Most clinical guidelines for the assessment and management of atrial fibrillation emphasize the importance of decision support provided by Patients Decision Aids, but they are to be used and evaluated only in the context of Shared Decision-Making. Detailed examination of 10 clinical decision support tools reveals that many do not engage with patient’s preferences at all. Only two take them seriously in terms of their formation, elicitation and processing, aimed at identifying the optimal personalised decision for the patient. This failure is traced to a reluctance to accept the ontological nature of preferences, as instantiations of comparative magnitudes, and to set them in an analytical framework that facilitates their transparent integration with individualised evidence.