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SIR—Rothel and Radford [1] and Mazurek et al. [2] raise a barrage of minor methodological points regarding our analysis [3] of the tuberculin-stimulated interferon-γ release assay (IGRA) for the diagnosis of latent tuberculosis infection (LTBI). This test is currently being sold by Rothel and Radford's company under the name QuantiFERON-TB (QFT; Cellestis). Their comments, which repeatedly undermine the validity of the tuberculin skin test (TST) for the diagnosis of LTBI, appear to be an attempt to distract readers from a central, critical fact—a half-century of clinical trials demonstrate that treatment of LTBI, diagnosed on the basis of TST results, prevents active tuberculosis and thereby saves lives (reviewed in [4] and [5]). The same has not been shown for the IGRA.

Given that dozens of very large studies pertaining to the diagnostic and prognostic abilities of the TST have been conducted over a period of many decades, it hardly seems reasonable to consider the IGRA, which has been the subject of only a handful of relatively small studies, as the appropriate “gold” standard. Our “bias” toward the TST is founded on very good reasoning, hundreds of thousands of subjects, millions of person-years of posttest follow-up experience in evaluating tests, and sound statistics. In contrast, the IGRA has been assessed among a small number of persons in cross-sectional studies, and virtually no longitudinal studies have been conducted to determine its ability to predict who will develop tuberculosis.

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Correspondence
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