
Those TST positives that became IFN-γ positive support the hypothesis that IFN-γ detect only effector T-cells whereas the TST is unable to discriminate between memory and effector Tcells.
Given that the risk of developing active TB is higher in the first 2 years after infection, the detection of recent TB infection by means of IFN-γ tests in HCWs (mainly immunocompetent) seems to be very useful for targeting the high risk population that really need LTBI.This is more important than being able to discriminate between active and latent TB, a notion which seems to have possessed and dominated some thought.
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