Differential Diagnosis in ICD-11 – A Clinical Guide
Differential diagnosis is the process in which a clinician compares several possible diagnoses and selects the one that best explains a patient's symptom picture. This guide explains what ICD-11 differential diagnosis involves, why it matters and how to resolve the most difficult dilemmas in Chapter 6 of the classification, that is, the part devoted to mental disorders.
What differential diagnosis is and why it matters
Differential diagnosis is the systematic comparison of several diagnoses that could fit a clinical picture (the set of symptoms, how long they have lasted and the context of the patient's life). The clinician does not stop at the first hypothesis that comes to mind, but deliberately sets it against other, similar possibilities and checks which of them best explains the full range of reported difficulties. Only after such a comparison can a diagnosis be made with adequate confidence.
In psychiatry and clinical psychology, differential diagnosis is especially demanding because many disorders share some of their symptoms. Low mood, anxiety, sleep problems or difficulty concentrating appear in dozens of diagnoses. What distinguishes one disorder from another is usually not a single symptom, but its severity, duration, configuration with other symptoms and the circumstances in which it appeared.
An accurate diagnosis has real consequences. It determines the choice of treatment, the prognosis (the expected course over time), as well as the way the patient understands their own experience. A differential error can lead to ineffective and sometimes harmful treatment – a classic example being the treatment of bipolar disorder (an illness with alternating episodes of low and abnormally elevated mood) as if it were ordinary depression. Differential diagnosis is therefore not an academic formality, but the foundation of safe care.
The ICD-11 classification, that is, the International Statistical Classification of Diseases and Related Health Problems in its eleventh revision, developed by the World Health Organization, provides a shared language and a shared set of criteria. Its Chapter 6 covers mental, behavioural and neurodevelopmental disorders. It is accompanied by detailed diagnostic guidance known as the CDDR (Clinical Descriptions and Diagnostic Requirements), which describes how to understand each category and how to distinguish them from one another.
The general method of differentiation in ICD-11
A good process of differentiation begins with a broad gathering of information, not with narrowing down. The clinician collects the full picture: current symptoms, their severity, the moment they appeared, their course over time, the history of earlier episodes, the family and social context, and the impact of the difficulties on everyday functioning. Only on this basis is a list of diagnostic hypotheses formed, that is, the disorders that come into consideration at the outset.
Next, the order of differentiation is important. ICD-11 recommends first ruling out causes that require a different course of action: a physical illness (for example, thyroid disorders that can mimic depression or anxiety), the effects of psychoactive substances and medications, and neurological conditions. Only once these avenues have been checked does one proceed to differentiating between the individual mental disorders.
Crucial here are the so-called exclusion criteria and symptoms with high differentiating value. Some symptoms are decisive – their presence shifts the diagnosis in one direction while ruling it out in another. An example is even a single clear episode of mania or hypomania (a period of abnormally elevated mood and energy), the mere presence of which moves the diagnosis from the group of depressive disorders to the group of bipolar disorders. The clinician actively looks for such symptoms, because it is they that settle any doubts.
ICD-11 also allows for the co-occurrence of several disorders at the same time – this is called comorbidity. Differentiation therefore does not always consist of choosing one category at the expense of the others; sometimes the correct answer is that two diagnoses are present in the patient at once. The clinician then decides whether the symptoms of one disorder are fully explained by the symptoms of the other, or whether they constitute a separate, independent problem requiring a diagnosis of its own. All ICD-11 differential diagnosis rests, moreover, on conversation and observation, and not on the automatic adding-up of symptoms from a list.
Unipolar depression versus bipolar disorder
This is one of the most important and most often missed dilemmas. Depressive disorder, that is, unipolar depression, involves episodes of low mood, loss of interest and loss of energy, but without periods of abnormally elevated mood. Bipolar disorder, by contrast, is characterised by a course in which, alongside depressive episodes, episodes of mania or hypomania also occur. The problem is that patients most often present to a specialist precisely during a depressive phase, and they may recall periods of elevated mood as simply good, productive weeks.
The decisive factor is a history of episodes of mania or hypomania. Mania is a period of clearly elevated, expansive or irritable mood combined with increased energy and activity, lasting at least a week and significantly disrupting functioning. Hypomania is a milder and shorter form of the same state, which does not cause such serious consequences. If even one clear episode of this type has occurred in the past, the diagnosis shifts to the bipolar disorders – even if the patient is currently depressed.
ICD-11 distinguishes bipolar type I disorder, in which at least one full episode of mania has occurred, from bipolar type II disorder, in which episodes of hypomania and depression are present but without full mania. Helpful pointers suggesting a bipolar basis include, among others, an early onset of the illness, numerous recurrences of depression, depression with excessive sleepiness and slowing, as well as a family history of bipolar disorder. None of these elements settles the matter on its own, but together they build up a picture.
The stakes of this differentiation are high, because the treatment of the two groups of disorders proceeds differently. For this reason, with every depressive episode the clinician should actively ask about earlier periods of unusually high energy, reduced need for sleep or increased activity. You can find out more on the pages devoted to diagnoses 6A70 and 6A71 (depressive disorders) and 6A60, 6A61 and 6A62 (bipolar disorders and cyclothymia).
PTSD versus complex post-traumatic stress disorder
ICD-11 introduced a clear distinction between post-traumatic stress disorder, designated as PTSD, and its more severe form, that is, complex post-traumatic stress disorder. Both disorders are a reaction to a traumatic event, that is, one that is exceptionally threatening or terrifying. Both share a core set of symptoms: re-experiencing the trauma (for example, in the form of intrusive memories or nightmares), avoidance of reminders of it, and a persistent sense of threat, that is, heightened vigilance and excessive reactivity to stimuli.
What distinguishes complex post-traumatic stress disorder is an additional group of three areas of difficulty, referred to as disturbances in self-organisation. First, problems with emotion regulation, that is, strong and hard-to-control emotional reactions or, conversely, emotional numbing. Second, a persistently negative self-concept, marked by feelings of guilt, shame or being a lesser person. Third, persistent difficulties in maintaining close relationships and a sense of connection with other people.
Complex post-traumatic stress disorder is usually associated with prolonged or repeated trauma from which it was difficult to escape – for example, abuse in childhood, long-term domestic violence, or experiences of captivity. This is not, however, a rigid rule: the diagnosis is determined by the symptom picture, not by the type of event itself. The clinician therefore checks whether, in addition to the three core symptoms of PTSD, persistent disturbances in self-organisation are also present.
In ICD-11 differential diagnosis it is important that only one of these disorders is diagnosed – either PTSD or its complex form, not both at once. The distinction has practical significance, because complex post-traumatic stress disorder usually requires longer therapy directed also at emotion regulation and relationships, and not solely at working through the trauma itself. See the diagnosis pages 6B40 and 6B41.
ADHD versus anxiety disorders
Attention deficit hyperactivity disorder, abbreviated as ADHD, and anxiety disorders can present a surprisingly similar surface picture. In both cases there appear difficulty concentrating, restlessness, a sense of inner tension and trouble finishing tasks. In both situations the patient may say that they cannot focus and that their thoughts wander.
The mechanism of these difficulties is, however, different. In ADHD the problems with attention are present constantly, independently of the level of stress, and usually begin in early childhood. They arise from enduring features of how the person functions, rather than from a reaction to current worries. In anxiety disorders, by contrast, the difficulty concentrating is secondary to anxiety: attention is occupied by worry and tension, and when the anxiety lessens, concentration usually improves.
Key pointers are provided by the onset and the course. ICD-11 requires that the symptoms of ADHD appear during the developmental period, that is, in childhood, and persist across many life situations – at school or work, at home and in relationships. If difficulties with attention appeared only in adulthood and are clearly linked to a period of heightened anxiety, an anxiety disorder is more likely. It also helps to ask whether the impulsivity and hyperactivity typical of ADHD are present, or whether instead the worry and tension typical of anxiety predominate.
It must be remembered that ADHD and an anxiety disorder can co-occur – one does not exclude the other. In some people with ADHD, years of difficulty and failure lead secondarily to the development of anxiety. The clinician then decides which symptoms are primary and which are their consequence, and whether two separate diagnoses are warranted. More can be found on the page for diagnosis 6A05 and on the pages of disorders in the 6B00–6B04 group.
Generalised anxiety disorder versus panic disorder
These two disorders belong to the same group of anxiety or fear-related disorders, but they differ in the rhythm and character of the anxiety. Generalised anxiety disorder consists of persistent, diffuse worry concerning many different areas of life – health, work, family, everyday duties. The anxiety here has the character of a background: it is constantly present, hard to control and persists on most days for many months.
Panic disorder runs differently. Its axis is panic attacks, that is, sudden, very intense surges of anxiety that build up over a few minutes and are accompanied by strong bodily symptoms: palpitations, shortness of breath, dizziness, trembling, a feeling of loss of control or fear of dying. Between attacks there additionally appears anticipatory anxiety, that is, fear of another attack occurring.
The simplest way to tell them apart is to ask whether the anxiety is constant and diffuse, or rather paroxysmal and focused around the attacks themselves. In generalised anxiety disorder the content of the worries consists of real-life matters. In panic disorder the central fear is the attack itself and its bodily symptoms – the patient is afraid of their own body and of what happens to it during an attack. It is worth remembering that single panic attacks can also occur in the course of other disorders, so their mere appearance does not settle the diagnosis.
Both disorders can co-occur, and the differentiation also includes ruling out physical causes of the bodily symptoms, for example heart or thyroid problems. See the diagnosis pages 6B00 and 6B01.
Schizophrenia versus schizoaffective disorder
Schizophrenia and schizoaffective disorder belong to a group of disorders in which psychotic symptoms are present, that is, delusions (strong, uncorrectable beliefs at odds with reality) and hallucinations (perceptions without a real stimulus, for example hearing voices). Differentiating between them rests on the temporal relationship between the psychotic symptoms and the mood symptoms, that is, episodes of depression or mania.
In schizophrenia the psychotic symptoms are the primary phenomenon and can persist independently of the state of mood. Episodes of low or elevated mood may appear, but they do not dominate the picture and do not span a substantial portion of the duration of the illness. In schizoaffective disorder, by contrast, psychotic symptoms and a clear mood episode – depressive or manic – occur simultaneously within the same episode of illness, and this co-occurrence is the defining feature.
The clinician therefore reconstructs the timeline: when the delusions and hallucinations appeared, when the mood symptoms did, how long they lasted and whether they overlapped. If the psychotic symptoms persist for a longer time also when the mood is balanced, the picture shifts towards schizophrenia. If, however, a clear mood episode and psychosis go hand in hand for most of the duration of a given episode, a schizoaffective diagnosis is more appropriate.
Within the same differential field also fall delusional disorder, in which fixed delusions predominate without the other typical symptoms of schizophrenia, and severe episodes of depression or mania with psychotic symptoms, where the psychosis appears only within the mood episode and subsides together with it. See the diagnosis pages 6A20, 6A21 and 6A24.
Borderline personality disorder versus complex post-traumatic stress disorder
This differentiation is among the most difficult, because the borderline pattern and complex post-traumatic stress disorder share many features: strong and hard-to-control emotions, a negative self-concept and problems in close relationships. Both categories are also often linked to difficult experiences early in life, which further blurs the boundary.
In ICD-11, personality disorder is diagnosed on the basis of the severity of the difficulties – mild, moderate or severe – and then described by means of distinct personality traits. One of these is the borderline pattern. It is characterised, among other things, by instability of self-image, abrupt swings of emotion, intense fear of abandonment, unstable and stormy relationships, impulsivity, and recurrent self-harming behaviours or suicidal thoughts. This pattern is an enduring feature, present across many life situations, independent of any specific event.
Complex post-traumatic stress disorder, by contrast, is directly linked to a traumatic experience and includes the core symptoms of PTSD: re-experiencing the trauma, avoidance of reminders and a sense of constant threat. It is precisely the presence of these three symptoms that constitutes the key differentiating pointer – in the borderline pattern itself they are not required. It can also help to ask about the stability of the self-image: in borderline it is shaky and changeable, whereas in complex post-traumatic stress disorder the negative self-concept is rather steadily and consistently negative.
Both situations can co-occur, and the differentiation requires a careful conversation about the person's life history, trauma and patterns of functioning persisting over years. Resolving it matters for the treatment plan. See the diagnosis pages 6D10, 6D11 (including the borderline pattern) and 6B41.
Social anxiety versus ordinary shyness
Not every form of shyness is a disorder. Many people feel awkward in new social situations, prefer smaller groups or need time to warm up. Shyness in itself is a common and acceptable feature of temperament, not a health problem. Differential diagnosis here consists of distinguishing the norm from social anxiety disorder.
Social anxiety disorder is a clear and persistent fear of situations in which a person may be judged by others – of speaking, conversations, eating in company or simply being observed. The person fears that they will behave in an embarrassing way or that their symptoms of anxiety, for example blushing or a trembling voice, will be noticed and judged negatively. The anxiety is disproportionately strong relative to any real threat.
Two criteria help to distinguish the disorder from shyness. The first is the intensity of distress – in the disorder the anxiety is so strong that it becomes in itself a source of considerable discomfort. The second is the impact on functioning: the person avoids important situations, gives up occupational, educational or social opportunities, and their life becomes narrower. Ordinary shyness does not block a person in this way.
ICD-11 also requires that the difficulties persist for a longer time, usually at least several months. The boundary between a character trait and a disorder therefore runs where persistent distress and a real restriction of life appear. See the page for diagnosis 6B04.
Prolonged grief versus a depressive episode
Grief after the loss of a loved one is a natural reaction and is not in itself a disorder. ICD-11 does, however, distinguish a disorder referred to as prolonged grief disorder, where the reaction to loss becomes exceptionally intense, long-lasting and seriously impairs functioning. At the same time grief is sometimes confused with a depressive episode, because both situations involve sadness, crying, withdrawal and a decline in activity.
Prolonged grief is focused around the deceased person. Its axis is a persistent longing for the deceased or a constant preoccupation with thoughts of them, combined with intense emotional pain – disbelief, anger, a sense of having lost part of oneself, difficulty accepting the loss. ICD-11 notes that these symptoms persist for an unusually long time, far longer than the course of grief expected in a given cultural and social context, and continue to seriously disrupt everyday functioning.
A depressive episode, by contrast, has a more diffuse character. The low mood does not concern the loss alone, but encompasses a general loss of the capacity to feel joy and interest across many areas of life. Characteristic of depression, and less typical of uncomplicated grief, is also a persistent sense of one's own worthlessness, as well as recurrent suicidal thoughts unrelated to a wish to reunite with the deceased. In grief, sadness usually comes in waves and alternates with moments of good memories.
Importantly, both diagnoses can co-occur – a depressive episode may also develop after a loss. The clinician then assesses whether the full picture of depression is present, or whether the distress is concentrated above all around the loss. See the diagnosis pages 6B42 and 6A70 and 6A71.
Autism spectrum disorder versus ADHD
Autism spectrum disorder and ADHD are two neurodevelopmental disorders, that is, ones whose onset falls within the developmental period in childhood. They can present a similar picture: a child may be restless, have difficulty in relationships with peers, cause behavioural problems and struggle to cope within a group. The sources of these difficulties are, however, different.
In autism spectrum disorder the core consists of persistent difficulties in social communication and reciprocal relationships – in reading social cues, in understanding the intentions of others, in establishing contact – together with restricted, repetitive patterns of behaviour and interests, and often also an atypical sensitivity to sensory stimuli. The social difficulties here arise from a different way of processing social information. In ADHD, by contrast, the axis is the attention deficit, hyperactivity and impulsivity; problems in relationships are usually secondary to impulsivity and difficulties with self-control, rather than arising from a failure to understand social situations.
A helpful question is: does the child want and is able to make contact, but is hindered by impulsivity and restlessness, or is social contact itself difficult and hard for them to read. A pointer towards the autism spectrum is also repetitive behaviours, rigid routines and strong, narrow interests. A pointer towards ADHD is variability of attention depending on how engaging the task is, as well as marked hyperactivity.
ICD-11 explicitly allows the simultaneous diagnosis of both disorders – earlier classifications did not permit this, which led to one of them being overlooked. In many people the autism spectrum and ADHD co-occur, which is why the clinician does not treat them as mutually exclusive diagnoses. See the diagnosis pages 6A02 and 6A05.
Obsessive-compulsive disorder versus generalised anxiety disorder
Obsessive-compulsive disorder and generalised anxiety disorder are sometimes confused, because in both there appear intrusive, hard-to-control thoughts and unease. The difference, however, lies in the nature of these thoughts and in what follows them.
In obsessive-compulsive disorder there are present obsessions, that is, recurrent, unwanted thoughts, images or impulses, often experienced as alien, for example about contamination, harming someone or the need for symmetry. They are accompanied by compulsions, that is, repetitive actions or mental acts – checking, washing, counting, ordering – performed in order to reduce the tension caused by the obsession or to prevent an imagined catastrophe. It is precisely this obsession–compulsion link that is the defining feature, and there is usually no logical connection between the action and the fear.
In generalised anxiety disorder, by contrast, worry predominates, concerning real-life matters – health, finances, the safety of loved ones. The content of these worries is understandable to the patient and is experienced as their own thoughts, rather than as alien intrusions. They are also not accompanied by rituals of a compulsive character. The anxiety here is a diffuse background, and not a closed cycle of intrusive thought followed by compulsive action.
The simplest differentiating pointer: look for compulsions. If repetitive rituals performed in response to intrusive thoughts are present, the picture shifts towards obsessive-compulsive disorder. If there is diffuse worry without rituals, generalised anxiety disorder is more likely. Both can also co-occur. See the diagnosis pages 6B20 and 6B00.
How to use differentiation in practice
The dilemmas presented here illustrate a common principle: a diagnosis is rarely determined by a single symptom, and almost always by its context – the moment it appeared, its course over time, its configuration with other symptoms and its impact on life. ICD-11 differential diagnosis is a discipline of thinking that guards against a hasty conclusion and against fitting the patient to the first matching label.
This guide is educational in character and does not replace a consultation with a specialist. A diagnosis of a mental disorder is made by a qualified clinician – a psychiatrist or a clinical psychologist – on the basis of a direct examination, conversation and the patient's full history. Educational tools and materials of the ICD diagnostics type can help in understanding the logic of differentiation and in preparing for a conversation with a specialist, but they are not meant for self-diagnosis.
If you recognise in yourself or in a loved one symptoms described in any of the areas above, it is worth consulting a mental health professional. An accurate diagnosis is the first step towards effective help, and differentiation – demanding though it is – serves precisely the purpose of ensuring that this help is well chosen.