Standardisation and classification of medical language data are dependent on the use of controlled medical vocabularies. In the pharmacovigilance setting, adverse drug reactions (ADRs) are coded using terminological systems such as the WHO Adverse Reaction Terminology (WHO-ART), Coding Symbols for a Thesaurus of Adverse Reaction Terms (COSTART) and Hoechst Adverse Reactions Terminology System (HARTS). The Medical Dictionary for Drug Regulatory Affairs (MEDDRA), derived from the Adverse Drug Reactions Online Information Tracking (ADROIT) terminology, was accepted as the basis for development of a new terminology at the fourth International Conference on Harmonisation for the Registration of Pharmaceuticals for Human Use (ICH 4) in July 1997. MEDDRA was extensively modified to become MedDRA, the Medical Dictionary for Regulatory Activities project. MedDRA is now maintained, developed and distributed by the Maintenance and Support Services Organization (MSSO).
The purpose of MedDRA is to describe the steps of drug development and regulatory issues about human exposure. It includes terms for the description of ADRs, indications, signs and symptoms, family history, investigations and surgical procedures. It is used in the US FDA Adverse Event Reporting System (AERS) and Vaccine Adverse Event Reporting System (VAERS) databases, the European Medicines Agency (EMEA) Eudrawatch system and the Japanese prescription-event monitoring system. Since January 2003, all electronic exchange of case reports of ADRs in the EU, Iceland, Liechtenstein and Norway should be coded with MedDRA.
MedDRA terms are organised in five levels: system organ class (SOC), high level group term (HLGT), high level term (HLT), preferred term (PT) and low level term (LLT). The preferred level of analysis is the PT. PTs are classified according to SOCs, which are clinical conditions (medical procedures or patient history) relevant for the pre- and post-marketing of drugs, and SOCs used to describe diseases or findings. PTs are grouped in HLTs and then in HLGTs in the same SOC.
Each PT is intended to represent a unique medical concept: it is the term preferred for use in the regulatory environment and is formatted according to MedDRA conventions. Eponymous terms may be used if recognized internationally. A PT may describe a single syndrome, even though a syndrome represents a collection of signs and symptoms. Each PT is duplicated as an LLT and may have subordinate to it one or more other LLTs that are synonyms, lexical variants or alternative spellings of the PT. In addition, some LLTs describe conditions that are more precise or specific than the PT to which they are linked; whilst not synonymous, they are considered not to warrant PT status from a pharmacovigilance perspective. Each PT is represented only once under a particular SOC, to which it is connected vertically via a single High Level Term (HLT), which in turn is fixed in location and represented only once in that SOC under one High Level Group Term (HLGT). It is a rule that there is only one route from LLT to PT to HLT to HLGT within the SOC.
