Revise Diagnostic criteria for obsessive-compulsive disorder (OCD) for Cambridge 9990 (1.5.1)

The diagnosis of Obsessive-Compulsive Disorder (OCD) is not a subjective judgement but is guided by strict criteria outlined in major classification systems. The two most influential manuals are the American Psychiatric Association's 'Diagnostic and Statistical Manual of Mental Disorders, 5th Edition' (DSM-5) and the World Health Organisation's 'International Classification of Diseases, 11th Revision' (ICD-11). These manuals provide a standardised set of observable symptoms and criteria, ensuring that a diagnosis of OCD is consistent and reliable across different clinicians and countries. This common language is essential for both clinical practice and research. It is crucial to remember that a formal diagnosis can only be made by a qualified mental health professional following a thorough clinical assessment.

The cornerstone of an OCD diagnosis is the presence of obsessions, compulsions, or both. Obsessions are defined as recurrent and persistent thoughts, urges, or images that are experienced as intrusive and unwanted, causing marked anxiety or distress. The individual attempts to ignore or suppress them, often with another thought or action. Compulsions are repetitive behaviours (e.g., hand washing, ordering) or mental acts (e.g., praying, counting) that the person feels driven to perform in response to an obsession or according to rigid rules. The aim of these compulsions is to prevent or reduce anxiety, but they are not connected in a realistic way with what they are designed to neutralise, or are clearly excessive.

This criterion distinguishes the clinical disorder from everyday habits or worries. For a diagnosis of OCD, the obsessions or compulsions must be significantly disruptive. This is defined in two ways: either they are time-consuming, taking up more than one hour per day, or they cause clinically significant distress or impairment in social, occupational, or other important areas of functioning. For example, a student may be unable to complete their homework due to constant checking rituals, or an adult may avoid social situations due to contamination fears. This criterion is essential as it establishes that the symptoms are severe enough to warrant a clinical diagnosis and intervention, moving beyond simple personality quirks or preferences for order.

Accurate diagnosis involves a process of elimination. Criterion C specifies that the symptoms cannot be attributable to the direct physiological effects of a substance (e.g., medication, illicit drugs) or another medical condition (e.g., a brain injury). Criterion D ensures the symptoms are not better explained by another mental disorder. This is known as differential diagnosis. For instance, excessive worries about real-life problems are characteristic of Generalised Anxiety Disorder, not OCD. Preoccupation with one's appearance is a feature of Body Dysmorphic Disorder. By ruling out these other potential causes, a clinician can be more confident that OCD is the correct diagnosis, which is vital for planning the most effective treatment.