Tuberculosis Screening - Interferon Gamma Release Assay (IGRA) Testing Available
QuantiFERON® (ELISA) - Sampling
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.
- Immediately after blood collection, invert tubes 10-12 times to thoroughly mix blood with antigens (coating the inside of each tube).
- At this point, you have 2 options:
- 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);
- 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
- Draw 4- 8ml of blood in lithium heparin or sodium heparin blood collection tubes (green tops). Note: EDTA blood tubes are not recommended.
- Gently mix by inverting the tubes 8-10 times to ensure that the whole blood is mixed thoroughly with the anticoagulant.
- 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.
- 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.
- 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:
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).
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:
Phone: (713) 441-4411 or 1 (855) 522-3282 (LABDATA)
Fax: (713) 441-4412
Please label the sample tubes with:
Patient's date of birth
Hospital or clinic identification number
Date and time of blood collection
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
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:
Director, HMHRI Molecular Tuberculosis Laboratory
Department of Pathology and Genomic Medicine
Houston Methodist Research Institute, R6-117
Houston, TX 77030