Estimating the number of AIDS-defining opportunistic illness diagnoses from data collected under the 1993 AIDS surveillance definition

Expansion of the surveillance definition for AIDS in the United States in 1993 caused a substantial distortion in the trend in AIDS incidence, mainly because CD4-positive (CD4(+)) T-lymphocyte criteria were added to the definition. To evaluate trends in the rate at which HIV-infected persons develop the opportunistic illnesses Listed in the AIDS surveillance definition (AIDS-OIs), we developed a procedure for estimating the incidence of these diseases. This estimate is based primarily on the probability distributions of the time from a CD4(+) count in given ranges to the diagnosis of the first AIDS-OI. Our estimates of AIDS-OI incidence change by <4% during most calendar quarters during 1991 through 1995 if we also include the estimated effects of unreported AIDS-OIs among persons with AIDS reported based on the CD4(+) criteria. Our procedure eliminates the transient effect of adding the CD4(+) criteria to the AIDS surveillance definition and permits us to evaluate trends in the incidence of AIDS-OIs.