Modelling the time to detection of urban tuberculosis in two big cities in Portugal: a spatial survival analysis

SETTING: Portuguese National Tuberculosis Control Programme. OBJECTIVE: To examine delays in tuberculosis (TB) diagnosis using a spatial component in two high-incidence cities, Lisbon and Oporto, in Portugal, a low-incidence country. DESIGN: A retrospective nationwide study was conducted based on official TB data between 2010 and 2013 to analyse diagnostic delays at the lowest administrative level (freguesias) using spatial survival analyses, taking into account individual level covariates. RESULTS: Median diagnostic delays in Lisbon (n=2706 cases) and Oporto (n = 1883) were respectively 62 (range 1-359, mean 81.01) and 60 days (range 1-3544, mean 79.5). In both cities, case detection rates initially rose until 50 days, then stabilised, but rose again at about 200 days. Diagnostic delay was significantly shorter among males and human immunodeficiency virus positive individuals in both cities, but was significantly longer among migrants in Lisbon. There is evidence of spatial correlation between freguesias; different spatial patterns were observed in diagnostic delays and in likelihood of case detection. CONCLUSION: These results are concordant with existing literature. The two study areas present considerable spatial variations in diagnostic delay, highlighting the fact that large cities should not be treated as homogeneous entities. The potential of spatial survival methods in spatial epidemiology is highlighted.