Medical classification, or medical coding, is the process of transforming descriptions of medical diagnoses and procedures into universal medical code numbers. The diagnoses and procedures are usually taken from a variety of sources within the health care record, such as the transcription of the physician's notes, laboratory results, radiologic results, and other sources.
Diagnosis codes are used to track diseases and other health conditions, whether they are chronic diseases such as diabetes mellitus and heart disease, to contagious diseases such as norovirus, the flu, and athlete's foot. These diagnosis and procedure codes are used by government health programs, private health insurance companies, workers' compensation carriers and others.
- statistical analysis of diseases and therapeutic actions
- reimbursement; e.g., based on diagnosis-related groups
- knowledge-based and decision support systems
- direct surveillance of epidemic or pandemic outbreaks
Many different medical classifications exist, though they occur into two main groupings: Statistical classifications and Nomenclatures.
A statistical classification brings together similar clinical concepts and groups them into categories. The number of categories is limited so that the classification does not become too big. An example of this is the International Statistical Classification of Diseases and Related Health Problems (known as ICD). It groups diseases of the circulatory system into one "chapter" (known as Chapter IX, covering codes I00–I99). Within this chapter, there is for instance, code I47.1. The code title (rubric) is Supraventricular tachycardia. However, there are several other clinical concepts that are also classified here. Among them are paroxysmal atrial tachycardia, paroxysmal junctional tachycardia, auricular tachycardia and nodal tachycardia.
Another feature of statistical classifications is the provision of residual categories for "other" and "unspecified" conditions that do not have a specific category in the particular classification.
In a nomenclature there is a separate listing and code for every clinical concept. So, in the previous example, each of the tachycardia listed would have its own code. This makes nomenclatures unwieldy for compiling health statistics.
Types of coding systems specific to health care include:
- Are used to determine diseases,disorders, and Symptoms
-Can be used to measure morbidity and mortality
-They are numbers or alphanumeric codes used to identify specific health interventions taken by medical professionals.
-Are used to identify medications
-Are codes that indicate a specific location in the body
WHO Family of International Classifications
The World Health Organization (WHO) maintains several internationally endorsed classifications designed to facilitate the comparison of health related data within and across populations and over time as well as the compilation of nationally consistent data. This "Family of International Classifications" (FIC) include three main (or reference) classifications on basic parameters of health prepared by the organization and approved by the World Health Assembly for international use, as well as a number of derived and related classifications providing additional details. Some of these international standards have been revised and adapted by countries for national use.
- International Statistical Classification of Diseases and Related Health Problems (ICD)
- ICD-9 (9th revision, published in 1977)
- ICD-9-CM (Clinical Modification, used in the US)
- ICD-10 (10th revision, in use by WHO since 1994)
- ICD-9 (9th revision, published in 1977)
- International Classification of Functioning, Disability and Health (ICF)
- International Classification of Health Interventions (ICHI) (previously known as International Classification of Procedures in Medicine)
Derived classifications are based on the WHO reference classifications (i.e. ICD and ICF). They include:
- International Classification of Diseases for Oncology, Third Edition (ICD-O-3)
- ICD-10 for Mental and Behavioural Disorders
- Application of the International Classification of Diseases to Dentistry and Stomatology, 3rd Edition (ICD-DA)
- Application of the International Classification of Diseases to Neurology (ICD-10-NA)
Related classifications in the WHO-FIC are those that partially refer to the reference classifications, e.g. only at specific levels. They include:
- International Classification of Primary Care (ICPC)
- International Classification of External Causes of Injury (ICECI)
- Anatomical Therapeutic Chemical Classification System with Defined Daily Doses (ATC/DDD)
- Technical aids for persons with disabilities: Classification and terminology (ISO9999)
- International Classification for Nursing Practice (ICNP)
Other medical classifications
The categories in a diagnosis classification classify [ and medical signs. In addition to the ICD and its national variants, they include:
- Diagnostic and Statistical Manual of Mental Disorders (DSM)
- International Classification of Headache Disorders 2nd Edition (ICHD-II)
- International Classification of Sleep Disorders (ICSD)
- Online Mendelian Inheritance in Man, database of genetic codes
- Read codes
- Systematized Nomenclature of Medicine - Clinical Terms (SNoMed-CT)
- Australian Classification of Health Interventions (ACHI)
- Canadian Classification of Health Interventions (CCI) 
- Chinese Classification of Heath Interventions (CCHI)
- Current Procedural Terminology (CPT)
- Health Care Procedure Coding System (HCPCS)
- ICD-10 Procedure Coding System (ICD-10-PCS)
- Office of Population, Censuses and Surveys Classification of Surgical Operations and Procedures (OPCS-4)
- Classification of Pharmaco-Therapeutic Referrals (CPR)
- Logical Observation Identifiers Names and Codes (LOINC), standard for identifying medical laboratory observations
- Medical Dictionary for Regulatory Activities (MedDRA)
- Medical Subject Headings (MeSH)
- Nursing Interventions Classification (NIC)
- Nursing Outcomes Classification (NOC)
- TIME-ITEM, ontology of topics in medical education
- TNM Classification of Malignant Tumors
- Unified Medical Language System (UMLS)
- Victoria Ambulatory Coding System (VACS) / Queensland Ambulatory Coding System (QACS), Australia
Library classification that have medical components
- Dewey Decimal System and Universal Decimal Classification (section 610–620)
- National Library of Medicine classification
ICD, SNOMED and Electronic Health Record (EHR)
What is SNOMED?
The Systematized Nomenclature of Medicine (SNOMED) is the most widely recognised nomenclature in healthcare. Its current version, SNOMED Clinical Terms (SNOMED CT), is intended to provide a set of concepts and relationships that offers a common reference point for comparison and aggregation of data about the health care process. SNOMED CT is often described as a reference terminology. SNOMED CT contains more than 311,000 active concepts with unique meanings and formal logic-based definitions organised into hierarchies. SNOMED CT can be used by anyone with an Affiliate License, 40 low income countries defined by the World Bank or qualifying research, humanitarian and charitable projects. SNOMED-CT is designed to be managed by computer, and it is a complex relationship concepts.
What is ICD?
The International Classification of Disease (ICD) is the most widely recognized medical classification maintained by the World Health Organization (WHO). Its primary purpose is to categorise diseases for morbidity and mortality reporting. The United States has used a clinical modification of ICD (ICD-9-CM) for the additional purposes of reimbursement. ICD-10 was endorsed by WHO in 1990, and WHO Member states began using the classification system in 1994 for both morbidity and mortality reporting. In the US, however, it has only been used for reporting mortality since 1999. Because of the US delay in adopting its version of ICD-10, it is currently unable to compare morbidity data with the rest of the world. ICD has a hierarchical structure, and coding in this context, is the term applied when representations are assigned to the words they represent. Coding diagnoses and procedures is the assignment of codes from a code set that follows the rules of the underlying classification or other coding guidelines.
SNOMED CT vs ICD
SNOMED CT and ICD are designed for different purposes and each should each be used for the purposes for which it was designed. As a core terminology for the EHR, SNOMED CT provides a common language that enables 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-9-CM, ICD-10-CM, and ICD-10-PCS 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, document whatever is needed for patient care and has better clinical coverage than ICD. ICD’s focus is statistical with less common diseases get lumped together in “catch-all” categories, which result in loss of information. SNOMED CT is used directly by healthcare providers during the process of care, whereas ICD is used by coding professionals after the episode of care. SNOMED CT had multiple hierarchy, whereas single hierarchy for ICD. SNOMED CT concepts are defined logically by their attributes, whereas only textual rules and definitions in ICD.
Data Mapping of SNOMED and ICD
SNOMED and ICD can be coordinated. The National Library of Medicine (NLM) maps ICD-9-CM, ICD-10-CM, ICD-10-PCS, and other classification systems to SNOMED.  Data Mapping is the process of identifying relationships between two distinct data models. The full value of the health information contained in an EHR system will only be realised if both systems involved in the map are up to date and accurately reflect the current practice of medicine.
Clinical Coding in Australia
Medical coding and classification systems are expected to become increasingly important in the health care sector. Together with and as an integrated part of the electronic health information systems, the coding and classification systems will be used to improve the quality and effectiveness of the medical services.
What is clinical coding?
Clinical coding is the translation of written, scanned and/or electronic clinical documentation about patient care into code format. For example, hypertension is represented by the code 'I10'; general anaethesia is represented by the code'92514-XX'.
A standardised classification system, The International Statistical Classification of Diseases and Related Health Problems, 10th Revision, Australian Modification (ICD-10-AM), is applied in all Australian acute health facilities. It is based on the World Health Organisation (WHO) ICD-10 system, updated with the Australian Classification of Health Interventions (ACHI), Australian Coding Standards (ACS). Clinical coding is a specialised skill requiring excellent knowledge of medical terminology and disease processes, attention to detail, and analytical skills.
What does clinical coder do?
A clinical coder is responsible for abstracting relevant information from the medical record and deciding which diagnoses and procedures meet criteria for coding as per Australian and State Coding Standards. The coder then assigns codes for these diagnoses and procedures based on ICD-10-AM conventions and standards. 
What is coded data used for?
The assigned codes and other patient data are processed by grouper software to determine a Diagnosis Related Group (DRG) for the episode of care, which is used for funding and reimbursement. This process allows hospital episodes to be grouped into meaningful categories, helping us to better match patient needs to health care resources. 
- Acronyms in healthcare
- Ambulatory Payment Classification, US billing system for outpatient services
- Biological database
- Classification of mental disorders
- Clinical coder
- German Institute for Medical Documentation and Information
- Health information management
- Health informatics
- Human resources for health information system
- List of international common standards
- Medical dictionary
- North American Nursing Diagnosis Association (professional organization)
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- Canadian Institute for Health Information. ICD-10-CA. Accessed 12 July 2011.
- New Zealand Health Information Service. ICD-10-AM. Accessed 12 July 2011.
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- Canadian Classification of Health Interventions. CCI.
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- WHO Family of International Classifications official site
- Medical terminologies at the National Library of Medicine