I know that ICD-10 is a diagnosis classification system.
I am not sure what SNOMED is and how to it is different from ICD-10. Why do we need two different systems?
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There are many systems for classifying diseases and medical terminology. Some are international, while many countries have their own systems.
The purpose of medical classification systems is to ensure that the accurate encoding of digital medical records so that terminology has the correct meaning across different regions and languages and for the gathering of statistical information for the purposes of research or healthcare planning.
The International Classification of Disease version 10 (ICD-10) is the tenth revision of the International Statistical Classification of Diseases and Related Health Problems (ICD), a medical classification list by the World Health Organization (WHO), who oversee health issues for the United Nations (UN).
It contains codes for diseases, signs and symptoms, abnormal findings, complaints, social circumstances, and external causes of injury or diseases.
The WHO also maintains several other internationally-recognised classifications, such as the International Classification of Functioning, Disability and Health (ICF).
Work began on ICD-10 in 1983 and it was first used by member states in 1994. The next major iteration (ICD-11), was ratified this year (2019) and will come into use in 2022.
You can browse the most recent ICD hierarchy on the WHO ICD site.
Whereas ICD-10 is primarily (though not exclusively) a classification of diseases, the Systematized Nomenclature of Medicine (SNOMED) is a system for classifying many types of medical data. The current version is known as SNOMED Clinical Terms, or SNOMED CT. It is the most widely recognised nomenclature in healthcare.
SNOMED CT (or SNOMED Clinical Terms) is a systematically organized computer processable collection of medical terms providing codes, terms, synonyms and definitions used in clinical documentation and reporting. SNOMED CT is considered to be the most comprehensive, multilingual clinical healthcare terminology in the world.
The purpose of SNOMED CT:
The primary purpose of SNOMED CT is to encode the meanings that are used in health information and to support the effective clinical recording of data with the aim of improving patient care. SNOMED CT provides the core general terminology for electronic health records.
So SNOMED CT has codes covering not just diseases, but also clinical findings, symptoms, diagnoses, procedures, body structures, organisms and other aetiologies, substances, pharmaceuticals, devices and specimens.
Other medical classification systems
There are many other examples of medical classification systems (such as the Diagnostic and Statistical Manual of Mental Disorders (currently in its fifth edition, DSM-V), published by the American Psychiatric Association. As the name suggests, this classification system is focussed on mental health problems.
The Wikipedia page on medical classifications provides a useful overview and comparison of ICD, SNOMED and other systems. I have provided some useful excerpts from the comparison.
SNOMED CT and ICD are designed for different purposes and each should be used for the purposes for which they were designed. SNOMED CT provides a consistent language that enables a consistent way of capturing, sharing, and aggregating health data across specialties and sites of care. It is highly detailed terminology designed for input, not reporting.
Classification systems such as ICD-10 group together similar diseases and procedures and organise related entities for easy retrieval. They are typically used for external reporting requirements or other uses where data aggregation is advantageous, such as measuring the quality of care, monitoring resource utilisation or processing claims for reimbursement.
SNOMED is clinically-based, documents whatever is needed for patient care and has better clinical coverage than ICD. It is used directly by healthcare providers during the process of care.
ICD’s focus is statistical with less common diseases get lumped together in “catch-all” categories, which result in loss of information. It is used by coding professionals after the episode of care.
Different systems can be mapped to each other if needed, although there are limitations where one system may not have an equivalent term in the other.
As for why there are so many different systems, it is probably the same reason why there are often many overlapping or competing standards in other fields like technology; large-scale cooperation can be challenging!