Title: Questions in the Role of Chest CT Scanning in TB Outbreak Investigation
Abstract: The study by Won Lee and colleagues1Lee SW Jang YS Park CM et al.The role of chest CT scanning in TB outbreak investigation.Chest. 2010; 137: 1057-1064Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar from South Korea in the recent issue of CHEST (May 2010) has generated a number of important issues and questions that need to be highlighted. The authors have diagnosed active pulmonary TB (PTB) in about 21% of patients, and it is really hard to believe this unusually high rate of active disease, especially when all the patients are young soldiers who are immunocompetent and healthy. It is quite surprising that none of the 18 cases of active PTB that were diagnosed using high-resolution CT (HRCT) scans showed positive acid-fast staining of the sputum specimens. Among the nine cases of active PTB diagnosed on the basis of CT scanning alone, two patients had neither any symptoms nor a positive sputum test result for acid-fast bacilli culture. Did these soldiers really have active PTB? How often do we see patients with active PTB who have no symptoms? Was administering a full course of antitubercular treatment for 6 months to these patients justified? The authors have claimed symptomatic or radiographic improvement in all cases of active PTB. Therefore, in those nine patients with normal chest radiographs and active PTB, follow-up HRCT scans of the thorax must have been done to assess radiologic improvement. How do we view this very high dose of radiation exposure in these young soldiers? It is well known that radiation generates highly reactive free radicals and is carcinogenic, teratogenic, and mutagenic. It has been associated with cancers of the thyroid, bone, lung, breast, and leukocytes.2Jacobson JA Radiation injury.in: Beers M Porter R Jones T The Merck Manual of Diagnosis and Therapy. 18th ed. Merck Research Laboratories, Whitehouse Station, NJ2006: 2601-2609Google Scholar Like lead and asbestos, radiation has no safe threshold. Therefore, performing HRCT scans of the thorax in only those patients who have symptoms suggestive of active PTB seems to be a more pragmatic approach. The authors have relied heavily on the results of interferon-γ (INF-γ) release assay in a few patients for the diagnosis of active PTB, even in the absence of clinical symptoms. It is well known that higher production of INF-γ correlates to some extent with the activity of Mycobacterium tuberculosis infection, but its low sensitivity and specificity for distinguishing active vs latent TB has been demonstrated by various studies worldwide, and therefore it is not recommended as a diagnostic tool for active TB.3Browatzki A Meyer CN Interferon-gamma release assay on suspicion of active tuberculosis? [in Danish].Ugeskr Laeger. 2009; 171: 2631-2635PubMed Google Scholar, 4Dewan PK Grinsdale J Kawamura LM Low sensitivity of a whole-blood interferon-gamma release assay for detection of active tuberculosis.Clin Infect Dis. 2007; 44: 69-73Crossref PubMed Scopus (128) Google Scholar, 5Janssens JP Roux-Lombard P Perneger T Metzger M Vivien R Rochat T Quantitative scoring of an interferon-γ assay for differentiating active from latent tuberculosis.Eur Respir J. 2007; 30: 722-728Crossref PubMed Scopus (90) Google Scholar We ought to be more judicious with the use of antitubercular therapy, especially when the threat of multidrug resistance and extensively drug-resistant TB is hovering over us.