|
Interferon-gamma release assays (IGRAs) are diagnostic tools for latent tuberculosis infection (LTBI). They are surrogate markers of ''Mycobacterium tuberculosis'' infection and indicate a cellular immune response to ''M. tuberculosis''. IGRAs cannot distinguish between latent infection and active tuberculosis (TB) disease, and should not be used as a sole method for diagnosis of active TB, which is a microbiological diagnosis. A positive IGRA result may not necessarily indicate active TB; however, a negative IGRA result rules out the possibility of both active and latent tuberculosis. Because IGRAs are not affected by Bacille Calmette-Guérin (BCG) vaccination status, IGRAs are useful for evaluation of LTBI in BCG-vaccinated individuals, particularly in settings where BCG vaccination is administered after infancy or multiple (booster) BCG vaccinations are given. In contrast, the specificity of tuberculin skin test (TST) varies depending on timing of BCG and whether repeated (booster) vaccinations are given. QuantiFERON, also known as QFT, is the registered trademark of the test for tuberculosis infection or latent tuberculosis. QFT is an interferon-γ release assay (IGRA) used in tuberculosis diagnosis. The QFT-GIT assay is an ELISA-based, whole-blood test that uses peptides from three TB antigens (ESAT-6, CFP-10, and TB7.7) in an in-tube format. The result is reported as quantification of IFN-gamma in international units (IU) per mL. An individual is considered positive for ''M. tuberculosis'' infection if the IFN-gamma response to TB antigens is above the test cut-off (after subtracting the background IFN-gamma response in the negative control). == QuantiFERON-TB (QFT) == QuantiFERON-TB Gold In-Tube (QFT-GIT), the third generation test, has replaced QuantiFERON-TB (QFT) and QuantiFERON-Gold, which are no longer marketed. According to the U.S. Centers for Disease Control,〔(【引用サイトリンク】title=Guidelines for Using the QuantiFERON-TB Test for Diagnosing Latent ''Mycobacterium tuberculosis'' Infection )〕 in 2001, the QuantiFERON-TB test (QFT) was approved by the Food and Drug Administration (FDA) as an aid for detecting latent ''Mycobacterium tuberculosis'' infection. This test is an in vitro diagnostic aid that measures a component of cell-mediated immune reactivity to ''M. tuberculosis''. The test is based on the quantification of interferon-gamma (IFN-γ) released from sensitized lymphocytes in whole blood incubated overnight with purified protein derivative (PPD) from ''M. tuberculosis'' and control antigens. Tuberculin skin testing (TST) has been used for years as an aid in diagnosing latent tuberculosis infection (LTBI) and includes measurement of the delayed type hypersensitivity response 48–72 hours after intradermal injection of PPD. TST and QFT do not measure the same components of the immunologic response and are not interchangeable. Assessment of the accuracy of these tests is limited by lack of a standard for confirming LTBI. As a diagnostic test, QFT: # requires phlebotomy # can be accomplished after a single patient visit # assesses responses to multiple antigens simultaneously # does not boost anamnestic immune responses (see Latent tuberculosis#Boosting). Compared with TST, QFT results are less subject to reader bias and error. In a CDC-sponsored multicenter trial, QFT and TST results were moderately concordant (overall kappa value = 0.60). The level of concordance was adversely affected by prior bacille Calmette-Guérin (BCG) vaccination, immune reactivity to nontuberculous mycobacteria (NTM), and a prior positive TST. In addition to the multicenter study, two other published studies have demonstrated moderate concordance between TST and QFT. However, one of the five sites involved in the CDC study reported less agreement. Although there have been studies confirming the increased future risk of active TB in individuals with positive TST, the same was not true for those with a positive IGRA result. A recently published study demonstrated that a positive IGRA result is predictive of future active TB risk. Moreover, IGRA was at least as sensitive and was more specific compared to traditional TST. In this study of immunocompetent recently exposed close contacts of active TB cases, the progression rate to active disease among untreated QFT positive individuals was significantly greater than for untreated TST positives (14.6% versus 2.3%). Although the numbers were small, all of the close contacts who went on to develop active TB were QFT positive, but only 83% were TST positive. As noted above, prior BCG vaccination can produce false positive TST results. In a study of military personnel returning from missions, about one-half of the positive TSTs were falsely positive. In a more recent study of military returning from missions, Franken et al. reported evidence suggesting false positive TST results are common and that QFT testing could guide more targeted treatment and alleviate unnecessary anti-tuberculous treatment. The FDA's cutpoint for a positive result was established at >0.34 International Units/millilitre (IU/ml), though this has proven functionally problematic in low prevalence areas, such as among US and Canadian healthcare workers. In areas of low risk and low prevalence, the positive predictive value of any test is diminished. In the case of serially screened North American healthcare workers, QFT results just above this cutpoint produce false-positive test results that upon repeat testing revert to negative, where tuberculosis screening is often mandated on an annual basis (MMWR June 2010, http://www.cdc.gov/mmwr/pdf/rr/rr5905.pdf). Research at Stanford University and the Veterans Administration has reported the use of a retesting (or borderline) zone below 1.1 IU/ml mitigates 76% of the false-positives, or reversions., Limitations of QFT include the need to draw blood and process it within 12 hours after collection and limited laboratory and clinical experience with the assay. There is need for further study of the utility of QFT in predicting the progression to active tuberculosis, particularly in children and immunocompromised hosts. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「QuantiFERON」の詳細全文を読む スポンサード リンク
|