Tuberculosis contact tracing in Boston homeless shelters: estimating exposure risk using electronic bed logs
O'Brien, John P.
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Public health departments struggle to maintain costs while sensitively targeting contacts of patients with infectious tuberculosis. The homeless populations in shelters have high degrees of contact exposure and are difficult to trace and treat. Once left untreated, a contact can continue the spread of tuberculosis and worsen a tuberculosis outbreak. It is important for public health workers to quickly identify all the at-risk contacts and to contain costs by specifically excluding any contacts with insignificant exposure. The Boston Public Health Commission utilizes electronic bed logs in the homeless shelters to measure the exposure duration and proximity. From this, it was desired to create a tier system in which a level of exposure could be linked to risk of tuberculosis infection. Two cases that occurred in 2006-2007 in Boston homeless shelters were studied. Electronic bed records were gathered for all nights when each index case stayed at their respective shelter. An exposure score was assigned as the sum of proximity-based ranks over the total number of nights within three beds of the index case. Priority risk groups were assigned from these scores. Tuberculin skin test (TST) converters had the highest mean (12.3, range 0.5-35) exposure score, followed by contacts with only a negative baseline TST (4.0, range 0.5-30). The lowest scores (2.0) were seen in the group with no TST results and in those with a documented prior positive TST. Among contacts with two appropriately timed TSTs, persons with exposure scores >10 had 24 times the odds of converting their TST compared to those with exposure scores <2. Increasing exposure scores were associated with increasing odds ratio of conversion (0.84, 4.80, and 24.0). Only exposure scores >10 were significantly associated with TST conversion. Our work suggests that an exposure score may provide a simple quantified estimate of the duration of exposure. Primitizing follow-up to those persons with higher exposure scores reduces the number of persons in whom testing is needed. Including the group of those with no TST information and those with only a negative baseline TST, targeting exposure scores of > 4.5 reduces the target group for whom testing was recommended by 77% (548 to 124), while maintaining high sensitivity for potential converters.
Thesis (M.A.)--Boston University