Houston Methodist. Leading Medicine.
Houston Methodist. Leading Medicine

Tuberculosis Testing

Tuberculosis Screening - Interferon Gamma Release Assay (IGRA) Testing Available


As of 2010, there are two FDA approved assays (QuantiFERON®-TB Gold In-Tube; Qiagen - Valencia, CA) and the T-SPOT®.TB; Oxford Immunotec - Abingdon, UK) that indirectly identify a person with Mycobacterium tuberculosis infection (and possibly disease). These two assays use different diagnostic platforms (ELISA and ELISPOT) and are collectively known as interferon gamma release assays (IGRAs), reflecting the release of interferon gamma (IFN-γ) from immune cells of individuals infected with Mycobacterium tuberculosis. IGRAs have been shown to be more sensitive and specific that the tuberculin skin test (TST) and are not affected by the BCG vaccine. IGRAs are used in conjunction with risk assessments, chest radiography, sputum and physical examination to determine tuberculosis infection or disease. The most recent CDC guidelines for the use of IGRAs can be found here: Centers for Disease Control and Prevention. Updated Guidelines for Using Interferon Gamma Release Assays to Detect Mycobacterium tuberculosis Infection --- United States, 2010. MMWR 2010; 59(No. RR-5:1-26).

QuantiFERON® (ELISA) - Sampling

  1. Collect approximately 1ml of whole blood in the 3 antigen-coated tubes in the following order: Nil (gray cap), Antigen (red cap), and Mitogen (purple cap). Note: Blood should be filled to the black line located on each tube (see red top tube in figure). Over-filling (purple top tube in figure) or under-filling (grey top tube in figure) of tubes with blood can affect results and the specimens may be rejected by the laboratory. Blood should not be obtained from PICC or central lines, but should be drawn with a non-heparinized phlebotomy instrument (needle, syringe or butterfly). High-altitude tubes should be used at altitudes above 3,350 feet. Tubes should be stored between 17°C to 25°C (63°F to 77°F) at the time of blood filling.
  2. Immediately after blood collection, invert tubes 10-12 times to thoroughly mix blood with antigens (coating the inside of each tube).
  3. At this point, you have 2 options:
    1. Either transport the sample to TMH Laboratory Central Specimens Receiving (CSR) before 2:30PM and within 16 hours of blood draw (transport at 22°C±5°C);
    2. Or incubate the sample at your site at 37°C±1°C for 16-24 hours. After incubation, transport to TMH Laboratory Central Specimens Receiving.

T-Spot.®TB Assay (ELISPOT) - Sampling

  1. Draw 4- 8ml of blood in lithium heparin or sodium heparin blood collection tubes (green tops). Note: EDTA blood tubes are not recommended.
  2. Gently mix by inverting the tubes 8-10 times to ensure that the whole blood is mixed thoroughly with the anticoagulant.
  3. Transport or ship to TMH Laboratory Central Specimens Receiving before 2:30PM. This assay may be more specific for pediatric, transplanted, immunocompromised, or HIV patients. For these patients, 8mL of blood is the minimum volume required for the test. Note: A minimum of 1 million peripheral blood mononuclear cells (PBMC) are needed for each T-Spot.TB assay.
  4. If the sample is received after 2:30 PM Monday through Wednesday, the specimen will be processed the next day (within 32 hours of blood draw) and the FDA approved T-Cell X-Tend® reagent will be used to enrich the PBMC.
  5. Samples may be also shipped overnight to arrive in the morning. If the sample cannot be shipped to arrive the same day, the T-Cell X-Tend® reagent will be used on these specimens.

IGRA testing is performed Monday through Friday. Samples are time-sensitive and must be received by Thursday at 2:30 PM, but incubated QuantiFERON samples may be received by Friday at 2:30 PM. Any T-Spot specimen received after 2:30 on Thursday will be rejected.

Interpretation of IGRA results:

Diagnosing or excluding tuberculosis disease and assessing the probability of LTBI require a combination of epidemiologic, historic, medical and diagnostic findings that should be taken into account when interpreting the IGRA results. See general guidance on the diagnosis and treatment of TB disease and LTBI at http://www.cdc.gov/tb/topic/testing/default.htm.

Test Results:

Both the quantitative and qualitative IGRAs results are reported, usually within 72 hours. Results are faxed to clients, but can also be received electronically or received via a web-based user interface when approved.

Gamma interferon concentrations are expressed in IU/mL. The cut-point for the QuantiFERON assay is 0/35 IU/mL. A positive QuantiFERON result occurs when the TB antigen minus the nil ≥ 0.35 IU/mL and ≥ 25% of the nil value and the nil value ≤ 8.0 IU/mL.

The cut-point for the T-Spot assay is 8 spots. A positive T-Spot result occurs when the Panel A antigen - nil or the Panel B antigen - nil ≥ 8 spots and the Nil control has less than 10 spots (see figure).

IGRA Limitations:

A false negative result can be caused by incorrect sample collection or improper handling of the specimen, affecting lymphocyte function.

While the IGRA antigens are absent from BSG strains of Mycobacterium bovis and from most environmental mycobacteria, it is possible that a positive IGRA result may be due to infections with Mycobacterium kansasii, Mycobacterium szulgai or Mycobacterium marinum. Alternative tests would be required if these infections are suspected.

A negative IGRA result does not exclude the possibility of exposure to, or infection with M. tuberculosis. Patients with recent exposure to TB infected individuals and exhibiting a negative IGRA should be considered for retesting within 6 weeks or if other relevant clinical symptoms indicate possible infection.

A positive IGRA result does not rule in active TB disease; additional tests should be performed to confirm the diagnosis of active TB disease such as sputum smear and culture, PCR and chest radiography.

For more details see "QuantiFERON" and "T-Spot" under our Searchable Test Catalog.

To obtain sampling tubes for either assay, please contact HMH Laboratory Client Services:

Hours: Monday - Friday 7:00 AM - 6:00 PM (CST)
Phone: (713) 441-4411 or 1 (855) 522-3282 (LABDATA)
Fax: (713) 441-4412
Email: hmdl@HoustonMethodist.org

Please label the sample tubes with:

Patient's name (Last name, First name)
Patient's date of birth
Hospital or clinic identification number
Date and time of blood collection

Test Request Form:

Please download and fill out a Molecular Tuberculosis Laboratory Clinical Test Request Form. Place the sample and Request Form in a sealed specimen (biohazard) bag. Please be sure to provide a detailed history and any specific questions to be answered.

Shipping:

Pack the sample with the test request form.
Send overnight by Fedex or other courier at 22°C±5°C with sample tracking.
Email the tracking number to laboratory personnel: jdlew@HoustonMethodist.org

Shipping Address:

Houston Methodist - CSR
Attention: Molecular Tuberculosis Laboratory (Graviss Lab)
6565 Fannin, Room D2-109, Houston, TX 77030
Tel 713-441-4411 or 713-441-1854
Fax: 713-441-4412 or 713-441-1802

For general questions: - contact our Laboratory Client Services: 713-441-4411

For clinical questions please contact:

Dr. Edward A. Graviss
Director, HMHRI Molecular Tuberculosis Laboratory
Department of Pathology and Genomic Medicine
Houston Methodist Research Institute, R6-117
6670 Bertner
Houston, TX 77030
713-441-4323 (phone)
713-441-3599 (fax)
eagraviss@HoustonMethodist.org

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