Chest Radiographic Patterns and the Transmission of Tuberculosis: Implications for Automated Systems

Abstract

BackgroundComputer-aided detection to identify and diagnose pulmonary tuberculosis is being explored. While both cavitation on chest radiograph and smear-positivity on microscopy are independent risk factors for the infectiousness of pulmonary tuberculosis it is unknown which radiographic pattern, were it detectable, would provide the greatest public health benefit; i.e. reduced transmission. Herein we provide that evidence.Objectives1) to determine whether pulmonary tuberculosis in a high income, low incidence country is more likely to present with “typical” adult-type pulmonary tuberculosis radiographic features and 2) to determine whether those with “typical” radiographic features are more likely than those without such features to transmit the organism and/or cause secondary cases.MethodsOver a three-year period beginning January 1, 2006 consecutive adults with smear-positive pulmonary tuberculosis in the Province of Alberta, Canada, were identified and their pre-treatment radiographs scored by three independent readers as “typical” (having an upper lung zone predominant infiltrate, with or without cavitation but no discernable adenopathy) or “atypical” (all others). Each patient’s pre-treatment bacillary burden was carefully documented and, during a 30-month transmission window, each patient’s transmission events were recorded. Mycobacteriology, radiology and transmission were compared in those with “typical” versus “atypical” radiographs.FindingsA total of 97 smear-positive pulmonary tuberculosis cases were identified, 69 (71.1%) with and 28 (28.9%) without “typical” chest radiographs. “Typical” cases were more likely to have high bacillary burdens and cavitation (Odds Ratios and 95% Confidence Intervals: 2.75 [1.04–7.31] and 9.10 [2.51–32.94], respectively). Typical cases were also responsible for most transmission events—78% of tuberculin skin test conversions (p<0.002) and 95% of secondary cases in reported close contacts (p<0.01); 94% of secondary cases in “unreported” contacts (p<0.02).ConclusionAs a group, smear-positive pulmonary tuberculosis patients with typical radiographic features constitute the greatest public health risk. This may have implications for automated detection systems.

Effectiveness of Canada's tuberculosis surveillance strategy in identifying immigrants at risk of developing and transmitting tuberculosis: a population-based retrospective cohort study

“…Similarly, some might call the 30 month transmission window into question. However, our method has been previously published and described, 19,20 with sensitivity analyses corroborating its appropriateness. Findings from other studies 30,31 also suggest that this length of time is likely to capture most transmission events.…”

“…Our group has previously reported on this method of transmission identification. 19,20 Type 1 secondary cases were individuals who were listed as a contact of the source case, diagnosed with active tuberculosis within a transmission window extending from 6 months to before 24 months after the date of diagnosis of the source case, and culture positive with an isolate of M tuberculosis that was a genotypic match to the fingerprint of the putative source case. Type 2 secondary cases were close contacts of the source case who were diagnosed with active tuberculosis within the 30 month transmission window, but were culture negative (or cultures could not be obtained).…”

TST positivity in household contacts of tuberculosis patients: a case-contact study in Malawi

“…[13, 27, 28]. However, prior studies have not focussed on discriminatory analysis amongst strongly SSP patients and our data also illustrates that extent of radiological disease can usefully triage infectiousness amongst a subset of individuals who are all known to have multi-bacillary disease.…”

Impact of radiology reports on timely tuberculosis diagnosis

“…Patients whose radiographic report mentioned ‘tuberculosis’ or ‘cavity’ were more likely to have a positive sputum acid-fast smear, but not all smear-positive patients had one of these terms included in the report. As a group, patients with smear-positive pulmonary tuberculosis with typical radiographic features are more likely to transmit tuberculosis, and constitute the greatest health risk 10. Failing to diagnose these patients has huge implications for both the individual and the community.…”