Joint model robustness compared with the time-varying covariate Cox model to evaluate the association between a longitudinal marker and a time-to-event endpoint.

BACKGROUND: The recent progress in medical research generates an increasing interest in the use of longitudinal biomarkers for characterizing the occurrence of an outcome. The present work is motivated by a study, where the objective was to explore the potential of the long pentraxin 3 (PTX3) as a prognostic marker of Acute Graft-versus-Host Disease (GvHD) after haematopoietic stem cell transplantation. Time-varying covariate Cox model was commonly used, despite its limiting assumptions that marker values are constant in time and measured without error. A joint model has been developed as a viable alternative; however, the approach is computationally intensive and requires additional strong assumptions, in which the impacts of their misspecification were not sufficiently studied. METHODS: We conduct an extensive simulation to clarify relevant assumptions for the understanding of joint models and assessment of its robustness under key model misspecifications. Further, we characterize the extent of bias introduced by the limiting assumptions of the time-varying covariate Cox model and compare its performance with a joint model in various contexts. We then present results of the two approaches to evaluate the potential of PTX3 as a prognostic marker of GvHD after haematopoietic stem cell transplantation. RESULTS: Overall, we illustrate that a joint model provides an unbiased estimate of the association between a longitudinal marker and the hazard of an event in the presence of measurement error, showing improvement over the time-varying Cox model. However, a joint model is severely biased when the baseline hazard or the shape of the longitudinal trajectories are misspecified. Both the Cox model and the joint model correctly specified indicated PTX3 as a potential prognostic marker of GvHD, with the joint model providing a higher hazard ratio estimate. CONCLUSIONS: Joint models are beneficial to investigate the capability of the longitudinal marker to characterize time-to-event endpoint. However, the benefits are strictly linked to the correct specification of the longitudinal marker trajectory and the baseline hazard function, indicating a careful consideration of assumptions to avoid biased estimates.

Formal and informal care after stroke: A longitudinal analysis of survivors' post rehabilitation hospital discharge.

AIMS: To describe the type and the amount of formal and informal care received during the first year after home discharge and to identify the baseline predictors of the formal and informal care needs of stroke survivors. DESIGN: Longitudinal study. Data were collected between June 2013-May 2016. METHODS: Survivors (N = 415) were enrolled during discharge from rehabilitation hospitals and interviewed at 3 (T1), 6 (T2), 9 (T3), and 12 (T4) months. The linear mixed effects model with random intercept and random slopes was used to trend for the amount of formal and informal care received by survivors during the four observation times. RESULTS: Regarding formal care, only physiotherapy and speech therapy decreased significantly over time. Stroke survivors received a mean of 17 hr of paid informal care per week at T1 and these hours did not significantly decrease after one year from discharge, while unpaid informal care decreased significantly over time. Higher numbers of paid informal caregiving were predicted by older age, higher education levels, lower physical functioning, and living without unpaid informal caregivers while higher numbers of unpaid informal care were predicted by lower physical functioning and living with unpaid informal caregivers. CONCLUSIONS: Stroke has a great effect on survivors' lives. During the first few months after rehabilitation hospital discharge, survivors need further care because they are often discharged before achieving independent functioning. IMPACT: The results of this study could be important to guide future interventions aimed at imporving stroke survivors' conditions after post rehabilitation hospital discharge.