Properties of tuberculin skin test and interferon-γ release assays (IGRAs)
| Tuberculin skin test | IGRA | ||
| ELISPOT (e.g. T-SPOT.TB#) | ELISA (e.g. QuantiFERON-TB Gold In-tube¶) | ||
| Internal controls | None | Nil and PHA | Nil and PHA |
| Antigens | PPD | Peptides from CFP-10 and ESAT-6 | Peptides from CFP-10, ESAT-6 and TB7.7 |
| Test substrate | Skin | PBMC | Whole blood |
| Time required for result | 48–72 h | 20 h incubation and 3 h for the final results | 20 h incubation and 3–4 h for the final results |
| Cells involved | Neutrophils, memory CD4 T-cells and CD8 T-cells; cells need to home and transmigrate out of capillaries into the skin | CD4 T-cells in the wells+ | CD4 T-cells in whole blood tubes+ |
| Cytokines involved | IFN-γ, TNF-α, TNF-β | IFN-γ | IFN-γ |
| Read-out | Measure of diameter of dermal induration, transverse to long axis of arm | Enumeration of IFN-γ spots | Measure of secreted IFN-γ, measured via ELISA optical density values of IFN-γ production |
| Read-out units | Millimeters | IFN-γ spot forming cells | IU·mL−1 |
| Outcome measure | Level of induration | Number of IFN-γ producing T-cells | Plasma concentration of IFN-γ produced by T-cells |
| Effect of treatment on test response | No effect, unless given soon after exposure | Decline in response with treatment at the population level but wide interindividual variation in rate of decline | Decline in response with treatment at the population level but wide interindividual variation in rate of decline |
ELISPOT: enzyme linked immune spot; PHA: phytohaemagglutinin; PPD: purified protein derivative; CFP: culture filtrate protein; ESAT: early secreted antigenic target; PBMC: peripheral blood mononuclear cells; IFN: interferon; TNF: tumour necrosis factor. #: Oxford Immunotec, Abingdon, UK; ¶: Cellestis, Carnegie, Australia; +: as suggested by flow cytometry 11, 12, 36, 37.