MULTIPLE IMPUTATION METHOD FOR ESTIMATING INCIDENCE OF HIV-INFECTION

Background. CD4+ T-lymphocyte (CD4) and platelet counts are good predi ctors of the 'maturity' of HIV infection and can be used to impute the date of infection/seroconversion in individuals for whom this date is unknown. Methods. Data from the Italian Seroconversion Study were use d to develop a Weibull regression model for time since seroconversion as a function of the haematologic markers. The model was used to imput e time since HIV infection/ seroconversion in individuals from a preva lent cohort, recruited through the Lazio regional HIV surveillance sys tem. Results. The range of the imputed calendar times of infection/ser oconversion in 2599 HIV prevalent individuals was 1972-1992; the earli est seroconversions occurred among injecting drug users (IDU). The pea k of incidence was reached in 1986 with 340 seroconversions. Among mal es, the estimated median time from seroconversion to HIV diagnosis was shorter in IDU (30 months) as compared to non-IDU (36 months), This d ifference was smaller for females (26.6 versus 28.4 in IDU and non-IDU , respectively). Conclusions. This method permits the estimation of po pulation-based curves of HIV incidence, using data from surveillance, The results support the hypotheses of an early spread of the epidemic among IDU in the Lazio region, and of shorter lead times in this popul ation.