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 an interferon gamma release assay (IGRA) for tuberculosis diagnosis manufactured by QIAGEN.
According to the U.S. Centers for Disease Control,[2] 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.
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.
A recently published study[7] demonstrated that a positive IGRA result is predictive of future active TB risk.
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.
[11] 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.
If the patient is infected with M. tuberculosis, their white blood cells will release IFN-gamma in response to contact with the TB antigens.
Detection of interferon-y by enzyme-linked immunosorbent assay (elisa) is used to identify in vitro responses to these peptide antigens that are associated with mycobacterium tuberculosis infection.According to the FDA approved package insert[20] Quantiferon TB Gold In Tube has a consistent specificity of >99% in low risk individuals and a sensitivity as high as 92% in individuals with active disease, depending on setting and extent of disease.
The 2010 guidelines establish a new benchmark because they recommend IGRAs as the preferred TB testing method in many patients, including those who are BCG vaccinated or are unlikely to return for TST reading.