Participation in External Quality Assessment programmes, such as Proficiency Testing Scheme (PTS) studies, has been recognised as a mandatory component of a Quality Management System. As part of a Quality Management System, any unsatisfactory result in a PTS study should be treated as a non-conformity and must be carefully investigated for causative factors and followed by the implementation of corrective and preventive actions to prevent reoccurrence. This document provides information on the general management of a non-conformity. It primarily aims at guiding laboratories of blood establishments on root-cause analysis of an unsatisfactory result and/or performance that may occur in a PTS study.