Certain methodological limitations of the study must be considered. Several important factors that have been suggested as risk factors for TB mortality, such as homelessness, history of incarceration, injection drug use, multidrug-resistant TB, compliance with DOT, and delay in diagnosis, could not be ascertained directly by this administrative database. Specifically, missed diagnosis and delayed treatment after hospitalization have been shown to occur more often in hospitals with low TB hospital admission rates and were strongly associated with in-hospital death in Canada. The measure of income was an ecologic rather than patient-specific measure, which can lead to the misclassification of income status. Given that persons with TB tend to have relatively lower socioeconomic status (SES) than their demographic or geographic counterparts, assigning SES values to individuals based on geographic means may actually overestimate their income and, in turn, underestimate the association between SES and TB outcomes. In addition, the results of this study rely on the accuracy of the diagnosis codes. The NIS database does not include patient identifiers, thus validation of the accuracy of the hospital discharge records was not feasible. However, several stud-ies have successfully assessed patient outcomes using the NIS database.
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