This article examines one aspect of the new test of effective participation at trial proposed by the Law Commission of England and Wales. This proposal aims to replace the current criteria for fitness to plead originating from Pritchard and developed more recently in M (John). Specifically, this article offers a critical examination of the Commission’s refusal to incorporate a so-called ‘diagnostic threshold’ within their proposed test. After reviewing the arguments for and against this decision, attention is drawn to the clear presence of diagnostic thresholds within other areas of law, such as the mental condition defence of insanity. Overall, the Commission’s proposals are a vast improvement upon the archaic rules of present day, and, contrary to the views of some scholars, their decision to omit a diagnostic criterion is no exception to this. In fact, the implications of this decision reach far beyond the particular context of unfitness proceedings and ultimately cast doubt on the significance of diagnostic thresholds in all areas of law. By focusing exclusively on the relationship between unfitness to plead and the defence of insanity, this piece demonstrates how both tests can be reformulated so as to avoid any explicit reference to a diagnostic criterion.