From the Department of Pulmonary Medicine (Dr A. K. Khurana) and the Department of Pathology (Dr U. Khurana), Government Medical College and Hospital.
Correspondence to: Alkesh Kumar Khurana, MD, DNB, FCCP, Department of Pulmonary Medicine, Government Medical College and Hospital, Section 32, Chandigarh, 160030 India; e-mail: firstname.lastname@example.org
Financial/nonfinancial disclosures: The authors have reported to CHEST that no potential conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.
Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestpubs.org/site/misc/reprints.xhtml).
© 2010 American College of Chest Physicians
The study in the May 2010 issue of CHEST by Gallant et al,1 whereby they simultaneously compared the quantitative results of in vivo and in vitro assays for TB infection generates a lot of interest. There are a couple of issues that need to be addressed in this study.
The authors have used Bacillus Calmette-Guérin (BCG), purified protein derivative (PPD), and early-secreted antigenic target-6 (ESAT-6) as the triggers for the release of interferon (IFN)-γ in the in vitro assays. The authors have justified the use of PPD in both the in vitro and in vivo assays because it may avoid the antigen specificity as the confounding factor, but the use of BCG as an in vitro trigger remains questionable because the BCG status of the population in question is not known.
ESAT-6 and complement fixation protein-10 (CFP-10) have been suggested as standard triggers for IFN-γ release from lymphocytes,2 but the employment of BCG and PPD for this purpose warrants further research. The higher specificity of ESAT-6 and CFP-10 (ESAT-6 in this study) as compared with other triggers is reflected in the authors’ observation that among the 100 patients with tuberculin skin test induration of < 5 mm, only 7% showed IFN-γ production in response to ESAT-6. On the other hand, 74% of the BCG subgroup and 65% of the PPD subgroup had positive IFN-γ production. This rather highlights the redundant nature of in vitro and in vivo assays, provided the antigens used are only ESAT-6 and CFP-10, although the higher specificity of these IFN-γ release assays is well proven.2
Latent TB infection has been a gray area for decades now because of the heterogenous prevalence of the disease and infection around the globe. We need further studies to reach definite conclusions.
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