November 2008

Document Type


Degree Name



Dept. of Public Health and Preventive Medicine


Oregon Health & Science University


Background: In March 2007, an employee at a large call center in Oregon was admitted to the hospital with active tuberculosis disease (TB). The Local Health Department initiated a contact investigation and determined that the patient had been symptomatic since October 2006. Objective: We conducted a worksite investigation to identify the number of coworkers who had TB infections resulting from exposure to the index case and to examine the risk of infection by proximity and duration of that exposure to an infectious person. Setting: The investigation took place at a call center with >1500 employees. Initial worksite testing of 18 co-workers who participated in new employment orientation with the index case in October revealed that 60% of those screened had new LTBI infection. Methods: We compiled a list of all employees who worked at the call center from October 2006 to March 2007. We encouraged current and former employees to be tested with mailings, phone calls, and on-site outreach in an effort to find as many employees infected with TB (latent and active disease) as possible. Information on proximity to the index case and duration of exposure was collected for everyone tested and used as predictors of risk for infection. Data collection: tuberculin skin tests (TST’s) and/or Quantiferon-gold (QFT-G) blood tests were offered to all employees. We gave employees a self-administered questionnaire that solicited information about demographics, natality, medical history, past TB exposures, smoking history, location during the exposure period, and the duration of their exposure. Outcome measure: positive QFT-G or TST results were considered a definitive positive for TB infection. Results: During the study period, 1,641 people worked at the call center. Of these, 531 employees were fully evaluated for TB infection and associated risk factors 65 of which tested positive for new infection (12%). In addition, three secondary cases were diagnosed among the index patient’s co-workers. Proximity to the index case was classified as sitting within 1-25 feet of the index case or >25 feet while duration was classified as exposed to the index case for > or < 60 days. Employees who sat close to the index case had a new infection prevalence of 41% and had a relative risk of 3.1 (95%CI: 1.8-4.7); in addition, employees who had worked at the call center >60 days had a new infection prevalence of 17%, and the relative risk of those long-term employees was 2.5 (95%CI: 1.3-4.2). Combining proximity and duration of exposure into a single ordinal variable showed a dose response effect, with those sitting closest to the case for the longest having a new infection prevalence of 43% compared to those sitting elsewhere for a shorter period (new infection prevalence 6%). Conclusions: This large open workspace contact investigation showed that (excluding previously infected people) asymptomatic TB infection among employees was no less than 11%. Taken together, proximity and duration of exposure can be a useful dimension for prioritizing contacts to be evaluated in the course of contact investigations after large open workspace exposures. INSTITUTIONS: Washington County Department of Health and Human Services, Hillsboro Oregon; Multnomah County Health Department, Portland Oregon; Tuberculosis Program, Oregon State Public Health Division: TB/HIV department; Oregon Health and Sciences University.




School of Medicine



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