Interferon Gamma Release Assay (IGRA) TB blood (QuantiFERON-GOLD) Testing Protocol

Purpose

To serve as a test to identify employees with latent Mycobacterium Tuberculosis infection.   Interferon Gamma Release Assay (IGRA) QFT-Gold will be used as a supplement /alternate to tuberculosis skin testing (TST) in the tuberculosis screening program.

Scope

The QFT-Gold can be used in all circumstances in which the TST is used but for the purposes of this protocol will be used for conditions in which the TST is inconvenient or may be difficult to interpret.  The QFT-Gold test measures interferon-gamma in the testee’s blood after incubating the blood with specific antigens from M. Tuberculosis proteins.  QFT-Gold results are based on the amount of IFN-gamma that is released in response to the antigens.  It cannot distinguish between latent tuberculosis infection and TB disease.  As with any laboratory test, the result will be interpreted in light of the sensitivity and specificity of the test and epidemiologic and clinical factors in the patient.

Procedure

  1. The test will be offered to the following types of employees:
    • Have an allergic sensitivity to tuberculin skin testing protein that may results in a test that is difficult to interpret
    • As part of a clinical assessment for employees with history of a positive TST such that repeat testing is not advisable
    • Have a reactive skin test and history of receiving the BCG vaccination
    • Had a recent reactive TB skin test but more than 12 weeks since known exposure
    • Have schedules that make skin testing or 2 step skin testing difficult to place and read in appropriate time frame
  2. QFT-G collection. Samples should arrive in lab within 8 hours of draw at room temperature.  Samples will be rejected if not received within 16 hours or by 5:00 p.m. on Fridays.

    • DO NOT refrigerate or centrifuge tubes
    • Label each tube with Name, Time, and Date of collection
    • Order of draw is important – Gray, Red, Purple
    • Tubes will fill slowly to the 1 mL black line, keep tube on needle until it stops filling.  Top of line is 1 mL – must fill to at least bottom of black line to be acceptable draw
    • Use of a butterfly needle is NOT recommended.  If necessary, utilize a waste tube to fill butterfly line prior to filling QuantiFERON tubes
    • Invert tubes 10 times just firmly enough to ensure that the entire surface of the tube has been coated with blood for thorough mixing of the tubes content with the blood.  
    • Place tubes back in sample transport bag and send to lab

      The green Special Chemistry Lab requisition should indicate QFTB next to the section Marked “Other”.    

  3. Evaluation for latent/active tuberculosis will be conducted for any person with positive      TB skin tests or positive QFT-Gold testing.  This includes but may not be limited to a    TB Reactor Assessment and chest x-ray.   If active disease is ruled out, employees for    whom latent TB treatment is recommended will review this with the clinic nurse.     Treatment may be coordinated through UM- Occupational Health Services, their      personal physician or the Health Department.  A copy of their skin test results,    and/or QFT-Gold result and chest radiograph report will be given to the employee.    Employees with a positive QFT result should not be tested by PPD or QFT as the    results will remain positive.

  4. If the test result is negative, the employee will have the QFT-G testing repeated on an  annual basis in place of skin testing.

  5. Persons with indeterminate results will be retested.  If repeatedly indeterminate,    assessment by a clinic provider will be made as to follow up.

Contraindications/Cautions for QFT testing

Interpretation of the QFT-Gold is difficult in persons with immunodeficiency or on immunosuppressive therapy, persons recently exposed to tuberculosis and children 17 years or younger.  Caution should be used with persons that have clinical conditions that increase the risk of progression to active TB (such as HIV).  An indeterminate result does not provide useful information regarding the likelihood of M. tb infection.  The reason for “indeterminate” result should be obtained, such as lack of mitogen response to control or high background levels of IFN-gamma.