Análisis espacial de casos de tuberculosis en los inmigrantes y los residentes permanentes, Beijing, 2000-2006

The data on all the TB cases reported in Beijing from 2000 through 2006 were obtained from the Beijing Institute for Tuberculosis Control, which specializes in TB prevention and research and is responsible for supervision of TB control in 18 districts of metropolitan Beijing. The cases that met the diagnostic criteria of TB issued by Ministry of Public Health in 2003 were included in the analyses.

The data include information on age, origin, current address, and date of TB onset. To assess the contribution of the migrant population from different areas, the case origins were divided into 4 zones, according to economic status and geography, i.e., western zone (including Shanxi, Gansu, Qinghai, Ningxia, Inner Mongolia, Xinjiang, Tibet, Sichuan, Chongqing, Guizhou, Guangxi, Yunnan Provinces, or other administrative regions), middle zone (including Heilongjiang, Jilin, Shanxi, Henan, Anhui, Hubei, Hunan, Jiangxi Provinces), eastern zone (including Liaoning, Hebei, Shandong, Jiangsu, Zhejiang, Fujian, Guangdong, Hainan Provinces), and 2 municipalities (Tianjin and Shanghai).

The zonal classification corresponded to that of the Report on Nationwide Survey on Epidemiology of Tuberculosis in 2000 (18) and thus was easily used for comparison. The case data have been stratified by age, gender, origin, and onset date of TB; age was divided into 3 groups: 1) 0-14 years, 2) 15-64 years, and 3) [greater than or equal to] 65 years. All the TB cases were coded according to the address where they resided (geo-coded) and matched to a 1:100,000 digital map of Beijing by using ArcGIS version 9.1 software (ESRI Inc., Redlands, CA, USA).

The demographic data of permanent residents and migrant population for each district were obtained from the 2000-2006 censuses, provided by Beijing Municipal Public Security Bureau (8,9,19-23). The 18 districts of Beijing, covering a total surface area of [approximately equal to] 16,800 [km.sup.2], had 11,976,900 permanent residents and 5,475,000 migrants in 2006 (9). On the basis of these data, the population densities of each district in different years were calculated and displayed on the digital map of Beijing.

The permanent residents in our study were defined as those who reside in Beijing with registered hukou in Beijing, and the migrant population was defined as those who had been residing in Beijing >1 month but whose hukou were still held in their homelands. Persons originally from other countries were beyond the scope of our current study. A shape file of property boundary data of 18 districts in Beijing 2003 obtained from the Ministry of Water Resources of the People's Republic of China was used to generate visual presentations with 1:100,000 scale by using ArcGIS 9.1.

Statistical Analysis

The dynamic changes in population densities and the TB case notification rate of both migrant population and permanent residents from 2000 to 2006 were displayed by district on the digital map of the Beijing municipality. Global Moran's 1 statistics with z score test and Getis's [G.sup.*.sub.i] statistics, which specify 10 km as the threshold of distance, have been used to detect the spatial distribution and the hot spots of TB in the 2 populations . Global Moran's I is used to discern spatial autocorrelation of TB cases in the study area and disclose the spatial pattern of disease with z score at the district level. A statistically significant (z score [greater than or equal to] 1.96) estimate of I indicates that neighboring districts (within 10 km) have a similar prevalence rate of TB and that the cases are likely to cluster at the district level (24). Getis's [G.sup.*.sub.i] statistics only assess positive spatial autocorrelation and are used to detect hot spots in the study area. A calculated value of [G.sup.*.sub.i] [greater than or equal to] 1.96 indicates that district i and its neighboring districts (within 10 km) have a TB prevalence rate that is statistically significantly different (higher) than other districts. District i is the center of the area with the higher TB prevalence rate, and is defined as a TB hot spot .

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