In Chapter 1, we outlined the events leading up to the adoption of the Consensus Diagnostic Criteria (CDC). A weighted score of eight or more for ages four years and above (five or more for ages less than four years), based on the presence of eight major (score 1) and 11 minor (score 0.5) symptoms, is considered sufficient for a clinical diagnosis of PWS. According to these diagnostic criteria, there is no requirement that any particular one (or more) of the criteria are present, rather that the total number present exceeds a given threshold. This was a relaxation of the implicit criteria of individual physicians. As noted in Chapter 1, some early reports, such as Hall & Smith and Pipes & Holm, did consider that the presence of some criteria were critical in the diagnosis of PWS.

Before PWS was recognised as a genetic syndrome, diagnosis was necessarily based on clinical criteria, and the primary role of such criteria was to ensure as accurate a diagnostic process as possible. When it was recognised that at least half of the people diagnosed with PWS had a deletion on the long arm of chromosome 15, clinical criteria and genetic criteria were used interchangeably.