Are psychiatric diagnoses of psychosis scientific and useful? The case of schizophrenia

Background: Insurance systems, scientific journals, professional groups, educational initiatives, regulatory bodies and organisation of services appear to be fixed in 19th century diagnostic metaphors for psychosis, providing natural protection against change. Furthermore, the existence of two separate diagnostic systems in psychiatry, DSM and ICD, produces a bias that is conservative rather than anti-conservative, reducing the probability of non-cosmetic change. Methods: A qualitative review of the validity, usefulness and acceptability of the diagnosis of 'schizophrenia', in order to assess possible discrepancies between actual diagnostic practice in mental health services and alternatives dictated by scientific evidence and societal developments. Results: A 21st century concept of psychotic disorder should refer to an experience that can be understood as a variation of normal human mentation that can be expressed quantitatively. For the purpose of diagnosis, use can be made of scientific evidence of specificity yielding high diagnostic likelihood ratios rather than evidence of weak mean differences yielding low diagnostic likelihood ratios. In the case of psychosis, the evidence appears to favour a syndromal system of classification combining categorical and dimensional representations. The concept of 'salience' has the potential to make the public recognise psychosis as relating to an aspect of human mentation and experience that is universal. Conclusions: The debate about alternatives to diagnose expressions of psychosis is often misunderstood as a misguided attempt to change societal stigma. In reality, however, it is about reduction of iatrogenic stigma occasioned by the use of unscientific and mystifying terminology. Scientific and societal developments have largely caught up with diagnostic traditions applied to psychosis phenotypes and although the traditional diagnostic system is deeply ingrained in all aspects of clinical and academic activities, change is possible if professional bodies actively promote a modern system of evidence-based diagnostic practice.