2.3. Test descriptions

The following technologies were included in the evaluation:

  • TST (RT23 PPD or PPD-S);
  • QuantiFERON-TB Gold In-Tube (QFT-GIT, QIAGEN, Australia); and
  • T-SPOT.TB (T-Spot, Oxford Immunotec, United Kingdom).

The original tuberculin material used by Mantoux in his first studies of tuberculin reactions was a heterogeneous mix of substances from killed Mtb. This so-called old tuberculin was replaced in the 1960s by a standardized preparation of purified protein, derived (hence the term “PPD”) from Mtb. Florence Seibert produced a single standard lot of this material, termed PPD-S; subsequently, all newly produced tuberculin material has been produced using the same methods, and tested against PPD-S, measuring induration in sensitized guinea pigs.

PPD-S contains a mix of antigens, including some that are specific to Mtb, but also many that are found in NTM and BCG. Hence, false positive reactions to PPD-S have been described in people with NTM disease, in those sensitized to NTM antigens, or in those who have received BCG vaccination (particularly if they received BCG more than once, or after infancy).

All of the assays work on the principle that the T-cells of an individual who has acquired TB infection will respond to re-stimulation with Mtb-specific antigens by secreting IFN-γ. The QuantiFERON-TB Gold and the newer version QuantiFERON-TB Gold In-Tube are whole-blood based enzyme-linked immunosorbent assays (ELISAs) that use whole blood and measures the amount of IFN-γ produced in response to specific Mtb antigens (QFT-G: ESAT-6 and CFP-10; QFT-GIT: ESAT-6, CFP-10 and TB7.7). In contrast, the enzyme-linked immunospot (ELISPOT)- based T-Spot measures the number of peripheral mononuclear cells that produce INF-g after stimulation with ESAT-6 and CFP-10.

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