Volunteer bias in nonrandomized evaluations of the efficacy of needle-exchange programs

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Abstract
Nonrandomized comparisons of the incidence of HIV and hepatitis B and C between injection drug users (IDUs) who do and do not attend voluntary needle-exchange programs may be subject to bias. To explore possible sources of bias, we examined characteristics associated with voluntarily beginning or ceasing to participate in the Seattle needle exchange. In a cohort of 2,879 IDUs, a standardized questionnaire measured characteristics present at enrollment. We examined the relation of these characteristics to the proportion of IDUs who began to use the program during the ensuing 12-month follow-up period and to the proportion of current exchangers who dropped out during that period of time. Of the 494 never-exchangers at baseline, 32% attended the exchange program during follow-up; those who reported sharing syringes or who were homeless at enrollment were more likely to become new exchange users (adjusted risk ratio [ARR] for becoming an exchange user = 1.8 for those who shared syringes, and ARR=2.2, for those who were homeless). Of 1,274 current exchangers, 16% stopped using the exchange during follow-up, with daily injectors (ARR=0.6) and those who reported backloading (ARR=0.6) being relatively less likely to drop out of the exchange. The analysis suggests that IDUs participating in needle-exchange programs at a given point in time may include a particularly high proportion of those injectors whose pattern of drug use puts them at elevated risk of blood-borne viral infections.