An examination of spatial concentrations of sex exchange and sex exchange norms among drug users in Baltimore, Maryland.

Baltimore, Maryland consistently ranks highest nationally in rates of sexually transmitted diseases and HIV infection. Prior studies have identified geographic areas where STI and HIV infection in the city is most prevalent. It is well established that sex exchange behavior is associated with HIV and STIs, yet it is not well understood how sex exchangers are spatially distributed within the high-risk areas. We sought to examine the spatial distribution of individuals who report sex exchange compared to those who do not exchange. Additionally we examined the spatial context of perceived norms about sex exchange. Data for the study came from a baseline sample of predominately injection drug users (n=842). Of these, 21% reported sex exchange in the prior 90 days. All valid baseline residential addresses of participants living within Baltimore city boundaries were geocoded. The Multi-Distance Spatial Cluster Analysis (Ripley's K-function) was used to separately calculate the K-functions for the addresses of participants reporting sex exchange or non-sex exchange, relative to the recruited population. Evidence of spatial clustering of sex exchangers was observed and norms aligned with these clusters. Of particular interest was the high density of sex exchangers in one specific housing complex of East Baltimore, which happens to be the oldest in Baltimore. These findings can inform targeted efforts for screening and testing for HIV and STIs and placement of both individual and structural level interventions that focus on increasing access to risk reduction materials and changing norms about risk behaviors.