Home1.2.1. Delayed diagnosis
Many outbreak investigations, as well as a root-cause analysis exploring factors contributing to TB exposures in a tertiary-care hospital in Canada, identify delay in making the diagnosis of TB as the most common reason for the exposures (see Appendix 1 and Appendix 2, Table 2c).11,12,16–18,20–23,33–41 The root-cause analysis noted failures to consider TB as a possible diagnosis and failure to obtain or correctly interpret imaging findings as common errors, with 80% of the errors being preventable.20 A Canadian study examining IPC failures contributing to bronchoscopy-associated exposures found a failure to obtain pre-procedure sputum or determine whether the patient was known to have a positive sputum smear prior to the procedure.42 Even when TB has been initially considered in the diagnosis, precautions may be inappropriately discontinued if HCWs have not been systematic in determining whether the diagnosis has been accurately excluded. The importance of considering a diagnosis of TB, and not prematurely excluding it, in an individual presenting with sub-acute or chronic respiratory symptoms, even when an alternate diagnosis is plausible, cannot be over-emphasized.
1.2.2. Number of patients with respiratory TB
It seems intuitive that a larger number of hospitalized patients with respiratory TB is an important determinant of higher institutional transmission risk. However, results from a study involving 17 acute care hospitals in Canada showed that institutional risk of M. tuberculosis transmission was better correlated with delayed diagnosis and treatment which in turn was associated with having a small number of admissions with respiratory TB disease.43 The study results suggest that healthcare facilities with more experience managing patients with TB are, not surprisingly, less likely to miss a diagnosis of respiratory TB and that facilities seeing fewer patients with TB need ongoing reeducation on recognizing a patient who might have respiratory TB.
1.2.3. Inadequate ventilation
The exchange of indoor air with outdoor air reduces the risk of infection transmission by diluting the concentration of viable airborne M. tuberculosis bacteria present. Several studies report inadequate ventilation as a risk factor that contributes to transmission.11,22,23,44
1.2.4. Duration of exposure and proximity to infectious patient
The risk of TB infection varies with duration of exposure, form of tuberculous disease, and type of patient care activity. Even when the relative risk of infection is low, close proximity and repeated exposure can lead to a higher cumulative risk.24,34
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