Table 3. Proposed groups classified by IGRA response following TST administration, with suggested possible immune mechanisms.

Group by IGRA response	IGRA assay QFT-GIT/T-SPOT.TB		Supposition
	Pre-TST	Post-TST
Persistently negative	−/−	−/−	No evidence of previous or current infection
Persistently positive	+/+	++/++	Strong stimulation from persisting antigen accompanied with robust T cell responses to PPD
Concordant conversions	−/−	+/+	Expansion of T cell memory population after PPD stimulation on a background of historic infection
Discordant conversions	−/+	+/++	Expansion of T cell memory population after PPD stimulation on a background of historic infection
Discordant reversions	−/+	−/−	Weak T cell response induced after limited (temporal or quantitative) M.tb antigen exposure

IGRA  =  interferon-gamma release assay; TST  =  tuberculin skin test; QFT-GIT  =  QuantiFERON Gold in-Tube; PPD  =  purified protein derivative; +  =  positive response; −  =  negative response.