Affiliations: Tuberculosis and Chest Service, Department of Health,
Tuberculosis and Chest Unit, Grantham Hospital, Hong Kong, China
Correspondence to: Chi Chiu Leung, MB, FCCP, Department of Health, 4/F Shaukiwan Jockey Club Clinic, 8 Chaiwan Rd, Shaukiwan, Hong Kong, China; e-mail: firstname.lastname@example.org
We read with great interest the article by Kobashi et al1 on the clinical utility of QuantiFERON TB-2G (Cellestis Ltd; Carnegie, VIC, Australia) for elderly patients with active tuberculosis (TB). Despite a relatively limited sample size, the article confirmed that the sensitivity of QuantiFERON TB-2G was not significantly reduced among very old subjects with culture-confirmed TB as compared to younger subjects (77% vs 87%, p = 0.185), in sharp contrast with the tuberculin skin test (TST). However, conflicting results have been reported regarding the clinical utility of the interferon-γ release assay (IGRA) in the diagnosis of active TB, depending on the methodology.1–4 Very high sensitivity (> 90%) and specificity (> 95%) were similarly reported in a case-control study2involving bacteriologically confirmed TB cases in a low-prevalence area. In two studies3–4 of patients with suspected disease, the negative predictive values for active TB were approximately 85%, but the positive predictive values were only 23% and 60%.
The differing sensitivity of TST and T-SPOT.TB (Oxford Immunotec; Oxford, UK) for latent tuberculosis infection (LTBI) among elderly subjects has been reported in another study.5 Despite this, the clinical utility of IGRA in confirming the diagnosis of active TB depends critically on the prevalence of disease among subjects in the sample tested. In contrast with LTBI, untreated active TB carries high morbidity and mortality. Missing 15 of 100 patients with active TB is quite unacceptable. While the IGRA may be useful for establishing LTBI, it cannot accurately differentiate between infection and disease. Indeed, none of the existing LTBI tests can rule in or rule out active TB. Their roles may be further limited among the elderly with high background LTBI prevalence in most Asian countries.1–5 When bacteriologic confirmation is not available, the overall clinical picture must be taken into account in arriving at a diagnosis sensibly.
The authors have no conflicts of interest to disclose.
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