Estimating the size of key populations at higher risk of HIV infection: a summary of experiences and lessons presented during a technical meeting on size estimation among key populations in Asian countries.

Population size estimates (PSE) for key populations at higher risk have been recognized as a critical piece of strategic information for understanding the trajectory of the HIV epidemic, planning and monitoring an effective response, especially for countries with concentrated epidemics such as those in Asia and the Pacific. Methodological approaches and practical lessons learned from implementing PSE in 13 countries from Asia were reviewed. These include local innovations such as adjusting for duplication and hidden sub-groups; extrapolating local estimates to national level; and setting ranges for national estimates. Building capacity to collect, analyze, and use PSE data requires engagement of the key population community, cultivation of a standing technical review group, and implementation of a transparent, well-documented process. Development of new methods appropriate for non-venue based key populations remains an area requiring more investment and collaborative efforts  between countries and among partners.