CHANGE-POINTS IN THE SERIES OF T4 COUNTS PRIOR TO AIDS

The absolute number of T4 cells has been established as an important c linical marker of disease progression to acquired immunodeficiency syn drome (AIDS) in persons infected with human immunodeficiency virus (HI V). Series of T4 counts are analyzed from the 131 homosexual men who e ntered the New York Blood Center Study in 1984, mostly seropositive fo r HIV, and who developed AIDS as participants by 1990. These series ex hibit a gradual decline of the log(T4) count followed by a more rapid decline close to the time of the development of AIDS. Empirical Bayes and hierarchical Bayes change point models are proposed to estimate th e distribution of the time before AIDS when this rapid decline begins. Results using the EM Algorithm and Markov chain Monte Carlo indicate that the mean change point occurs approximately 1 year before diagnosi s with a standard deviation of 9 months. Detection of a change point m ay indicate that an AIDS diagnosis is increasingly likely for an indiv idual HIV-positive but AIDS-free.