Search Results: Displaying all Positive QFT Results (5)

Question Date Updated
Is a patient with a positive QFT response contagious? 2013 The QFT test is both a test for LTBI and a helpful aid for diagnosing M. tuberculosis infection in sick patients where there is clinical suspicion of active TB disease. A positive result supports the diagnosis of TB disease; however, it does not differentiate between recently acquired or old infection, or between LTBI and active tuberculosis. If active TB is suspected, other diagnostic evaluations are necessary to confirm TB disease (e.g. culture of M. tuberculosis) and the patient should be considered at risk of transmitting TB disease. A person with a positive QFT test result—but with no symptoms compatible with active TB—likely has LTBI and is not contagious. However, all people who return a positive QFT test result should undergo clinical evaluation for active TB before they can be assumed infectious or not.
How should a QFT positive response, without information about a recent contact be interpreted? 2013 A positive QFT result is meaningful and even without history of recent contact indicates that M. tuberculosis infection is very likely. However, QFT does not differentiate between recently acquired or old infection, or between LTBI and active tuberculosis. Additionally, infections by other mycobacteria (eg. M. kansasii) can also potentially lead to positive results. As with the TST, a positive QFT response should be not be interpreted in isolation but in conjunction with risk factors. In this situation the person with a positive QFT result may have been infected some time ago and thus have a positive response. However, exposure to someone with active TB may not always be recognized by a person testing positive, and this is one of the factors to be taken into account by the clinician.
Does a positive QFT mean there is a greater risk of progressing to active TB than does a positive TST? 2013 The fact that QFT is more specific than the TST tells us that those with a QFT positive test result are very likely to be truly infected with M. tuberculosis. Therefore, as QFT has been shown to be at least as sensitive as the TST, logic suggests that those with QFT positive test results will be more likely to progress to active TB than those with TST positive test results—on a population basis. Recently, there has been significant growth in the body of evidence confirming that QFT accurately identifies individuals who will progress to active TB disease. In a landmark study published in the American Journal of Respiratory and Critical Care Medicine, QFT had a predictive value for developing TB disease of 12.9%, more than 4 times greater than the 3.1% for the TST. In this study, both TST and QFT were used in a TB contact investigation involving 954 individuals. 66.3% (604) had a positive TST, but only 20.8% (198) of the exposed individuals were QFT positive. Of the QFT positive patients who completed preventative treatment (n=51) none progressed to active TB. The study followed patients for two years post testing, and 19 patients (all untreated) developed active TB disease. QFT had detected all 19 and the TST only 17 (cut-point 5 mm). There were 413 contacts who were TST positive, but QFT negative, and none of these developed TB. Further, the progression rate was 28.6% (6 of 21) for QFT-positive children and significantly higher than 10.3% (13 of 126) for adults. This German study builds on a previously published work by Higuchi et al which showed that after 3.5 years of follow-up, none of 91 QFT negative (but TST positive) contacts had developed TB disease. This indicates that the risk of progression of QFT negative individuals in this BCG vaccinated population is low, even if they are TST positive. All these studies suggest that with the use of QFT, doctors can now treat only a fraction of the individuals they would have had to based on the TST—with the knowledge that they are preventing TB disease.
Can the level of a positive QFT result be used to give an indication of the likelihood of active disease in the future? 2013 QFT is a qualitative (not quantitative) test of TB infection. With current knowledge, the magnitude of IFN-γ response cannot be correlated to stage or degree of infection, level of immune responsiveness, or likelihood for progression to active disease. However, it has been postulated that those with higher QFT responses will be more likely to progress to active TB. This is supported by the study of Diel et al which found that all individuals who subsequently developed TB disease had IFN‑γ responses greater than 10 IU/mL soon after being infected. Higuchi et al also found an association between magnitude of response and development of active TB.
Can you explain the occasional change in QFT results for people with responses close to the cut-off when the test is repeated? 2013 A QFT result above the population-derived cut-off (0.35 IU/mL) is meaningful and suggests likely M. tuberculosis infection. But on an individual basis, small changes in the level of response between two different testing points should be expected. These changes may be due to the inherent variability of the test itself (< 15% CV), variation in the individual’s immune response over time, perhaps a laboratory artefact, or a change in their infection status. However, it is difficult to determine if small changes around the test’s cut-off are meaningful. For example, a change from 0.34 IU/mL to 0.36 IU/mL is unlikely to have clinical significance, but probably suggests that the person has not changed infection status between testing points. To account for laboratory variations in performing the ELISA, the following recommendation appears in the QFT Package Insert: “Where M. tuberculosis infection is not suspected, initially positive results can be confirmed by retesting the original plasma samples in duplicate in the QFT ELISA. If repeat testing of one or both replicates is positive, the individual should be considered test positive.” If, in serial testing, a person changes from a negative to a positive result, this result is an aid to the clinician making the final diagnosis and possible treatment decision. Ultimately, diagnosis and treatment decisions should be made in light of all available clinical and historical information. This is akin to interpretation of the TST when repeat test results are available.


You haven't selected a question.