Tuberculosis Screening Protocols using QFT (QuantiFERON-GOLD)
Purpose
To serve as a test to identify employees with latent Mycobacterium Tuberculosis infection. 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 schedules that make skin testing or 2 step skin testing difficult to place and read in appropriate time frame
- 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
2. QFT-G will require a minimum of 5cc of blood in green top tube. The sample tube
should be gently mixed immediately after the blood draw and transported to Central
Distribution at room temperature. The green Special Chemistry Lab requisition should
indicate QFTB next to the section marked “Other”. Sample draw times will be limited
to Mon- Thurs. 0730-1200. The specimen should not be refrigerated or centrifuged.
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 INH treatment is recommended will be review this with the clinic nurse.
Treatment may be coordinated through employee health, their personal physicians or
the Health Department. A copy of their skin test results, and/or QFT-Gold result and
chest radiograph will be given to the employee.
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 NP or MD 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.
