Search Results: Displaying all Positive QFT Results (2)

Question Date Updated
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.


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