Writing in the
Australian Prescriber Anastasios Konstantinos, Director of Queensland TB Control Centre made the following comment on the performance of interferon gamma release assays
many studies show that tuberculin skin testing and interferon gamma release assays perform similarly in non-BCG vaccinated people at high risk for recent tuberculosis infection, if an appropriate cut-off (for example 10 mm induration) is used for tuberculin skin testing.
The reference for this observation was the
Pai Menzies Meta Analysis which clearly states that
the results of IGRAs and the TST were frequently discordant.
The latest
Diel longitudinal study into the predictive value of IGRA for developing active TB
clearly states
Many countries worldwide have adopted a higher TST cutoff for screening close contacts than the 5-mm cutoff recommended by bodies such as the U.S. Centers for Disease Control and Prevention (16) and the German Central Committee against Tuberculosis (17). Our results demonstrate that although the use of a greater than 10-mm cutoff may mitigate the specificity problems inherent with the greater than 5-mm TST cutoff, it introduces a dangerous compromise in terms of sensitivity. Nine of our 19 (47%) close contacts who progressed to active TB would have been overlooked if we had limited our concern to persons with indurations of 10 mm or more; resulting in an NPV for progression of only 98.7%. Clearly such information is highly relevant for policy makers who are considering replacing the TST with an IGRA.
