A review of the recurrence risk literature uncovers several paradoxical observations: thrombophilias predispose for a first episode of deep venous thrombosis (DVT) but not for recurrence (the thrombophilia paradox1); aspirin users are at lower risk for cardiac events, but higher risk of recurrence (the aspirin paradox2); and although obesity is an established risk factor for coronary artery disease, it appears to protect against recurrent coronary events (the obesity paradox3). Although several hypotheses have been proposed to explain these paradoxes, it is not generally appreciated that all recurrent risk analyses are prone to a particular bias that may induce such paradoxical results. This phenomenon could be referred to as index event bias because it arises in studies that select patients based on the occurrence of an index event.
Dahabreh IJ, Kent DM. Index Event Bias as an Explanation for the Paradoxes of Recurrence Risk Research. JAMA. 2011;305(8):822–823. doi:10.1001/jama.2011.163
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