# 03 - Development of the MMS and the CDDR for ICD 1

# Development of the MMS and the CDDR for ICD-11 mental, behavioural and neurodevelopmental disorders

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be useful to non-specialist health professionals (e.g. primary care physicians, nurses), who in 
many countries provide a substantial proportion of total mental health services. They will also 
be useful for other professionals in clinical and non-clinical roles who need to understand the 
nature and symptoms of these disorders even if they do not personally diagnose them. Finally, the 
CDDR are intended to provide students and trainees in variety of mental health and other health 
fields with comprehensive guidance and information to support their development as competent 
diagnosticians or interdisciplinary team members.
It is important to note that the organization of ICD-11 into chapters is not intended to reflect 
the scope of practice of specific medical specialties or clinical professions. For example, mental, 
behavioural and neurodevelopmental disorders and diseases of the nervous system are in separate 
chapters, but WHO does not intend this as a statement that psychiatrists should not be allowed 
to assess and treat headache disorders or that neuropsychologists should not be permitted to 
evaluate which disease process may be causing a particular case of dementia given the pattern of 
symptoms. Similarly, certain mental disorders (e.g. neurocognitive disorders such as dementia 
or delirium, or dissociative disorders such as dissociative neurological symptom disorder 
or dissociative amnesia), frequently come to the attention of a variety of health professionals 
(e.g. primary care physicians, neurologists) who may be equipped to evaluate and diagnose them 
using the CDDR as a guide.
Health-care professionals using the CDDR to make diagnoses should be qualified to do so by 
their clinical training and experience, and are expected to have the necessary clinical expertise 
and understanding of mental disorders to identify symptoms and to distinguish disorders from 
normal variation, from one another, and from transient responses to stress or environmental 
circumstances. The CDDR are written to allow for the exercise of clinical judgement, and it is 
the diagnosing health professional who is responsible for developing a diagnostic formulation 
appropriate for an individual patient, considering the patient’s individual, social and cultural 
context as well as the characteristics of the health system. It is equally important to note that 
diagnostic classification is only a part of patient assessment. The CDDR are not a guide to patient 
care, nor a comprehensive textbook of psychiatry, nor a manual of how to conduct clinical 
assessments and differential diagnoses. The focus of the CDDR is on the classification of disorders 
and not the assessment and treatment of people, who are frequently characterized by multiple 
disorders and diverse needs (5).
Development of the MMS and the CDDR for ICD-11 mental, 
behavioural and neurodevelopmental disorders
As noted above, the WHO Department of Mental Health and Substance Use led the development 
of the ICD-11 chapter on mental, behavioural and neurodevelopmental disorders, including 
both the MMS and the CDDR. Over the course of more than a decade, the Department 
coordinated an intensive, systematic, international process that involved a wide range of key 
stakeholder groups including scientific and public health experts, clinicians, representatives of 
WHO Member States, scientific and professional societies, service users and their carers, and 
other nongovernmental organizations. All expert contributors to this document were asked to 
complete a WHO Declaration of Interests (DoI) form prior to their contribution. Once received, 
the WHO Secretariat reviewed the DoI forms and evaluated whether there were any conflicts of 
interest and, if so, whether these required a management plan. Prior to the finalization of this 
document, all contributors were asked to complete another DOI form. No conflicts of interest 
requiring a management plan were identified.
Introduction

Clinical Descriptions and Diagnostic Requirements for ICD-11 Mental, Behavioural or Neurodevelopmental Disorders
Formation of the ICD-11 International Advisory Group
In 2007, the WHO Department of Mental Health and Substance Use appointed the International 
Advisory Group for the Revision of ICD-10 Mental and Behavioural Disorders (5), comprising 
scientific experts from all WHO regions and representatives from relevant international scientific 
societies. This Advisory Group was tasked with scientific oversight for the entire revision process. 
Its specific functions were to advise WHO on the key guiding principles and goals of the revision, 
the steps involved in the revision, and identification of other groups of relevant consultants and 
stakeholders required to develop the classification of mental disorders and other relevant parts 
of ICD-11; and to facilitate the implementation of global field studies to assess and improve the 
revisions to ICD-11 and the CDDR. The Advisory Group also functioned as one of approximately 
30 topic advisory groups for the overall development of ICD-11, each focusing on a different area 
or cross-cutting theme (e.g. gastroenterology, ophthalmology, quality and safety) and represented 
on the Revision Steering Committee – working with the WHO Classifications and Terminology 
and Data Standards Team responsible for coordinating the overall effort.
Development of the CDDR by ICD-11 working groups
The International Advisory Group advised the WHO Department of Mental Health and Substance 
Use on the appointment of more than a dozen working groups tasked with reviewing the 
relevant evidence and making proposals for changes to the structure and content of the ICD-10 
classification of mental, behavioural and neurodevelopmental disorders. Most of the working 
groups were established in relation to a particular subset of mental disorders (e.g. psychotic 
disorders, mood and anxiety disorders, personality disorders). Others were appointed to make 
proposals regarding how cross-cutting themes (e.g. presentations in children and adolescents, 
presentations in older adults, cultural factors) would be handled in the CDDR. Members of the 
working groups were expert multidisciplinary mental health professionals with relevant scientific 
and/or clinical expertise. The working groups were constituted such that experts from all WHO 
regions were included, with substantial representation from low- and middle-income countries.
The working groups’ charge included reviewing the extant body of basic science, clinical and public 
health research relevant to their area of responsibility for use as a basis for their recommendations 
for revisions to the classification of mental disorders in ICD-10. The working groups used these 
reviews to recommend changes to enhance the validity of ICD-11 (e.g. addition or deletion 
of categories, changes in thresholds or diagnostic requirements). They also reviewed available 
evidence related to the clinical utility of proposed changes, such as whether the revised diagnostic 
descriptions would enhance the identification of individuals who need mental health services or 
their usefulness in treatment and management decisions in a range of global health-care settings, 
particularly in low- and middle-income countries. Working groups proposed specific changes to 
the structure of the classification of mental disorders in ICD-11; what categories and specifiers 
were included; and the MMS brief descriptions and the CDDR for the area under their purview. 
They were asked to provide the rationale, evidence base and expected impact on clinical utility of 
any proposed change via a structured, documented process (16).
Working groups were instructed to emphasize considerations of clinical utility and global 
applicability in developing their recommendations. Clinical utility is important because health 
classifications represent the interface between health encounters and health information. A system 
that does not provide clinically useful information at the level of the health encounter will not 
be faithfully implemented by clinicians. In that case, data aggregated from health encounters 
will not be optimal and perhaps not even valid, affecting the usefulness and validity of summary 
health encounter data used for decision-making at the health system, national and global level. 
The WHO Department of Mental Health and Substance Use operationalized the clinical utility

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of a category in the ICD-11 chapter on mental, behavioural and neurodevelopmental disorders 
as depending on:
• its value in communicating (e.g. among practitioners, patients, families, administrators);
• its implementation characteristics in clinical practice, including its goodness of fit 
(i.e. accuracy of description), its ease of use and the time required to use it (i.e. feasibility); 
• and its usefulness in selecting interventions and in making clinical management decisions.
Global applicability was addressed via the diverse global membership of the working groups and 
by the international nature of the field studies implemented as part of the development of the 
CDDR, and by specific attention to culture as a part of the CDDR (see the sections on public 
review and field testing and on cultural factors below).
In order to generate relatively uniform information and a consistent structure across groupings 
and categories of the CDDR (see the next section on using the ICD-11 classification of mental, 
behavioural and neurodevelopmental disorders in clinical settings), working groups collated 
diagnostic information using a standardized template (referred to as a “content form”), with 
relevant references. This information served as source material for the development of the 
CDDR, with the final editorial responsibility vested in the WHO Department of Mental Health 
and Substance Use. The brief descriptions in the MMS and the more detailed essential features 
in the CDDR were developed together in order to be fully compatible, though designed for 
different purposes. The MMS brief descriptions are typically summaries of the essential features 
for the corresponding entity in the CDDR, although these brief descriptions alone do not provide 
sufficient information for implementation in clinical settings.
Public review and field testing
Proposals developed by working groups were described in the scientific literature (e.g. 20–26), 
made available for public review (27), and tested via a systematic programme of global field 
studies (see the section on field studies below). Scientific oversight for the field studies was 
provided by the ICD-11 Field Studies Coordination Group (FSCG) (28), comprising global 
leaders in clinical care, scientific research and public health representing all WHO regions, with 
substantial representation from low- and middle-income countries. FSCG members not only lent 
their technical expertise to the design, analysis and interpretation of the field studies but also 
served as essential facilitators by successfully engaging global clinicians to participate in ICD-11 
field studies around the world. Many of them directed international field study centres, which 
conducted field studies in routine clinical settings with real patients. The FSCG and relevant 
working groups were also involved in proposing changes to the CDDR based on the results of the 
field studies (17).
WHO’s comprehensive programme of field testing to assess the reliability, clinical utility and 
global applicability of the proposed CDDR was a major area of innovation, employing novel study 
designs and new methodologies for collecting information. Global participation was a defining 
characteristic of the ICD-11 CDDR field studies, which engaged multidisciplinary clinicians 
working in diverse contexts across the world, and were conducted in multiple languages. A key 
strength of the research programme was that studies were conducted within a time frame that 
allowed the results to be used as a basis for further revision of the CDDR prior to publication.
Early in the revision process, two major international, multilingual surveys were conducted – one 
of psychiatrists, conducted in collaboration with the World Psychiatric Association (29) and the 
other of psychologists, conducted in collaboration with the International Union of Psychological 
Science (30). These surveys focused on participants’ use of diagnostic classification systems in 
clinical practice, and the desirable characteristics of a classification of mental disorders. The 
professionals overwhelmingly preferred more flexible guidance to allow for cultural variation 
Introduction

Clinical Descriptions and Diagnostic Requirements for ICD-11 Mental, Behavioural or Neurodevelopmental Disorders
and clinical judgement compared to a strict criteria-based approach, and were receptive to a 
system incorporating a dimensional component. Respondents described a number of categories 
as having poor clinical utility in practice, while others were recommended for addition or 
deletion (31,32). These data were used in the initial decisions about the content of the CDDR and 
in the development of diagnostic guidance by working groups.
In order to provide data to assist in developing an organizational structure that would be more 
clinically useful, two formative field studies were conducted to examine the conceptualizations 
held by mental health professionals around the world regarding the structure of mental disorders 
and the relationships among them (33,34). These data showed a high degree of similarity across 
countries, languages and regions, regardless of the classification system participants used in 
clinical practice, and informed decisions about the structure of the classification.
The proposed CDDR material was then tested in two main sets of evaluative field studies: 
case-controlled (internet-based) and ecological implementation (clinic-based) field studies. 
The case-controlled studies assessed the clinical utility of the proposed diagnostic material; 
most compared how global clinicians applied the proposed ICD-11 material versus the ICD-10 
diagnostic guidelines for a given diagnostic area in terms of accuracy and consistency of clinicians’ 
diagnostic formulations, using a scientifically rigorous vignette-based methodology (17,35). 
Other studies examined scaling for diagnostic specifiers (36) and how clinicians actually used 
classifications in their clinical practice (37), including how they combined multiple dimensions 
in making a categorical diagnosis (38).
Case-controlled studies were conducted in between three and six languages – Chinese, English, 
French, Japanese, Spanish and Russian – per study, with participation from thousands of 
clinicians from across the globe who were members of WHO’s Global Clinical Practice Network. 
The Network’s field studies have assessed a wide range of disorder groupings (e.g. 39–45). 
Two internet-based studies comparing ICD-11 to ICD-10 were also conducted in German (46,47). 
Overall, these studies have demonstrated incrementally superior – or in some cases equivalent – 
performance of the ICD-11 CDDR compared to the ICD-10 CDDG in the accuracy of diagnoses 
assigned to case vignettes by participating clinicians, as well as improvements in ratings of clinical 
utility. When a clinician’s diagnoses did not follow the expected pattern, the study methodology 
allowed for examination of which specific features of the diagnostic requirements accounted for 
underperformance, which in turn permitted modifications to the CDDR to address points of 
ambiguity or misunderstanding (e.g. 39,44). These studies also included analyses of results by 
region and language to identify potential difficulties in global or cultural applicability, as well as 
problems in translation. In addition to refining the CDDR, data from these studies have provided 
useful information for the development of training programmes on the new ICD-11 diagnostic 
material. For example, the German study examining performance on a coding task suggested a 
substantial need for training initiatives to support the use of ICD-11 by professional coders (47).
Two major ecological implementation (clinic-based) field studies were conducted. The first 
tested the proposed CDDR diagnostic material when applied by practising clinicians to adult 
patients receiving care in the types of clinical settings in which the CDDR will be implemented 
(18,19). The study was conducted in 14 countries – Brazil, Canada, China, Egypt, India, Italy, 
Japan, Lebanon, Mexico, Nigeria, the Russian Federation, South Africa, Spain and Tunisia – via a 
network of international field study centres. The study assessed the reliability and clinical utility 
of the CDDR for disorders that account for the highest percentage of global disease burden and 
use of mental health services in clinical settings among adults: schizophrenia and other psychotic 
disorders, mood disorders, anxiety and fear-related disorders, and disorders specifically 
associated with stress. A joint-rater reliability methodology, in which two clinicians were present 
during the patient interview but reported their diagnostic formulation and clinical utility ratings

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independently, was employed in order to isolate the effects of the CDDR from other sources of 
variance in diagnosis (e.g. changes over time, inconstancy in reporting). Importantly, the level 
of training on the ICD-11 CDDR received by participating clinicians was similar to what might 
be expected in routine clinical setting during ICD-11 implementation. Clinicians were given no 
instructions on how to conduct their diagnostic interviews other than to assess the areas that 
were required as part of the study protocol. Overall, intraclass kappa coefficients (a measure of 
reliability between raters) for diagnoses weighted by site and study prevalence ranged from 0.45 
(dysthymic disorder) to 0.88 (social anxiety disorder). The reliability of the ICD-11 diagnostic 
requirements was superior to that previously reported for equivalent ICD-10 guidelines (18). 
Clinician ratings of the clinical utility of the ICD-11 CDDR were very positive overall. The 
CDDR were perceived as easy to use, accurately reflecting patients’ presentations (i.e. goodness 
of fit), clear and understandable, and no more time-consuming than the clinicians’ standard 
practice (19).
A separate study of common child and adolescent diagnoses was conducted in four countries – 
China, India, Japan and Mexico – with children and adolescents from 6 to 18 years of age (48). 
The study focused on attention deficit hyperactivity disorder, disruptive behaviour and dissocial 
disorders, mood disorders, anxiety and fear-related disorders, and disorders specifically associated 
with stress, using a design that was analogous to the adult study. Kappa estimates indicated 
substantial agreement for most categories, with moderate agreement for generalized anxiety 
disorder and adjustment disorder. No differences were found between younger (6–11 years) and 
older (12–18 years) age groups, or between outpatient and inpatient samples. Clinical utility 
ratings for these diagnoses were positive and consistent across the domains assessed, although 
they were somewhat lower for adjustment disorder. Taken together, the results of the ecological 
implementation studies supported the implementation of the ICD-11 CDDR in clinical settings, 
and suggested that the results of the case-controlled studies were generalizable to clinical settings. 
Another clinic-based field study in three countries examining the novel behavioural indicators 
for the assessment of the severity of ICD-11 disorders of intellectual development found them 
to have good to excellent levels of inter-rater reliability, concurrent validity and clinical utility. 
This supported their use to assist in the accurate identification of individuals with disorders 
of intellectual development, particularly in settings where specialized assessment services are 
unavailable (49).
A separate field studies programme to test the section of the ICD-11 CDDR on disorders due to 
substance use and addictive behaviours involved field testing centres in 11 countries: Australia, 
Brazil, China, France, Indonesia, India, the Islamic Republic of Iran, Malaysia, Mexico, Switzerland 
and Thailand. The main aim of the studies was to explore the public health and clinical utility, 
feasibility and stability (comparability with ICD-10) of the proposed CDDR for disorders due 
to the use of psychoactive substances, as well as the newly designated subgrouping of disorders 
due to addictive behaviours (i.e. gambling disorder and gaming disorder). The mixed-methods 
approach used in these studies included key informant surveys and interviews, focus groups 
and consensus conferences at each study site. Across sites, more than 1000 health professionals 
participated in the survey, more than 200 participants were involved in 30 focus groups organized 
at the study sites, and 42 identified national experts in the field reviewed the draft CDDR.
Overall, this section of ICD-11 was judged to be major step forward compared to ICD-10 in 
terms of its utility for meeting clinical and public health, and its feasibility for implementation. 
There was broad support for major innovations in this area. For disorders due to substance use, 
this included the expansion of substance classes to reflect evolving patterns in global psychoactive 
substance use (e.g. synthetic cannabinoids, MDMA1 or related drugs), the introduction of new 
categories to capture episodes of harmful substance use, and the inclusion of the concept of “harm 
to health of others” in the definition of harmful substance use (25). There was also support for 
 3,4-methylenedioxy-methamfetamine, also known as “ecstasy”. 
Introduction

Clinical Descriptions and Diagnostic Requirements for ICD-11 Mental, Behavioural or Neurodevelopmental Disorders
integrating disorders due to addictive behaviour in the same overarching grouping as disorders 
due to substance use, and for the introduction of the new diagnostic category gaming disorder. At 
the same time, the field study results highlighted the overall increase in complexity of this part of 
ICD-11 and the need for training of health professionals in order to ensure a smooth transition. 
The field studies also yielded specific suggestions for better delineation of the boundaries among 
some diagnostic categories as well as better descriptions of new ones.
In addition, WHO commissioned a study (50) on the concordance among diagnoses for alcohol 
and cannabis use disorders based on ICD-11, ICD-10 and the Diagnostic and Statistical Manual 
of Mental Disorders, fourth and fifth editions (DSM-IV and DSM-5) (51,52). The results of 
the study demonstrated high concordance among the populations identified by the ICD-11 
diagnostic requirements compared to ICD-10 and DSM-IV. Concordance of between ICD-11 
and DSM-5 was substantially lower, in large part due to low agreement between the diagnoses of 
harmful pattern of alcohol use and harmful pattern of cannabis use in ICD-11 and mild alcohol 
use disorder and mild cannabis use disorder in DSM-5.
Development of the CDDR also included the involvement of mental health service users and 
carers through two studies in 15 countries representing diverse clinical contexts in multiple 
global regions (53,54). These studies constituted the first instance of a systematic research 
programme studying mental health service users’ perspectives during the revision of a major 
diagnostic classification system. The studies employed participatory research methodologies to 
systematically collate service user perspectives on key CDDR diagnoses that contribute to high 
disease burden, including schizophrenia, depressive episode, bipolar type I disorder, generalized 
anxiety disorder and personality disorder. Findings from these studies provided an understanding 
of how mental health service users respond to diagnostic content of the CDDR, and served 
as a basis for providing recommendations to WHO about potential enhancements of CDDR 
diagnostic material that may enhance its clinical utility (e.g. its usefulness in communicating 
with service users) and mitigating potential unintended negative consequences of the diagnostic 
material, including stigmatization of diagnosed individuals.
Coordination with the development of DSM-5
The development of ICD-11 overlapped with the development and publication of DSM-5 (52). 
The Chair and Co-Chair of the DSM-5 Task Force regularly attended meetings of the Advisory 
Group in an effort to facilitate “harmonization” of the two classifications. This was most successful 
in terms of the way that mental disorders are divided into groupings and how those groupings are 
ordered in the two classifications (referred to as the “metastructure”). In this regard, ICD-11 and 
DSM-5 are quite similar to one another, though not identical, and substantially different from ICD10 and DSM-IV. Most ICD-11 working groups included at least one member of the corresponding 
DSM-5 workgroup. ICD-11 working groups were asked to consider the clinical utility and global 
applicability of material being developed for DSM-5, with the goal of minimizing unintentional or 
arbitrary differences between the two systems. Intentional conceptual differences were permitted, 
however, and the working groups were asked to provide a justification for such differences where 
they were proposed. The differences between ICD-11 (both the MMS and the CDDR) and DSM-5 
are therefore conscious and intentional (16,55), and a number of such differences have stimulated 
valuable research that has enhanced our knowledge about psychopathology (56).