The International Statistical Classification of Diseases and Related Health Problems includes both physical and mental disorders. It originated from the 1893 International List of Causes of Death used to monitor global mortality and morbidity statistics (i.e. data on death and disease).  In 1948, The World Health Organisation (WHO) became involved and over the years the ICD has been updated many times.

The current version, ICD11 was published in 2018 and endorsed by all WHO members in 2019. It is a multilingual, freely available resource is used by clinicians and researchers, policy makers and patient organisations around the world.

It provides a ‘common language’ so that data collected in different countries can be usefully compared.

Mental disorders and ICD codes

Chapter 6 of ICD11 is called Mental, Behavioural and Neurodevelopmental Disorders. It contains more 20 different sub-categories.

Each disorder has a code, starting with 6 followed by a series of letters and numbers. The codes are hierarchical. for example, 6A20 refers to  schizophrenia, 6A20.0 refers to ‘schizophrenia – first episode’ and 6A20.00 to ‘schizophrenia – first episode – currently symptomatic’.  6B00 covers ‘anxiety or fear-related disorders’ and within that code 6B03 is ‘specific phobic disorder’.

Each additional digit or extension adds further detail. The coding system has been developed to allow new disorders to be added, without having to recode the other disorders. These codes are used for indexing medical records. This makes it easier to find people with specific conditions, when conducting research. The system attempts to include all known conditions whilst avoiding repetition or overlap.

Changes over the years

ICD-5 had just four categories of disorder in 1938 and although considerable changes were made to ICD-6, Erwin Stengel (1959) concluded that this section needed a major overhaul.

Reliability was a major issue – the presentation, communication and interpretation of symptoms is influenced by language and culture. This means that patients in one country might be given a different diagnosis to patients in another country, despite presenting very similar symptoms.

Likewise, patients with very different symptoms might be diagnosed with the same disorder. This posed problems for the international research community and led to the development of a globally accepted glossary of terms.

Creating a more reliable and valid system

A rigorous research programme was set up to review differences in diagnostic practice and differing use of diagnostic terminology across the world. Dozens of international conferences, seminars and workshops took place. Delegates from many countries diagnosed patients using video clips and discussed the issues with each other. 

International interview schedules were developed to facilitate a common assessment process, allowing comparisons to be made.  This process helped to reveal inconsistencies, ambiguities and overlaps between disorders, which were then removed.

Psychiatrists are encouraged to indicate whether a diagnosis is ‘provisional’ or ‘tentative’, i.e. in the absence of sufficient information or whether they are ‘confident’ in their decision.

Testing the ICD-10

Norman Sartorius and colleagues (1995) reported the findings of field trials involving eleven thousand patient assessments and nearly one thousand clinicians from 32 different countries. Most disorders showed good reliability but some had lower reliability e.g. mild recurrent depressive disorder, (Kappa 0.3).

Generally, clinicians found the ICD10 easy to use however they still felt that some symptoms overlapped in different disorders and disorder with mixed symptoms, e.g. mixed anxiety and depressive disorder were challenging to diagnose.

Cultural differences lead psychiatrists to make differing diagnosis

A study by Barry Gurland et al. in 1970 demonstrated that compared with psychiatrists from London (UK), psychiatrists in New York (US) were more likely to diagnose patients with schizophrenia than affective (mood) disorders. Gurland found that this difference was due to differences in the psychiatrists and not differences in the patients. This shows that individual differences in the cultural background, experience and training of psychiatrists can affect their interpretation of symptoms, leading to differential diagnosis.