The structure of MedDRA is very logical. There are five levels to the MedDRA hierarchy, arranged from very specific to very general. At the most specific level, called “Lowest Level Terms” (LLTs), there are more than 70,000 terms which parallel how information is communicated. These LLTs reflect how an observation might be reported in practice. This level directly supports assigning MedDRA terms within a user database.
Each member of the next level, “Preferred Terms” (PTs), is a distinct descriptor (single medical concept) for a symptom, sign, disease diagnosis, therapeutic indication, investigation, surgical or medical procedure, and medical social or family history characteristic. Each LLT is linked to only one PT. Each PT has at least one LLT (itself) as well as synonyms and lexical variants (e.g., abbreviations, different word order).
Related PTs are grouped together into “High Level Terms” (HLTs) based upon anatomy, pathology, physiology, etiology or function. HLTs, related to each other by anatomy, pathology, physiology, etiology or function, are in turn linked to “High Level Group Terms” (HLGTs).
Finally, HLGTs are grouped into “System Organ Classes” (SOCs) which are groupings by etiology (e.g. Infections and infestations), manifestation site (e.g. Gastrointestinal disorders) or purpose (e.g. Surgical and medical procedures). In addition, there is a SOC to contain issues pertaining to products and one to contain social circumstances.