TY - JOUR T1 - Whole blood IFN-γ assay for detecting TB in children JF - Thorax JO - Thorax SP - 919 LP - 921 VL - 61 IS - 10 AU - A J Radford AU - J S Rothel AU - G Sberna Y1 - 2006/10/01 UR - http://thorax.bmj.com/content/61/10/919.abstract N2 - Connell et al1 uses the QuantiFERON-TB Gold (QFT-Gold) test for tuberculosis (TB) infection, but it contains methodological inaccuracies. Unfortunately, the interpretation of the test by Connell et al was not that recommended by the manufacturer. The authors arbitrarily introduced a restriction on the nil control of the QFT-Gold test to <1 IU/ml interferon (IFN)-γ, falsely generating a number of “failed” tests. As a result, the data presented by Connell et al significantly overestimate the number of invalid test results and may be misleading to readers.The authors also present results that attempt to correlate increasing IFN-γ responses to mitogen (up to 80 IU/ml) with age by extrapolating data beyond the range of the QFT-Gold ELISA. In addition, Connell et al have attempted to compare TB antigen induced IFN-γ levels with disease state. This assumption may or may not be accurate but, as stated in the QFT-Gold package insert, IFN-γ levels above 15 IU/ml cannot be relied upon as they are extrapolations outside the operational range of the instrumentation. All diagnoses of TB infection and identification of low immune function are within the linear range of the test. As such, the observations of Connell et al of an age associated mitogen response and a correlation between IFN-γ levels and TB disease cannot be made from this study.Connell et al further express concerns on tuberculin skin test (TST) positive—even strongly TST positive—individuals who are QFT-Gold negative. This reflects a common misconception that the TST is a definitive diagnostic rather than an indicator of increased risk of TB infection. Large scale studies show that, for most populations, progression to active TB is low, even for large TST responders, and far below the often quoted 5–10%.2–,4 Because progression rate is an indicator of the true infection rate,5 it … ER -