QuantiFERON-TB Gold Plus test result interpretation

How are QFT-Plus test results interpreted?

QFT-Plus test results are interpreted based upon the quantification of interferon-gamma (IFN-y) production in each of the four QFT-Plus Blood Collection Tubes (Table 1). IFN-y is released by white cells (T cells) when a whole blood sample comes in contact with tuberculosis (TB)-specific antigens that coat the specialized QFT-Plus Blood Collection Tubes.

QFT-Plus is an indirect assay. The amount of background IFN-y measured in the negative control tube is subtracted from IFN-y that's generated from contact with TB-specific antigens. Therefore, a positive quantitative result implies tuberculosis infection is likely, while a negative result indicates that tuberculosis infection is not likely. An indeterminate result can be recorded if the positive or negative control tube did not return an IFN-y value that allowed a result determination. Indeterminate results are uncommon and

Optional QFT-Plus Analysis Software can be used to analyze raw test data and calculate the results.

Diagnosing or excluding tuberculosis disease, and assessing the probability of TB infection, requires clinical analysis including a combination of epidemiological, historical, medical and diagnostic findings.

QFT-Plus result interpretation table

Nil (IU/mL)	TB1 minus Nil (IU/mL)	TB2 minus Nil (IU/mL)	Mitogen minus Nil (IU/mL)	QFT-Plus Result	Report/interpretation
≤8.0	≥0.35 and ≥25% on Nil	Any	Any	Positive	M.tuberculosis infection likely
	Any	≥0.35 and ≥25% on Nil	Any	Positive	M.tuberculosis infection likely
	<0.35 or ≥0.35 and <25% of Nil	<0.35 or ≥0.35 and <25% of Nil	≥0.50	Negative	M.tuberculosis infection NOT likely
	<0.35 or ≥0.35 and >25% of Nil	<0.35 or ≥0.35 and <25% of Nil	<0.50	Indeterminate	Likelihood of M.tuberculosis infection cannot be determined
>8.0	Any			Indeterminate

For complete details of QFT-Plus test interpretation, consult the QuantiFERON-TB Gold Plus (QFT-Plus) Package Insert.

What are Mitogen and Nil in QuantiFERON Testing?

QFT-Plus has a four-tube format, with the tubes TB1, TB2, Mitogen and Nil.

TB Antigen Tube 1 (TB1) contains TB-specific ESAT-6 and CFP-10 antigens that stimulate IFN-gamma production from CD4 T cells.
TB Antigen Tube 2 (TB2) contains selected CD4 antigens and also proprietary CD8 antigens.
The Mitogen tube serves as a positive control and contains phytohemagglutinin.The Nil tube serves as a negative control and allows for background IFN-y production to be subtracted from the results.

The Mitogen tube is used as an IFN-γ positive control for each specimen tested. The Mitogen tube also serves as a control for correct blood handling and incubation. A low IFN-γ response to Mitogen may occur with insufficient lymphocytes, reduced lymphocyte activity due to improper specimen handling or an inability of the patient’s lymphocytes to generate IFN-γ in response to pathogens.

Elevated levels of IFN-γ in the Nil sample may occur with the presence of heterophile antibodies or in response to intrinsic IFN-γ secretion. The IFN-γ level of the Nil tube is subtracted from the IFN-γ level for the TB antigen tubes and the Mitogen tube to adjust for background IFN-γ secretion.