title: Survival analysis: time-dependent effects and time-varying risk factors.
abstract: In traditional Kaplan-Meier or Cox regression analysis, usually a risk factor measured at baseline is related to mortality thereafter. During follow-up, however, things may change: either the effect of a fixed baseline risk factor may vary over time, resulting in a weakening or strengthening of associations over time, or the risk factor itself may vary over time. In this paper, short-term versus long-term effects (so-called time-dependent effects) of a fixed baseline risk factor are addressed. An example is presented showing that underweight is a strong risk factor for mortality in dialysis patients, especially in the short run. In contrast, overweight is a risk factor for mortality, which is stronger in the long run than in the short run. In addition, the analysis of how time-varying risk factors (so-called time-dependent risk factors) are related to mortality is demonstrated by paying attention to the pitfall of adjusting for sequelae. The proper analysis of effects over time should be driven by a clear research question. Both kinds of research questions, that is those of time-dependent effects as well those of time-dependent risk factors, can be analyzed with time-dependent Cox regression analysis. It will be shown that using time-dependent risk factors usually implies focusing on short-term effects only.
authors.affiliation: Department of Clinical Epidemiology, Leiden University Medical Centre, Leiden, The Netherlands. f.w.dekker@lumc.nl
authors.forename: Friedo W
authors.initials: FW
authors.lastname: Dekker
authors.forename: Renée
authors.initials: R
authors.lastname: de Mutsert
authors.forename: Paul C
authors.initials: PC
authors.lastname: van Dijk
authors.forename: Carmine
authors.initials: C
authors.lastname: Zoccali
authors.forename: Kitty J
authors.initials: KJ
authors.lastname: Jager
mesh_terms: D015992:Body Mass Index; D006801:Humans; D016016:Proportional Hazards Models; D012307:Risk Factors; D016019:Survival Analysis; D013997:Time Factors