Roaming through methodology. XXXV. Bias in prevalent cohort studies of the natural course of disease]

The natural history of infectious diseases with a long asymptomatic incubation period has mainly been studied in cohorts of individuals already infected at study entry: the so-called prevalent cohort study. Because the time of infection is usually unknown in the prevalent cohort, in standard survival analysis it is common to use the time since entry into the cohort instead of the time since infection to study risk factors for disease progression. However, the use of the time since study entry may bias results. The two most important sources of bias are onset confounding and differential length-bias sampling. Because bias may occur, results derived from a prevalent cohort are not directly comparable to results derived from an incident cohort where the moment of infection is known.