Interpretation

The specimen is alloquoted into 4 tubes: nil, mitogen, TB1 and TB2 antigen tubes. These tubes are then incubated for 16-24 hours and then tested.

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 (e.g., heterophile antibody effects or non-specific IFN-γ production), which could cause a false-positive reading on the test.

The mitogen, or positive control, tube contains a mitogen (phytohemagglutinin-P), which is a non-specific 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.

The TB1 Antigen tube is coated with M tuberculosis-specific antigens designed to elicit CMI responses from CD4+ T-helper lymphocytes.

The TB2 Antigen 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.

The amount of IFN-γ produced by each tube (nil, mitogen, TB1 and TB2 antigen) is measured using a standard ELISA format. QuantiFERON-TB Gold Plus results are based on the amount of IFN-γ that is released in response to the antigens. The result “Negative” or “Positive” is calculated from these values using an FDA approved algorithm. “Indeterminate” results are generated when either of the control tubes (nil or mitogen) does not produce the intended values.

Per the package insert, “The magnitude of the measured IFN-γ level cannot be correlated to stage or degree of infection, level of immune responsiveness, or likelihood for progression to active disease. A positive TB response in persons who are negative to mitogen is rare, but has been seen in patients with TB disease. This indicates the IFN-γ response to TB antigens is greater than that to mitogen, which is possible as the level of Mitogen does not maximally stimulate IFN-γ production by lymphocytes.”