QuantiFERON^®-TB Gold Plus

The QuantiFERON-TB Gold Plus (QFT-Plus), also known as an interferon gamma release assay (IGRA), is an in vitro diagnostic test that utilizes a peptide cocktail simulating ESAT-6 and CFP-10 proteins to stimulate cells in heparinized whole blood. Detection of interferon gamma (IFN-g) by enzyme-linked immunosorbent assay (ELISA) is used to identify in vitro responses to these peptide antigens that are associated with Mycobacterium tuberculosis infection.

QFT-Plus is an indirect test for M tuberculosis infection (including disease) and is intended for use in conjunction with risk assessment, radiography, and other medical and diagnostic evaluations. This test does not differentiate between latent and active tuberculosis (TB).¹

This type of test is referred to as an IGRA.

Recommandations from the Centers for Disease Control and Prevention (CDC) state that^2,3

• "An IGRA may be used in place of (but not in addition to) a tuberculin skin test (TST) in all situations in which CDC recommends TST as an aid in diagnosing adult M tuberculosis infections."
• “An IGRA is preferred for testing persons who have received Bacillus Calmette-Guérin (BCG) as a vaccine or for cancer therapy.”
• “An IGRA or a TST may be used without preference to test recent contacts of persons known or suspected to have active TB with special considerations for follow-up testing. IGRAs offer the possibility of detecting M tuberculosis infection with greater specificity than with a TST. If IGRAs are to be used in contact investigations, negative results obtained prior to 8 weeks after the end of exposure typically should be confirmed by repeat testing 8 to 10 weeks after the end of exposure.” 

Up-to-date product-specific warnings and other information can be found at http://www.QuantiFERON.com.

The QFT-Plus assay is a blood test for M tuberculosis infection that is based on measurement of a cell-mediated immune (CMI) response. A peptide cocktail simulating the mycobacterial proteins ESAT-6 and CFP-10 stimulate the patient’s T cells in vitro to release IFN-γ, which is then measured using ELISA technology. The test detects infections produced by the M tuberculosis complex, which includes M tuberculosis, as well as certain non-tuberculous mycobacteria, including M kansasii, M szulgai, and M marinum. BCG strains and the majority of other non-tuberculous mycobacteria do not harbor ESAT-6 and CFP-10 proteins; thus, patients either vaccinated with BCG or infected with most environmental mycobacteria should test negative. Results should always be interpreted in conjunction with other clinical and laboratory findings.

• Results are not subject to reader bias

• Accuracy is not affected by prior BCG vaccination

• Requires only 1 patient visit (2-4 required for TST)

• Reduced staff time and avoidance of unnecessary follow-up, additional treatment, and treatment due to false-positive TST results²

An example of a QFT-Plus report is shown below:

• Nil or Negative Control Tube (gray cap)

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.

• Mitogen or Positive Control Tube (purple cap)

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.

• TB1 Antigen (green cap)

The inside of this tube is coated with M tuberculosis–specific antigens designed to elicit CMI responses from CD4+ T-helper lymphocytes.

• TB2 Antigen (yellow cap)

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.

• Result Evaluation

Results from each of the 4 tubes are evaluated in accordance with the algorithm below (Table 2) to provide a final result.:

Calculation of Positive, Negative, or Indeterminate

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

Per the package insert, “The magnitude of the measured IFN-g 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-g response to TB antigens is greater than that to mitogen, which is possible as the level of mitogen does not maximally stimulate IFN-g production by lymphocytes.”⁴

Conversion: With respect to TB antigens, conversion is the point at which IFN-γ becomes detectable above the established threshold or cutoff from a previous negative or unknown result.

Reversion: The opposite of conversion—a result that goes from above the established cutoff to below the cutoff (positive to a negative). While there are no data derived using the QFT-Plus test, there are numerous publications on conversion/reversion based on the QuantiFERON-TB Gold test.

In one study of the QuantiFERON-TB Gold assay, among individuals with quantitative IFN-γ values just above or just below the cutoff threshold, reversions and conversions were common.⁵ Per the package insert, it is prudent to confirm a positive QFT-Plus test by the same or alternate method before starting therapy if M tuberculosis complex infection is not suspected.

The 2 main causes of indeterminate results are¹

• Lack of incubating the samples properly (such as not incubating the sample at all or incubating longer than recommended)

• Insufficient or overly aggressive mixing of blood collection tubes

Other, less common, causes of indeterminate results are¹

• Presence of heterophile antibodies in the patient sample

• Compromised immune status of the individual being tested

• Intrinsic IFN-γ secretion

• Recent patient illness

• Recent vaccinations

• Lymphocytes responding indiscriminately (recent patient bouts with poison ivy, rheumatoid arthritis, etc)

• Lack of response to phytohemagglutinin (rare, <1 in 1,000 patients)

• Storage of filled blood collection tubes outside the recommended temperature range

• Compromised mitogen transport tubes

• Insufficient lymphocytes

• Inability of the patient’s lymphocytes to generate IFN-γ

• Immune-modulating therapies, especially in patients who are also receiving high-dose steroids

If an indeterminate result is obtained, the physician may choose to redraw a specimen or perform other procedures as appropriate.¹

No. QFT-Plus testing cannot be used to monitor TB therapy.⁶

Yes, a prior TST can boost IGRA results. However, boosting does not occur in uninfected individuals, because they do not have an established immune response to TB. Furthermore, the TST, despite its in vivo application, does not cause sensitization or establish a cell-mediated response. Therefore, it cannot cause subsequent boosting among persons without prior TB infection. The implications of a boosted response are unknown. However, a boosted response is thought to reflect the induction of a weak memory response from multiple points of stimulation. Boosting is a common phenomenon when a TST is repeated. Each TST can boost subsequent TST responses, due to remote TB infection, as well as infection with non-tuberculous mycobacteria or vaccination with BCG.^1,7,8

COVID-19 vaccines: The CDC previously recommended either the TST or IGRA be performed >4 weeks after completion of COVID-19 vaccination to minimize potential interference between vaccination and TB testing. This recommendation reflected an abundance of caution during a period when these vaccines were new. These tests may now be done without regard to timing of COVID-19 vaccination and can even be administered during the same encounter as COVID-19 vaccination.⁹

Similarly, COVID-19 vaccination should not be delayed because of testing for TB infection.⁹

Live-virus vaccines: Live-virus vaccines might affect IGRA test results.¹⁰ However, the effect of live-virus vaccination on IGRAs has not been studied.¹⁰ Until additional information is available, IGRA testing in the context of live-virus vaccine administration should be done as follows:¹⁰

• Either on the same day as vaccination with a live-virus vaccine or 4-6 weeks after the administration of the live-virus vaccine
• At least 1 month after smallpox vaccination

Yes. An IGRA can be used for children 2 years of age or older. It is preferred for children who have had the BCG vaccine or are unlikely to return for a TST to be read. For children younger than 2 years of age, the TST is the preferred test to detect TB infection.¹²