The nil value comes from the negative control tube, which contains no additives; it is used to determine if the patient has a preexisting immune response (eg, heterophile antibody effects or non-specific IFN-γ production), which could cause a false-positive reading on the test.
For a test to be valid, the nil tube must have a value of ≤8.0 IU/mL. In the example cited above, the nil value (0.07 IU/mL) is well within the recommended range of ≤8.0 IU/mL. The nil tube value is subtracted from the values of the mitogen, TB1, and TB2 tubes for the final result of the respective tubes.
The mitogen, or positive control, tube contains a mitogen (phytohemagglutinin-P), which is a nonspecific stimulator of T-cells. It is used to assure that the patient’s T cells are capable of producing IFN-γ and also serves as a control for correct blood handling and incubation. The mitogen tube is also used to detect false-negative readings.
For a test to be valid, the mitogen tube must have an IFN-γ value at least 0.5 IU/mL higher than the value of the nil tube. This indicates that the test is functioning properly.
In the example cited above, the mitogen tube value minus the nil tube value is well above the minimum breakpoint of 0.5 IU/mL.
The inside of this tube is coated with M tuberculosis–specific antigens designed to elicit CMI responses from CD4+ T-helper lymphocytes.
The inside of this tube is coated with the M tuberculosis–specific antigens designed to elicit CMI responses from CD4+ T-helper lymphocytes and also contains an additional set of peptides targeted to the induction of CMI responses from CD8+ cytotoxic T-lymphocytes.
Results from each of the 4 tubes are evaluated in accordance with the algorithm below (Table 2) to provide a final result.: