===== UCUM =====
The Unified Code for Units of Measure ([[http://unitsofmeasure.org/ucum.html|UCUM]]) is a code system intended to include all units of measures being contemporarily used in international science, engineering, and business. It provides the Standard Concepts for all units in the OMOP Standardized Vocabulary. 

Note that UCUM is not a fixed set of Concepts with a concept_code, like the majority of other vocabularies. Instead, it is a system of syntax and grammar for creating any possible valid unit. We incorporated a set of units that are commonly used in medical records and drug strength descriptions.

The basic rules are described here:

  * All units are derived from unit atoms, which are either proper units (e.g. centimeter, inch) or arbitrary units, which depend on the method of measurement (e.g. international units).
  * Square brackets ("[" and "]") depict unit atoms (e.g. [U], [drop], [yd_us] - yard US, [tb'U] - tuberculin unit), or annotations to units that make them new unit atoms (e.g. mm[Hg])
  * Curly braces ("{" and "}") contain annotations that are traditionally used in clinical labs (e.g. mg{creat} - mg creatinine, {RBC}/uL - red blood cells per microliter). Any annotation in curly braces can be omitted without changing the meaning of the unit.

==== Transformation =====
Since there is no fixed set of UCUM-based units, there is no transformation. All UCUM Concepts are manually curated, and additions and changes can be made on request.

==== Concept Names ====
Concept Names are the full non-abbreviated names of the unit.

==== Concept Codes ====
Concept Codes are not codes in the traditional sense of the OMOP Standardized Vocabulary, identifying a distinct semantic meaning within the source vocabulary. Instead, the Concept Codes contain the unit in UCUM notation.

==== Standard Concepts ====
All UCUM units are Standard Concepts.

==== Concept Classes ====
There are two types of Concept Classes in UCUM: Units and Canonical Units. Canonical Units are those that are derived from SI proper units. 

==== Domains ====
All UCUM Concepts are of the "Unit" domain.

==== Concept Relationships ====
There are only mapping relationships from equivalent units in other vocabularies. However, these mappings are not yet comprehensive.

==== Hierarchy ====
There is no hierarchical system in UCUM units.