The prevalent cohort study and the acquired immunodeficiency syndrome

The acquired Immunodeficiency syndrome (AIDS) Is caused by a retrovirus, the human immunodeficiency virus (HIV). A rapid and convenient method to Identify additional cofactors or risk modifiers and markers of disease progression is to study a cohort prevalent with HIV antibody. However, because the time of viral infection is usually unknown in the cohort, there are several potential sources of bias. Three sources of bias in a prevalent cohort study are identified assuming a proportional hazards model: onset confounding, differential length-biased sampling, and frailty selection. A number of problems In the interpretation of results on markers from a prevalent cohort also are considered. It is concluded that risk estimates derived from a prevalent cohort are not directly comparable to risk estimates derived from an Incident cohort.