Participation in patient support forums may put rare disease patient data at risk of re-identification.

Background: Rare disease patients often struggle to find both medical advice and emotional support for their diagnosis. Consequently, many rare disease patient support forums have appeared on hospital webpages, social media sites, and on rare disease foundation sites. However, we argue that engagement in these groups may pose a healthcare data privacy threat to many participants, since it makes a series of patient indirect identifiers 'readily available' in combination with rare disease conditions.