Search Results: Displaying all About QFT (11)

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What is QFT? 2013 QuantiFERON-TB Gold (QFT) is an in vitro laboratory test that measures responses to TB-specific peptide antigens in whole blood. It is an indirect test for M. tuberculosis infection. A modern replacement to the tuberculin skin test (TST), QFT provides clinicians with an accurate, reliable and efficient tool for aiding the diagnosis of TB infection. QFT is highly specific and sensitive; a positive result is strongly predictive of true infection with M. tuberculosis. However, like the TST and other Interferon-gamma release assays (IGRAs), QFT cannot distinguish between active tuberculosis disease and LTBI, and is intended for use in conjunction with risk assessment, radiography, and other medical and diagnostic evaluations.
What is QFT’s intended use? 2013 QFT is an in vitro laboratory diagnostic test using a whole blood specimen. It is intended for use as a diagnostic aid for M. tuberculosis complex infection, whether active tuberculosis disease or LTBI, and is intended for use in conjunction with risk assessment, radiography, and other medical and diagnostic evaluations. Like the TST or any diagnostic aid, QFT should never be used as a stand-alone test for diagnosis or treatment of active tuberculosis, and a negative QFT result should be used with caution when the patient is suspected of having active TB, or is immunosuppressed. Like any other diagnostic aid, QFT cannot replace clinical judgement
What are the clinical situations in which QFT can be used? 2013 QFT can be used for those being evaluated for possible M. tuberculosis infection, whether active disease or LTBI. According to the US Centers for Disease Control and Prevention (CDC), QFT can be used in many situations: CDC Specific Recommendations „„-IGRAs may be used in place of (but not in addition to) a TST in all situations in which the CDC recommends TST as an aid in diagnosing M. tuberculosis infection. „„- IGRA is preferred for testing persons from groups that historically have poor rates of return for TST reading. „„- IGRA is preferred for testing persons who have received Bacille Calmette-Guerin (BCG) (as a vaccine or for cancer therapy). „„- Either an IGRA or a TST may be used (without preference) to test recent contacts of persons with infectious tuberculosis with special considerations for follow-up testing. In contact investigations, negative results obtained prior to 8 weeks typically should be confirmed by repeat testing 8–10 weeks after the end of exposure. „„- Either an IGRA or a TST may be used (without preference) for periodic screening that addresses occupational exposure to TB (eg. surveillance programs for healthcare workers (HCW)) with special considerations regarding conversions and reversions (see full CDC guideline). Two-step testing is not required because IGRA testing does not boost subsequent test results. „„- TST is preferred for testing children younger than 5 years old, due to the relatively few published reports documenting the performance of IGRAs in young children. However use of an IGRA in conjunction with TST may increase diagnostic sensitivity in this age group. „„- While routine testing with both TST and an IGRA is not recommended, results from both tests may be useful in the following situations when the initial test is NEGATIVE: †† when the risk of infection, the risk of progression, and the risk of a poor outcome are increased (such as when persons with HIV infection, or children < 5 years old are at increased risk for M. tuberculosis infection), or †† when there is clinical suspicion of active tuberculosis (such as in persons with symptoms, signs, and/or radiographic evidence suggestive of active tuberculosis) and confirmation of M. tuberculosis infection is desired. „„- While routine testing with both TST and an IGRA is not recommended, results from both tests may be useful in the following situations when the initial test is POSITIVE: †† additional evidence of infection is required to encourage compliance (such as in foreignborn healthcare workers who believe their positive TST is due to BCG); or ††i n healthy persons who have a low risk of both infection and progression. „„- Repeating an IGRA or performing a TST may be useful when the initial IGRA result is indeterminate and a reason for testing persists. „„- Decisions should not be based on IGRA or TST results alone. A diagnosis of M. tuberculosis infection, and the decisions about medical or public health management should include epidemiological, historical, and other clinical information when using IGRA or TST results. „„- Persons with a positive TST or IGRA result should be evaluated for the likelihood of M. tuberculosis infection, for risks of progression to tuberculosis disease if infected, and for symptoms and signs of tuberculosis disease. „„- Neither an IGRA nor TST can distinguish LTBI from TB disease. A diagnosis of LTBI requires that tuberculosis disease be excluded by medical evaluation, which should include checking for suggestive symptoms and signs, a chest radiograph, and, when indicated, testing of sputum or other clinical samples for the presence of M. tuberculosis. „„- In persons with symptoms, signs, or radiographic evidence of TB disease, and in those at increased risk of progression to tuberculosis disease if infected, a positive result with either an IGRA or TST may be taken as evidence of M. tuberculosis infection. However, negative IGRA or TST results are not sufficient to exclude infection in these persons, especially in those at increased risk of a poor outcome if disease develops, and clinical judgment dictates when and if further diagnostic evaluation and treatment are indicated. „„- Both the standard qualitative test interpretation and the quantitative assay measurements should be reported, together with the criteria for test interpretation. „„- As with the TST, IGRAs generally should not be used for testing persons who have a low risk of infection and a low risk of disease due to M. tuberculosis. „„- IGRAs or TST should be used as aids in diagnosing infection with M. tuberculosis. These tests may be used for surveillance purposes or to identify persons who are likely to benefit from treatment. „- IGRAs should be performed and interpreted according to established protocols using FDAapproved test formats. IGRAs should be performed in compliance with Clinical Laboratory Improvement Amendment (CLIA) standards. „„- For BCG vaccinated persons who are not at increased risk for developing TB if infected, TST reactions <15mm may be reasonably discounted as false positives if the individual has a clearly negative IGRA result. „- If two different tests are performed, a positive result from either test should be taken as evidence of infection for those with suspected active TB or at high risk of progression.
Can QFT distinguish between active TB and LTBI? 2013 Like the TST and other IGRA tests, QFT cannot distinguish between active TB and LTBI. Anyone testing positive should be assessed for active TB with a medical evaluation, chest radiograph, and other tests indicated by the clinical symptoms and medical evaluation.
How does it work? 2013 QFT measures cell-mediated immune (CMI) response in TB-infected individuals. T-cells of these individuals are sensitized to TB, and respond to stimulation with peptides simulating those expressed by the TB causing bacteria by secreting a cytokine called interferon-gamma (IFN‑γ). QFT uses peptides from three proteins made almost exclusively by M. tuberculosis and the other mycobacteria of the tuberculosis complex. Those proteins are absent from all BCG vaccine preparations and from most non-tuberculous mycobacteria (NTM) (with the exceptions of M. kansasii, M. marinum, and M. szulgai). Special blood collection tubes coated with peptides from these three TB antigenic proteins are used for blood collection and incubation of the patient’s blood. IFN-γ is released when the blood from infected individuals is incubated with the antigens (16–24 hours at 37°C). This is not the case for individuals free from infection. An ELISA laboratory test is used to detect and quantify the amount of IFN‑γ that has been released.
Why measure interferon-gamma? 2013 M. tuberculosis is an intracellular pathogen primarily residing within macrophages. During the latent phase of the infection little—if any—antigen is expected to leave the macrophages to be available to B-cells to stimulate a humoral antibody response. However processed antigen is presented by infected macrophages to antigen-specific T-cells and triggers a cascade of immune responses leading to the generation of specialized effector T-cells, which will circulate in the individual’s blood stream. When blood is taken from an infected individual and stimulated with M. tuberculosis-specific antigens, effector T-cells release the cytokine IFN-γ. The production and subsequent measurement of IFN-γ by a rapid ELISA forms the basis of QFT.
How does QFT differ from the TST? 2013 Sensitivity and specificity: The tuberculin purified protein derivative (PPD) used in the TST is an ill-defined mix of proteins and protein fragments, of which some are specific for M. tuberculosis complex. However, the vast majority have homologs that are shared with environmental mycobacteria and BCG vaccine strains. It is largely for this reason that the TST test has poor specificity, especially in BCG-vaccinated individuals. The TST assesses in vivo delayed-type hypersensitivity (Type IV), whereas QFT measures in vitro release of IFN-γ. The TST measures response to PPD, a polyvalent antigenic mixture, whereas QFT measures responses to a well defined mix of synthetic peptides simulating three antigenic proteins that are specific for tuberculosis. Unlike the TST, an uninfected individual is not subject to boosting with a QFT. Moreover, QFT is not confounded by BCG vaccination and most common environmental NTM (except M. kansasii, M. marinum, and M. szulgai). Handling and interpretation: There are numerous differences between the TST and QFT: „„- The TST requires skill in placing PPD, whereas QFT requires routine phlebotomy. „„- The TST requires a person to return to have their test read 48 to 72 hours after administration. QFT requires only one visit to a healthcare provider for blood collection. „„- The TST is subjective in its interpretation—in respect to both measuring the induration on the individual’s arm and in deciding what cut-off to apply. QFT is an objective, laboratory based, test with interpretation determined by analysis of ELISA data by QFT analysis software. „„- Positive QFT results can be provided confidentially, whereas a positive inflammatory TST response can be a source of stigma since it is often visible, especially if redness accompanies the induration. „- Individuals can confound their own TST with something as simple as a hot shower or low dose over-the-counter corticosteroid cream.
How long does it take to get QFT results? 2013 This varies and depends on how frequently the laboratory in your area carries out the test. Results can be available in 24 hours. Unlike the TST, individuals do not need to return 2–3 days later in order to have the test read.
What is the minimum time necessary to wait between exposure to M. tuberculosis and QFT testing? 2013 Available data suggests that QFT returns a positive result at least as quickly as the TST following recent infection. A Japanese study concluded that the standard 3 month follow-up used for the TST should be used for QFT. In that study, individuals were tested at the time of diagnosis of the index, and at 2, 3, 4 and 6 months. Of those who developed positive responses, 2 contacts were positive at the time of diagnosis of the index, 5 more were positive at 2 months and 1 more at 3 months. In a contact investigation of Swiss military recruits, 14 out of 15 contacts were positive when tested 8 weeks after exposure. The CDC guidelines on the use of QFT recommend that recent contacts who test QFT negative prior to 8 weeks after the end of exposure, be retested 8 to 10 weeks later—similar to the recommendations for the TST. Many other national guidelines recommend a similar approach.
Why do you include a positive control? How does this work? 2013 The Mitogen tube is used as an IFN-γ positive control for each specimen tested. The Mitogen tube also serves as a control for correct blood handling and incubation. The mitogen used is phytohaemagglutinin-P (PHA), which is a non-specific stimulator of T-cells. While it is a direct activator of T-cells, unpublished data suggest that macrophages are also required for it to activate T-cells. A low IFN-γ response to Mitogen (< 0.5 IU/mL) indicates an indeterminate result when a blood sample also has a negative response to the TB antigens. This pattern may occur with insufficient lymphocytes, reduced lymphocyte activity due to improper specimen handling, or an inability of the patient’s lymphocytes to generate IFN-γ.
What is the evidence supporting QFT? 2013 „„- Over 900 publications in numerous international journals support the use of QFT in different clinical settings. „„- For a complete and up to date list of clinical papers and guidelines, please refer to, the online QuantiFERON library.


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