Estimating the size of hard-to-reach populations: a novel method using HIVtesting data compared to other methods

Objective: To estimate population size of hard-re-reach groups such as inje cting drug users and men who have sex with men. Design: Several different methods were used to estimate the size of these p opulations in Canada's three largest cities (Toronto, Montreal and Vancouve r). Methods: A novel method (referred to as the indirect method) was developed for use in Toronto and Vancouver that combines HIV serodiagnostic informati on with data on HIV testing behavior. Population size estimates were obtain ed by dividing the number of injecting drug users or men who have sex with men recorded in HIV serodiagnostic databases in a given year by the proport ion of the corresponding group that reported being tested in a 1-year perio d. Results of this method were compared with four other methods: (1) popula tion surveys; (2) capture-recapture (for injecting drug users only); (3) a modified Delphi technique; and (4) a method based on the proportion of neve r-married men aged 45 and over (for men who have sex with men only). Only t hese other methods were used in Montreal. Results: The survey method gave the lowest estimates which are best viewed as minimum estimates given the relative inability of surveys to access thes e populations and the reluctance of participants to admit to sensitive beha viors. The indirect method produced results more closely comparable with th ose obtained by other methods, but they are probably slight overestimates, at least for injecting drug users, due to possible underestimation of the p roportion tested for HIV. Point estimates using the indirect method were 17 700 and 17 500 for injecting drug users in Toronto and Vancouver, respecti vely, and 39 100 and 15 900 for men who have sex with men. In Toronto, resu lts for the other methods ranged from 12 300-13 360 for injecting drug user s and 18 800-35 000 for men who have sex with men. For Vancouver, these ran ges were 6400-11 670 and 7000-26 500, respectively. In Montreal, ranges wer e 4300-12 500 for injecting drug users and 18 500-40 000 for men who have s ex with men. Conclusions: This novel method provides estimates of population size of har d-to-reach groups such as injecting drug users and men who have sex with me n that are comparable with results derived by other methods. These estimate s may be useful for the purposes of planning, implementing and evaluating p revention and care services, especially when they are combined with the res ults of other estimation methods to improve the degree of confidence in the resulting estimates. (C) 2001 Lippincott Williams & Wilkins.