Specialized Testing

Interpretation of the TB Gold Quantiferon Assay

Quantiferon®-TB Gold, (Client Incubated)

Order Name: TB GOLD
Test Number: 5587325
Panel Details
TB Gold NIL Value
TB Gold Mitogen-NIL Value
TB Gold TB Antigen-NIL Value
Quantiferon® Interpretation

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The interpretation of the TB Gold Quantiferon Assay is very straight forward as long as it is Positive or Negative; however, an indeterminate result suggests an invalid result. Take into consideration that the assay is measuring the release of gamma interferon due to the activation of the T helper cells and macrophages by an antigen. Taking it step by step, first evaluate the Nil and then the mitogen result.

Significance of the NIL result: The NIL indicates the presence of any residual gamma interferon found in the patient’s blood due to an infection with an organism requiring cell mediated immunity/T cell immunity (e.g. virus, fungus, intracellular bacteria, etc.). The NIL tube value indicates if the patient has an ongoing immune response which can cause a false-positive result. The NIL tube must be a value of no greater than 8.0 IU/mL.

Significance of the MITOGEN result: The mitogen control tube addresses the immune competence of the patient’s immune cells. Thus a patient on immunosuppressive therapy/drugs , chemotherapy, lymphopenia, inability of patient’s lymphocytes to generate gamma interferon, etc., will have very little response to the mitogen indicating its inability to respond to an antigen.

Thus a low response to mitogen (<0.5 IU/ml) indicates an indeterminate result when a blood sample also has a negative respond to the TB antigen. Indeterminate results from low mitogen values would not be expected to change upon immediate retesting. If a physician desires to retest a patient on immunosuppressive therapy or chemotherapy, wait 30-60 days before retesting. The mitogen tube also serves as a control for indicating long or improper transport, incorrect blood handling and incubation. If technical issues are suspected with the collection or handling of blood samples, repeat the entire quantiferon test with new blood specimens. The mitogen tube acts as a false negative control. The mitogen tube must demonstrate a gamma interferon value of equal to or greater than 0.5 IU/ml higher than the value of the NIL tube.

Significance of the TB Antigen Result: The TB antigen tube is coated with Mycobacterium tuberculosis specific antigens. The results must always be interpreted in light of the clinical presentation of the patient. A positive quantiferon result is meaningful and even without a history of recent contact indicates that a M. tuberculosis infection is very likely. The quantiferon assay does not differentiate between recently acquired or old infection, or between a latent TB infection and active tuberculosis. Also interpret cautiously because infections by other mycobacterium (e.g. M. kansasii) can generate a positive result. For a test to be considered positive, the TB antigen tube value minus the NIL tube must be equal to or greater than 0.35 IU/ml.

Please call Gerald C. Miller, Ph.D. at 800-722-8077, 918-744-2553 or 918-744-3131 ext 15543 with any questions regarding the TB Gold quantiferon assay.