Impact of referral bias on prognostic studies outcomes: insights from a population-based cohort study on infective endocarditis.

PURPOSE:Prognostic studies derived from samples of patients managed in tertiary hospitals are subject to referral bias. We aimed to characterize this bias using the example of infective endocarditis. METHODS:We analyzed data from a French population-based cohort, which included 497 patients with infective endocarditis. Patients were admitted directly to a tertiary hospital (Group T), admitted to a non-tertiary hospital and referred to a tertiary hospital (Group NTT) or not (Group NT). We compared patients' characteristics, survival rates and prognostic factors between groups. RESULTS:Compared with Group T (n = 291), NTT patients (n = 144) were more often males (81.3% vs. 72.5%; P = .046), injection drug users (9.7% vs. 4.5%; P = .033), and had more frequent surgical indications (78.5% vs. 64.3%; P = .003). Compared with Group NT (n = 62), NTT patients were more often males (81.3% vs. 67.7%; P = .034) and had surgical indications more often (78.5% vs. 19.4%; P < .001). One-year survival was higher in NTT + T patients than in NT patients (73.0% vs. 56.1%; P = .01). Prognostic factors and hazard ratios estimates varied across groups. CONCLUSIONS:When derived from samples mixing patients admitted directly and those referred to tertiary hospitals, validity of characteristics description, survival estimates, and hazard ratios is threatened by referral bias.