When a concept is added to the Metathesaurus it receives a unique identifier and is placed in the Metathesaurus structure. This structure has four levels of specification:

Concept Unique Identifiers (CUI)
A concept is a meaning. A meaning can have many different names. A key goal of Metathesaurus construction is to understand the intended meaning of each name in each source vocabulary and to link all the names from all of the source vocabularies that mean the same thing (the synonyms). CUI contain the letter C followed by seven numbers. In the example on the right the CUI is C0018681.

Lexical (term) Unique Identifiers (LUI)
LUI link strings that are lexical variants. Lexical variants are detected using the Lexical Variant Generator (LVG) program, one of the UMLS lexical tools. LUI contain the letter L followed by seven numbers. In the example on the right there are three lexical variants, each given a seperate LUI.

String Unique Identifiers (SUI)
Each unique concept name or string in each language in the Metathesaurus has a unique and permanent string identifier (SUI). Any variation in character set, upper-lower case, or punctuation difference is a separate string, with a separate SUI. SUI contain the letter S followed by seven numbers. In the example on the right there are four strings with four different SUI.

Atom Unique Identifiers (AUI)
The basic building blocks or "atoms" from which the Metathesaurus is constructed are the concept names or strings from each of the source vocabularies. Every occurrence of a string in each source vocabulary is assigned a unique atom identifier (AUI). If exactly the same string appears multiple times in the same vocabulary, for example as an alternate name for different concepts, a unique AUI is assigned for each occurrence. AUI contain the letter A followed by seven numbers. In the example on the right there are five strings from five sources with five different AUI. The abbreviation for the source that contributed each string is noted in parentheses after the string.