ICD-11 diagnostic criteria – how to read and apply the CDDR
Diagnostic criteria are an agreed set of conditions that must be met before a given mental disorder can be diagnosed. In this guide we explain how ICD-11 (the eleventh version of the International Classification of Diseases prepared by the World Health Organization) describes these criteria and how to read them carefully, without over-diagnosing.
What diagnostic criteria are and why they exist
A diagnostic criterion is simply a condition that must be met before it can be accepted that a given person matches the description of a specific disorder. A set of such conditions together forms the definition of a diagnosis. Put more simply: criteria are a list of features against which a clinician (a psychiatrist or a clinical psychologist) checks whether the pattern of symptoms in a patient corresponds to a particular diagnostic entity.
Criteria exist so that different specialists, in different countries and at different times, understand the same diagnosis in the same way. Without a common language a diagnosis would depend solely on individual judgement and would be difficult to compare. Shared criteria allow clinicians to communicate, treatment to be planned, scientific research to be conducted and data on population health to be collected.
ICD-11 is the official, international standard for the classification of diseases. In the area of mental health it provides both lists of disorders together with codes (for example 6A20 for schizophrenia or 6B00 for generalised anxiety disorder) and detailed descriptions according to which a diagnosis is made. It is these descriptions that we call diagnostic criteria.
It is worth noting from the outset that meeting the criteria on paper is not the same as making a diagnosis. A diagnosis is a clinical process in which a specialist takes a history, assesses a person's life context, excludes other explanations and only then relates the whole to the criteria. This page describes the logic of the criteria itself and does not replace consultation with a specialist.
The CDDR – how ICD-11 describes diagnostic requirements
In ICD-11 the descriptions of mental disorders intended for clinicians are collected in a document called the CDDR (from the English Clinical Descriptions and Diagnostic Requirements). It is an extensive guide issued by the World Health Organization which, for every disorder, sets out exactly what has to be established in order to diagnose it.
The very name CDDR is significant. The creators of ICD-11 deliberately used the words diagnostic requirements rather than criteria in the sense of a rigid list of points to be ticked off. The intention was to emphasise the role of clinical judgement and flexibility, while still preserving the consistency of diagnoses across specialists.
The description of every disorder in the CDDR has a recurring structure. It includes, among other things, a brief clinical description of the disorder, a list of essential features (that is, those indispensable for the diagnosis), information about the boundary with normality and with other disorders, notes on the course and on differential diagnosis (that is, distinguishing it from similar states), and guidance concerning children, culture and gender. As a result, a clinician receives not only a list of symptoms but also the context needed for an accurate assessment.
The CDDR is linked to the structure of ICD-11 codes. Every disorder has its code, and codes form orderly groups. For example, mood disorders have codes beginning with 6A6 (such as 6A60 for bipolar type I disorder or 6A71 for recurrent depressive disorder), and anxiety or fear-related disorders begin with 6B0. This classification browser makes it possible to move from a code to the full description of a given disorder.
Essential features versus additional features
The most important distinction in the ICD-11 criteria is the division into essential features and additional features. Essential features are those elements of the clinical picture whose presence is required before a given diagnosis can even be considered. If even one of them is missing, the diagnosis usually cannot be made.
Additional features are symptoms and circumstances that often accompany a given disorder, support the diagnosis or help to describe its severity or subtype, but in themselves are not a necessary condition. They may or may not be present, and their absence does not rule out the diagnosis. They play an auxiliary role, refining the clinical picture.
For example, in generalised anxiety disorder (code 6B00) the essential feature is persistent, excessive anxiety or worry concerning many areas of life that is difficult to control. Symptoms such as muscle tension, sleep problems, irritability or difficulty concentrating are typical accompanying features that reinforce the picture but do not replace the essential feature.
This distinction has significant practical consequences. A person may experience many additional symptoms and yet not meet the criteria for a disorder if the essential feature is absent. And conversely – the presence of the essential features alongside few additional symptoms may still be sufficient for a diagnosis. Counting symptoms alone does not replace checking whether the core of the definition is met.
The approach based on essential features is one of the more important changes in ICD-11 compared with earlier classifications. Instead of requiring a rigid number of symptoms from a longer list, ICD-11 asks the clinician to assess whether the significant, defining picture of the disorder is present. This is a more flexible but also a more demanding approach.
Duration as a diagnostic threshold
Many mental disorders differ from transient states not in the type of symptoms but in how long the symptoms persist. For this reason the ICD-11 criteria often include requirements concerning duration – so-called time thresholds. A time threshold is the minimum period for which symptoms must be present before a given diagnosis can be considered.
The point of a time threshold is to separate a disorder from a short-lived reaction. Sadness after a loss, anxiety before an important event or insomnia after a stressful day are phenomena that usually resolve on their own. Only symptoms that persist long enough begin to indicate a state requiring clinical attention.
Time thresholds differ between disorders and stem from knowledge about their course. For example, diagnosing a depressive episode within mood disorders requires symptoms to persist for most of the day, nearly every day, over a period measured in weeks. Generalised anxiety disorder (6B00), in turn, requires anxiety and worry to persist for many months. On the other hand, acute and transient psychotic disorder (6A23) has, by definition, a short course, and the long persistence of psychotic symptoms directs the diagnosis towards other entities.
Time thresholds are not a rigid boundary applied mechanically. The CDDR allows clinical judgement when the clinical picture is exceptionally severe or rapidly worsening. Nonetheless, omitting the duration requirement is one of the more common errors – diagnosing a disorder on the basis of symptoms present for a few days is usually premature.
Symptom severity and severity qualifiers
The second important dimension of the criteria is symptom severity. The same symptoms may occur in a mild form that little disturbs life, or in a severe form that disorganises everyday functioning. ICD-11 takes this into account by means of so-called qualifiers – additional designations that refine the diagnosis.
Severity qualifiers make it possible to describe whether a disorder is mild, moderate or severe in character. This is clearly seen in the example of mood disorders, where a depressive episode is described as mild, moderate or severe, which translates into different codes and matters for treatment planning. Similarly, disorder of intellectual development (6A00) has separate categories for the mild, moderate, severe and profound degrees.
Besides severity, ICD-11 also uses other qualifiers, for example those describing the course (first episode, multiple episodes, continuous course) or the current state (symptomatic, partial remission, full remission). This is well visible in the elaborate structure of the codes for schizophrenia (6A20), where the further digits of the code specify precisely the course and the current state.
Symptom severity also matters at the boundary with normality. Mild, transient symptoms do not always mean a disorder. Only symptoms of sufficient severity, combined with duration and an impact on functioning, make up a full diagnostic picture. The mere presence of a symptom, in isolation from its intensity, says little.
Impact on functioning – a key threshold
The third pillar of the ICD-11 criteria, alongside the type of symptoms and their duration, is the impact on functioning. Functioning is a person's ability to cope in important areas of life: in family relationships, at work or in study, in social contacts and in independent everyday activity.
Most ICD-11 criteria require that symptoms cause significant distress or a significant deterioration of functioning. This requirement plays the role of a cut-off threshold: even if someone experiences symptoms corresponding to a given disorder but does not suffer because of them and copes in life without noticeable difficulty, the diagnosis is usually not justified.
The impact on functioning is what most often separates a personality trait or a lifestyle from a disorder. A shy, cautious or meticulous person does not have a disorder on that account, as long as these traits do not begin to genuinely hinder their life or cause distress. This also applies to the area of personality disorders (code 6D10), where the assessment of disturbances in relationships and in the fulfilment of life roles is a central element of the diagnosis.
It is worth emphasising that distress and deterioration of functioning are two separate, though related, conditions. Sometimes a person suffers despite coping well outwardly; sometimes functioning is clearly disturbed while the sense of distress is limited. ICD-11 usually regards the presence of either of these elements as significant, and the clinician assesses both.
Why the ICD-11 criteria are not a checklist of symptoms
A common misunderstanding is treating diagnostic criteria like a simple questionnaire: if someone ticks the right number of symptoms, they automatically have a diagnosis. ICD-11 deliberately moves away from such thinking. This is precisely why there is an emphasis on essential features and on diagnostic requirements instead of a rigid list of points.
A list of symptoms answers the question of what a person feels. Diagnostic criteria answer a much richer question: whether this set of symptoms, of this severity, persisting for this length of time, producing such consequences, and with other explanations excluded, corresponds to the definition of a specific disorder. This is the difference between gathering data and interpreting it clinically.
Accurate ICD diagnostics therefore requires something more than matching symptoms. It requires checking the course over time, assessing the life and cultural context, carrying out a differential diagnosis and excluding physical causes and the effect of substances. Screening tools and questionnaires are helpful, but they only indicate that it is worth looking at the problem more closely.
For this reason a diagnosis should be made by a qualified specialist. Educational materials, self-checks or classification browsers can help one to understand how a given disorder is thought about, but they do not replace a clinical interview and examination. Matching oneself to the criteria on one's own easily leads to mistaken conclusions in both directions – to over-diagnosis and to overlooking a real problem.
The boundary between a disorder and a normal reaction
One of the most difficult tasks in diagnosis is distinguishing a disorder from a normal, appropriate human reaction to difficult circumstances. Sadness, anxiety, anger, grief or tension are natural elements of human life, not symptoms of illness. Their mere presence does not indicate a disorder.
ICD-11 builds this boundary directly into the criteria. The descriptions in the CDDR contain separate notes on the boundary with normality, which expressly indicate which reactions are expected and proportionate to the situation. The clinician is obliged to assess whether a person's reaction still falls within the expected range or clearly exceeds it.
A good example is the reaction to stress and loss. Adjustment disorder (code 6B43) describes excessive, persistent difficulties in adapting to a significant stressful event, but only when the reaction is clearly disproportionate and disturbs functioning. An ordinary, though painful, reaction to a divorce, job loss or relocation is not a disorder. Similarly, grief after the loss of a close person is a normal process, and a separate diagnosis is considered only when it has an exceptionally prolonged and impairing character.
This boundary is always marked out by the same three dimensions: the type and severity of symptoms, their duration and the impact on functioning and the level of distress. A reaction proportionate to the situation, resolving within a reasonable time and not causing lasting disturbance is normal, even if it is very unpleasant. Pathologising ordinary human experiences is an error just as serious as overlooking a genuine disorder.
Differential diagnosis and excluding other causes
Meeting the criteria for one disorder is only half the road. Before a clinician regards a diagnosis as certain, they must carry out a differential diagnosis, that is, consider other states that could explain the same pattern of symptoms, and assess which explanation fits best.
Many mental symptoms are non-specific – they occur in various disorders. A depressed mood appears in depressive disorders, but also in the depressive phase of bipolar disorder (6A60), in adjustment disorder or as the effect of a physical illness. Anxiety accompanies both generalised anxiety disorder (6B00) and panic disorder (6B01), phobias or post-traumatic stress disorder (6B40). The clinician must determine which clinical picture is the leading one.
The CDDR supports this process by providing, for every disorder, notes on the boundaries with other entities. It indicates, for example, how to distinguish schizoaffective disorder (6A21) from schizophrenia with co-occurring mood symptoms, or how to separate obsessive-compulsive disorder (6B20) from worry in generalised anxiety disorder.
A separate, mandatory step is the exclusion of physical causes and the effect of substances. Mental symptoms may stem from general medical conditions, from the effect of medicines or from the use of psychoactive substances, including alcohol (disorders due to alcohol use have the code 6C40). If the symptoms are better explained by such a cause, a diagnosis of an independent mental disorder is not appropriate. For this reason a diagnostic examination also includes an assessment of physical health.
The most common errors in applying the criteria
The first common error is counting symptoms in isolation from the essential features. A person may recognise many accompanying symptoms in themselves and conclude that there is a disorder, even though the core, defining feature is absent. The ICD-11 criteria require the presence of the essential features, not merely a number of symptoms.
The second error is omitting the time threshold. Symptoms present for a few days, in reaction to a specific event, are usually not enough to diagnose a disorder which, by definition, requires persistence over weeks or months. A diagnosis made too early often turns out to be mistaken when the state resolves on its own.
The third error is ignoring the requirement of distress and impact on functioning. Features that do not cause distress or hinder life usually do not justify a diagnosis, even if in terms of description they resemble symptoms. Omitting this threshold leads to pathologising ordinary individual differences.
The fourth error is omitting the context, including the cultural and situational context. Behaviours and experiences must be assessed in relation to the norms of a person's environment, their age and their life circumstances. What in one context is a symptom may in another be the norm. The CDDR expressly draws attention to this.
The fifth error is treating the criteria as a tool for self-diagnosis. The ICD-11 diagnostic criteria are a description created for trained clinicians who can relate them to the full clinical picture. Educational materials help one to understand the logic of diagnoses, but diagnosing oneself or a loved one without consulting a specialist is unreliable.
How to use this service to understand the criteria
This service allows you to see, step by step, what the ICD-11 diagnostic criteria look like in practice. The pages of the individual disorders present a clinical description, the typical symptoms and the place of a given entity in the structure of the classification, which makes it easier to understand how one diagnosis differs from another.
The ICD-11 classification browser reproduces the hierarchy of codes, so one can move from a general group, for example mood disorders or anxiety disorders, to specific entities and their descriptions. This helps one to see how close together the states lie that are easy to confuse, and why differential diagnosis is so important.
The content on this service is educational and informational in character. It helps one to understand how mental health is thought about and how the criteria are constructed, but it is not a diagnostic tool and does not replace contact with a specialist. Reliable ICD diagnostics is always based on an examination carried out by a psychiatrist or a clinical psychologist.
If you recognise in yourself or a loved one the symptoms described on this service and they persist, intensify or hinder everyday life, the best step is to arrange a consultation with a specialist. Knowledge of the criteria helps you to describe your experiences better and to discuss them with a clinician with awareness.