Overdiagnosis in physical and mental health has become a fashionable and political issue. The UK health secretary, Wes Streeting, apologised recently for his prior statements that too many people with mental health and neurodevelopmental conditions were being diagnosed and receiving state benefits as a consequence. Suzanne O’Sullivan’s book The Age of Diagnosis is about this expanding diagnostic net of modern medicine, and the extent to which it is turning difference, risk or distress into disorders or conditions, without adequately considering the overall benefit to the person being labelled.

O’Sullivan, a neurologist, conveys the issues of overdiagnosis through patient stories in conditions including long COVID, Lyme disease, autism, attention-deficit hyperactivity disorder, genetic forms of cancer and hypermobile Ehlers–Danlos syndrome (hEDS). A young woman is relieved that her difficulties are explained by her mild autism, but a question mark hangs over whether the label will restrict her future potential. A mother of a son with severe autism is frustrated that expanding autism phenotypes have reduced his access to care, and a middle-aged man is angry that his autism is even considered a medical condition. Another patient with functional seizures has collected diagnoses of autism spectrum disorder, hEDS and postural orthostatic tachycardia syndrome but without anyone drawing connections between them. O’Sullivan examines whether these diagnoses alter treatment, prognosis or self-understanding in a way that genuinely helps the individual. She warns that ‘helpful’ can’t always be judged by whether the person is glad to have a diagnosis, if that diagnosis limits their horizons or hope.

Many previous book reviews have correctly used the word ‘brave’ to describe O’Sullivan’s candour. Diagnoses can rapidly become part of someone’s identity, and challenging them can be a threat to personal or group integrity. For conditions like long COVID or chronic Lyme disease, their diagnostic label signals unequivocal external causation in contrast to the patient being ‘blamed’ for symptoms. This is, of course, a false dichotomy, which O’Sullivan recognises, but ultimately, in my view, risks entrenching through an unabashed ‘psychologising’ and somewhat dichotomising approach.

This book will be uncomfortable reading for many patients with these diagnoses. Although O’Sullivan is careful to acknowledge the reality of patients’ suffering and the value diagnoses can have, her argument necessarily involves pushing back against the validity of some of them and against the primacy of ‘lived experience’ as their sole arbiter. But it is really health professionals who should be most uncomfortable reading it. They can be encouraged by a strong plea for preservation of clinical skills, as an essential tool to help people interpret a rapidly changing diagnostic landscape. But how many of us think enough about the harm we might be doing when we do tests and provide labels? It’s especially a warning to those of us tempted to give ‘disorder’ diagnoses at the mild end of the spectrum, however much it may please both patient and clinician.