Objective: To estimate the incubation-period distribution (time from seroconversion to AIDS) accounting for death before an AIDS diagnosis (DBAD) in a cohort of injecting drug users (IDU) in Amsterdam, The Netherlands and to compare these estimates with those previously obtained from a contemporaneous study of homosexual and bisexual men in Amsterdam carried out using the same facilities.
Design: Participants in a cohort study begun in Amsterdam at the end of 1985 have scheduled follow-up visits every 4 months. All participants of Dutch nationality and who had two or more follow-up visits before January 1996 from which CD4 measurements were available were included in this study. Data concerning AIDS diagnosis and death were verified through review of national and municipal registries.
Methods: Because time of seroconversion was unknown for study participants and because IDU are at substantial risk for DBAD, we used a Markov model with CD4-based stages that allows for DBAD. The parameters in this model were estimated using the method of maximum likelihood and confidence intervals were calculated using bootstrap methods.
Results: A total of 173 IDU (134 seroprevalent, 39 seroincident) made 1829 visits. Nearly 10% of the visits were non-consecutive. Forty-five IDU developed AIDS and 25 died without an AIDS diagnosis. We estimated that 24% [95% confidence interval (CI), 17-25%] of IDU die before an AIDS diagnosis. As a result, the median time from seroconversion to AIDS (10.5 years; 95% CI, 9.1-10.7 years) is considerably longer than the median time from seroconversion to death (8.3 years; 95% CI, 7.9-8.5 years). Conditional on survival to an AIDS diagnosis, the median time to AIDS is 8.2 years (95% CI, 7.7-8.7 years). The median survival time after a diagnosis of AIDS is estimated to be 1.0 years.
Conclusion: The high occurrence of DBAD in IDU has a considerable influence on estimates of the incubation-period distribution. Progression from seroconversion to death was faster in the IDU cohort than in a cohort of homosexual men in Amsterdam (median, 8.3 years and 9.6 years, respectively). However, progression to AIDS conditional on survival to an AIDS diagnosis seems to be similar in both the IDU cohort and in the cohort of homosexual men (median, 8.2 years and 8.3 years, respectively).