The incidence of AIDS-defining illnesses in 4883 patients with human immunodeficiency virus infection. Royal Free/Chelsea and Westminster Hospitals Collaborative Group.

Background: Acquired immunodeficiency syndrome (AIDS)-defining illnesses are known to occur at different levels of immunosuppression, and the incidence of diagnoses may also vary according to the CD4 lymphocyte count strata. Information about the incidence of disease at different levels of immunosuppression would help clinicians monitoring patients and would allow prophylaxis to be targeted at the most appropriate population.

Methods: Between 1982 and July 1995, 4883 patients testing positive for the human immunodeficiency virus were seen at either the Royal Free or Chelsea and Westminster Hospitals in London, England. The incidence of each diagnosis, both overall and stratified by CD4 lymphocyte count, was calculated using a person-years analysis. Patients who had no CD4 lymphocyte counts measured during follow-up were excluded from the analysis.

Results: During a median follow-up period of 27.6 months, 3875 AIDS-defining illnesses were reported in 1713 patients. The incidence of AIDS-defining illnesses ranged from 6.22 per 100 person-years of follow-up for Pneumocystis carinii pneumonia (95% confidence interval, 5.74-6.70) to 0.37 for extrapulmonary tuberculosis (95% confidence interval, 0.26-0.48). The incidence of each AIDS-defining illness increased as the CD4 lymphocyte count declined; diagnoses such as cytomegalovirus and Mycobacterium avium-intracellulare complex infection had a low incidence at CD4 lymphocyte counts above 0.05x10(9)/L (50/mm3), while Kaposi sarcoma, P carinii pneumonia, and esophageal candidiasis had a high incidence throughout all CD4 lymphocyte count strata.

Conclusions: This study provides important information about the risk of AIDS-defining illnesses at lower CD4 lymphocyte counts, enabling disease-specific prophylaxis to be targeted at the most appropriate population. In the future, as more prophylactic therapies are developed, this study will provide historical data of the incidence of diseases before specific prophylaxis was introduced.