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QuantiFERON®-TB Gold, (Draw Site Incubated)
Test code(s) 16603
Question 1. What is the QuantiFERON®-TB Gold, (Draw Site Incubated)?
An alternative to the tuberculosis skin test (TST).
The QuantiFERON-TB Gold, (Draw Site Incubated) is a blood test for use as an aid in diagnosing Mycobacterium tuberculosis infection, both latent tuberculosis infection and active tuberculosis disease. This test is considered an interferon gamma release assay (IGRA).
Centers for Disease Control and Prevention (CDC) states: “The greater specificity (as compared to the TST) of the Quantiferon test and the requirement for only one office visit are compelling advantages.”
CDC Specific Recommendations (June 25, 2010 / 59(RR05);1-
- “Interferon Gamma Release Assays (IGRAs) may be used in place of (but not in addition to) a TST in all situations in which the CDC recommends TST as an aid in diagnosing adult M. tuberculosis infections.”
- “IGRA is preferred for testing persons who have received Bacille Calmette-Guérin (BCG) as a vaccine or cancer therapy.”
- “An IGRA may be used in place of TST (without preference) to test recent contacts of persons with infectious tuberculosis with special considerations for follow-up testing. In contact investigations, negative results obtained prior to 8 weeks typically should be confirmed by repeat testing 8-10 weeks after the end of exposure.”
Question 2. How does QuantiFERON-TB Gold, (Draw Site Incubated) work?
The QuantiFERON-TB Gold, (Draw Site Incubated) is an indirect test for M tuberculosis infection that is based on measurement of a cell-mediated immune response. A cocktail of 3 mycobacterial proteins (ESAT-6, CFP-10, and TB 7.7) stimulate the patient’s T-cells in vitro to release interferon-gamma, which is then measured using enzyme linked immunoabsorbent assay (ELISA) technology. The test detects infections produced by the M. tuberculosis complex including M tuberculosis, M bovis, and M africanum. Bacilli Calmette-Guérin strains and the majority of other non-tuberculosis mycobacteria do not harbor ESAT-6, CFP-10, and TB 7.7 proteins; thus, patients either vaccinated with bacilli Calmette-Guérin or infected with most environmental mycobateria should test negative. Results should always be interpreted in conjunction with other clinical and laboratory findings.
Question 3. What are the advantages of QuantiFERON-TB Gold, (Draw Site Incubated) over tuberculin skin test (TST)?
- Improved specificity
- Results are not subject to reader bias
- Results are not affected by the booster phenomenon (i.e., increased sensitivity on subsequent test, leading to false-positive results)
- Accuracy is not affected by prior BCG vaccination
- Requires only 1 patient visit (2-4 required for TST)
- Cost savings from reduced staff time and avoidance of unnecessary follow-up testing and treatment
Question 4. How do you interpret the QuantiFERON-TB Gold, (Draw Site Incubated) test result?
An example of a QuantiFERON report is shown below:
Nil or Negative Control Tube (grey cap)
The Nil value comes from the negative control tube that contains no additives; it is used to determine if the patient has a preexisting immune response (i.e., heterophile antibody effects, non-specific gamma interferon production, etc.) which could cause a false-positive reading on the test.
In order 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.
Mitogen or Positive Control Tube (purple cap)
The mitogen control tube contains a mitogen (phytohaemogglutin-P), which is a non-specific stimulator of T-cells. It is used to assure the patient has a healthy immune status and also serves as a control for correct blood handling and incubation. It is used to detect false-negative readings.
In order for a test to be valid, the mitogen tube must have a gamma interferon value ≥0.5 IU/mL higher than the value of the Nil tube. This indicates that the mitogen control, as the positive control, 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.
TB Antigen (red cap)
The inside of this tube is coated with the M tuberculosis specific antigens described above.
For a test to be considered positive, the TB antigen tube value minus the Nil tube value must be ≥0.35 IU/mL (FDA cleared breakpoint). Low positive values (0.35–1.0 IU/mL) may not repeat as positive in some patient populations (see below table); some authorities recommend that the patient be retested to confirm low positive tests.
In the example cited above, the TB antigen tube value minus the Nil tube value is well above the positive breakpoint of 0.35 IU/mL. A positive/detected result suggests that the M. tuberculosis infection is likely. It does not determine if the infection is recent or old, or if the disease is currently active or latent.
Conversions and Reversions – Quantiferon*
Among individuals with quantitative values just above or just below the cut-off threshold, reversions or conversions were common. Retesting was performed within 3 weeks of the original test. Confirming a positive Quantiferon test is prudent before starting therapy.
*Per Susan Dorman, M.D. The John Hopkins University School of Medicine.
Calculation of Detected, Not Detected, or Indeterminate
The amount of gamma interferon produced by each tube (Nil, mitogen, TB antigen) is measured using a standard ELISA format. QuantiFERON-TB Gold, (Draw Site Incubated) results are based on the amount of INF-γ that is released in response to the antigens. The result “Not Detected” or “Detected” is calculated from these values using an FDA approved algorithm run on QuantiFERON software. Indeterminate results are generated when either of the control tubes does not produce their intended values.
Question 5. What causes an indeterminate response?
The 2 main causes of indeterminate results are:
- Lack of incubating the samples properly (such as not incubating the sample at all or over incubating)
- Insufficient mixing of blood collection tubes
Other less common causes are outlined below:
- Presence of heterophile antibodies in the patient sample
- 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 phytohaemagglutinin(occurs in less than 1 in 1,000 patients)
- Storage of filled blood collection tubes outside the recommended temperature range (22°C ± 5°C) prior to 37°C ± 1°C incubation
- Compromised mitogen transport tubes
- Compromised immune status of the individual being tested
- Insufficient lymphocytes
- Inability of the patient’s lymphocytes to generate IFN-γ
If an indeterminate result is obtained, the physician may choose to redraw a specimen or perform other procedures as appropriate.
Question 6. Can Quantiferon testing be used to monitor TB therapy?
No. Quantiferon testing cannot be used to monitor TB theapy.5
Question 7. Does prior TB skin testing affect a Quantiferon result?
Yes. IGRAs such as Quantiferon should be performed within 3 days after placing a skin test to avoid the boosting phenomenon.1
- Daley CL, Reves RR, et al. A summary of meeting proceedings on addressing variability around the cut point in erial nterferon-gamma release assay testing. ICHE 2013;34:625-630.
- Diel R, Loddenkemper R, Meywald-Walter K, et al. Comparative performance of tuberculin skin test, QuantiFERON-TB-Gold In Tube Assay, and T-Spot.TB Test in Contact Investigations for Tuberculosis. Chest. 2009;135:1010-1018.
- Diel RR, Loddenkemper R, Nienhaus A. Evidence-based comparison of commercial interferon-gamma release assays for detecting active TB: a metaanalysis. Chest. 2010;149:177-184.
- Horsburgh CR, Rubin EJ. Latent tuberculosis infection in the United States. N Engl J Med. 2011;364:1441-1448.
- Pollock, NR, Kashino SS, et al. Evaluation of the effect of treatment of latent tuberculosis infection on Quantiferon-TB Gold assay results. Infect Control Hosp Epidemiol. 2009;30:1123-1126.