In the ongoing battle against HIV/AIDS, it is critical that we are able to measure and monitor HIV incidence, that is the number of new infections during a period of time, usually expressed as number of infections/person-years of observation or as an annual percentage of the population that acquire infection. Knowledge of HIV incidence is necessary to understand transmission patterns; to provide a rational basis for targeting prevention efforts; to evaluate interventions to reduce transmission; and to predict or project the burden of HIV infection in different demographic and risk populations. Reliable information on HIV incidence is especially important to support prevention programs in the low-income and middleincome countries that continue to bear a disproportionate share of the global burden of the HIVepidemic. Improved estimates of HIV incidence are essential to evaluate ongoing HIV prevention and treatment programs in these resource-constrained settings and to guide the most effective use of the billions of dollars that will be spent on the epidemic in the coming years [1].

Direct measurement of incidence through prospective follow-up of cohorts of HIV-negative persons is expensive, generally unrepresentative of the larger population, and difficult to sustain, even in resource-rich settings. Furthermore, enrollment of persons into cohort studies can introduce selection bias or induce behavior change that can result in a lower observed HIV incidence than is representative of the population from which the cohort is drawn.