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QuantiFERON®-TB Gold In-Tube Test
Test code(s) 16603


Question 1. What is the QuantiFERON®-TB Gold In-Tube?

An alternative to the tuberculosis skin test (TST).

The QuantiFERON-TB Gold In-Tube 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);1-25):

  • “Interference-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 In-Tube work?

The QuantiFERON-TB Gold In-Tube 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 In-Tube 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 In-Tube 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 (0.35–0.7 IU/mL) may not repeat as positive; 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.

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 In-Tube 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 does an indeterminate response mean?

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

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 immune status of the individual being tested
  • Insufficient lymphocytes
  • Inability of the patient’s lymphocytes to generate IFN-γ

 

References

  1. 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.
  2. 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.
  3. Horsburgh CR, Rubin EJ. Latent tuberculosis infection in the United States. N Engl J Med. 2011;364:1441-1448.
This FAQ is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
Document: FAQS.04 Revision: 1