Hierarchy of MedDRA Terms: From LLT to SOC

Published on 21/12/2025

Understanding the Hierarchy of MedDRA Terms from LLT to SOC

Table of Contents

Introduction to the MedDRA Hierarchical System

The Medical Dictionary for Regulatory Activities (MedDRA) is a standardized terminology used globally to code adverse events in clinical trials and post-marketing surveillance. One of its most powerful features is its hierarchical structure, which allows for flexible analysis of safety data. The hierarchy ranges from very specific patient-reported terms (Lowest Level Terms, LLTs) to broad categories used in regulatory summaries (System Organ Class, SOC).

This structure ensures that data can be reviewed at varying levels of detail. For example, a regulatory reviewer may focus on SOC-level summaries such as “Nervous system disorders,” while a safety physician may need PT-level data on “Convulsions” to understand case-level safety implications. Without this hierarchy, harmonized signal detection and cross-study comparisons would be impossible.

MedDRA’s hierarchy is applied across regulatory submissions, including FDA IND safety reports, EMA EudraVigilance submissions, and DSURs. Its standardized nature ensures global consistency in how adverse events are categorized and reported.

The Five Levels of MedDRA Terms

MedDRA’s structure is comprised of five interconnected levels. Each level rolls up into a broader category, enabling flexibility in data analysis.

Lowest Level Term

(LLT): The most specific level, often the exact wording used by investigators. Example: “Heart attack.”

Preferred Term (PT): A distinct descriptor representing a single medical concept. Example: “Myocardial infarction.”

High Level Term (HLT): Groups of related PTs that share a common medical concept. Example: “Ischaemic coronary artery disorders.”

High Level Group Term (HLGT): Broader groups of related HLTs. Example: “Coronary artery disorders.”

System Organ Class (SOC): The highest level, grouping terms by organ system or etiology. Example: “Cardiac disorders.”

This structured roll-up allows sponsors and regulators to analyze adverse events at different levels of granularity. Investigators may use patient language (“chest pain”), but regulators require consistency across submissions, which MedDRA provides by coding these terms into PTs and SOCs.

Example of Hierarchical Mapping

Below is an illustrative example showing how terms map through the MedDRA hierarchy:

Investigator Term	LLT	PT	HLT	HLGT	SOC
Heart attack	Heart attack	Myocardial infarction	Ischaemic coronary artery disorders	Coronary artery disorders	Cardiac disorders
Seizures	Seizure	Convulsion	Seizures (incl. subtypes)	Central nervous system disorders NEC	Nervous system disorders
Low WBC	Leukopenia	Neutropenia	White blood cell disorders	Haematopoietic disorders	Blood and lymphatic system disorders

This example demonstrates how LLTs used by investigators are standardized into PTs and then progressively grouped into SOCs for high-level regulatory analysis.

Application of the Hierarchy in Clinical Trials

The MedDRA hierarchy is critical for both operational and regulatory purposes:

Case-level analysis: Coders assign PTs to investigator terms for precise classification.
Aggregate analysis: Regulators and safety teams review SOC-level data to detect system-wide toxicity patterns.
Signal detection: Aggregated PTs under HLTs and HLGTs allow statistical analysis for safety signal identification.
Regulatory submissions: IND safety reports, DSURs, and PSURs require tabulations at SOC and PT levels.

For example, in oncology trials, individual reports of “low platelets” coded as “Thrombocytopenia” (PT) roll up to the SOC “Blood and lymphatic system disorders,” enabling regulators to quickly assess hematologic toxicity across trials.

Challenges in Using the MedDRA Hierarchy

Despite its strengths, applying MedDRA’s hierarchy presents challenges:

Ambiguity at LLT level: Investigator terms like “felt unwell” require careful coding to avoid misclassification.
Overlapping SOCs: Certain PTs may link to multiple SOCs (primary and secondary linkages), complicating analysis.
Version updates: Twice-yearly MedDRA updates may alter mappings, requiring database reconciliation.
Consistency issues: Different coders may choose different LLTs for similar investigator terms.

These issues highlight the need for SOPs, coding conventions, and periodic coding reviews. Training coders on version updates and providing case-based examples reduces inconsistency and ensures compliance.

Regulatory Expectations and Audit Readiness

Regulatory agencies such as the FDA, EMA, and CDSCO expect sponsors to demonstrate traceability in their coding practices. During inspections, common findings include:

Incorrect LLT or PT assignment for investigator terms.
Failure to reconcile coding after MedDRA version updates.
Inconsistent mapping of similar events across different studies.
Lack of coding conventions documentation.

To avoid findings, sponsors must maintain coding guidelines, training logs, and reconciliation records. Regular audits and mock inspections can strengthen inspection readiness. Reference resources such as the EU Clinical Trials Register demonstrate regulatory emphasis on standardized coding practices.

Key Takeaways

The MedDRA hierarchy provides a structured approach to adverse event coding, ensuring accuracy, consistency, and global harmonization. Clinical teams must:

Understand the five levels of MedDRA: LLT → PT → HLT → HLGT → SOC.
Apply coding conventions consistently to avoid misclassification.
Maintain updated databases aligned with new MedDRA versions.
Document coding decisions for inspection readiness.
Train coders and CRAs to navigate the hierarchy effectively.

By mastering the MedDRA hierarchy, sponsors and CROs can ensure high-quality safety data, meet regulatory expectations, and protect patients through accurate pharmacovigilance practices.