Andrew N. Phillips, PhD; Caroline A. Sabin, MSc
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Phillips AN, Sabin CA. CD4 Counts as Surrogate Markers for Progression to AIDS. Ann Intern Med. 1994;120:87-88. doi: 10.7326/0003-4819-120-1-199401010-00018
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Published: Ann Intern Med. 1994;120(1):87-88.
TO THE EDITOR:
Choi and colleagues  use data from the AIDS Clinical Trials Group (ACTG) Protocol 019 Trial to investigate the degree to which zidovudine's effect on the risk for clinical disease acts via its effect on the CD4+ lymphocyte count (CD4+ percent). They fit the CD4+ lymphocyte count (CD4+ percent) as a time-dependent covariate (that is, the current value) in a Cox proportional-hazards model of the effect of zidovudine on the risk for progression to AIDS. The small degree of attenuation in the placebo-zidovudine relative risk estimate resulting from such adjustment led to the conclusion that only a small proportion of the effect of zidovudine on progression to AIDS was statistically explained by its effect on CD4+ lymphocyte levels (CD4+ percent). The range of possible values for the proportion is given as 0% to 37% (although we calculated this value to be 45% from the results presented). The former estimate relates to use of the CD4+ lymphocyte count and the latter to the use of the CD4+ percent.
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