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Unified Medical Language System® (UMLS®)

RxNorm Project History

In late 2001, the NLM began an experiment in modeling clinical drugs in the Unified Medical Language System. There were several motives in doing so: there was a suspicion that in the Metathesaurus there was considerable missed synonymy in naming of clinical drugs; while avoiding missed synonymy has frequently been problematic, the traditional methodologies of recognizing missed synonymy in the UMLS did not seem to be effective; there was hope that developing a new method might lead to improved interoperability of drug terminology; the area of clinical drugs was seen as important in the growing issues of patient safety; and there was a growing consensus in the HL7 vocabulary technical committee of what a model for clinical drugs should be. The HL7 model was based on what a clinician might order, and what type of order might be sent to the pharmacy. The dose form would be the form in which a drug was administered to a patient, as opposed to the form in which the manufacturer had supplied it. It was clearly distinct from the choices the pharmacy might make in fulfilling that order. This project has become known as the RxNorm project.

The RxNorm project approached clinical drug representation in a series of steps. The initial effort was a to define a Semantic Normal Form (SNF) to represent clinical drugs. SNFs for clinical drugs are canonical representations, as defined by their active ingredients, strengths, and orderable dose forms. SNFs make explicit and/or normalize every active ingredient, strength, unit of measurement, and dosage form for a given clinical drug preparation. Employing both relationships between concepts and attribute-value pairs, the data represent the semantics of a clinical drug concept. SNFs for clinical drugs use standardized (generic) ingredient names, units, and dose forms, and a set of rules for expressing strength in a set of standard units.

There are two SNFs created as UMLS concepts for every clinical drug. The SNF Drug Component (SCDC) has form:

CUI|ShortName|ActiveIngredient|PreciseIngredient|Basis|Strength|Units|Notes

For example, the following are SCDCs:

C0111111|APAP|Acetaminophen|Acetaminophen|B|325|MG|Component example#1
C0123456|Codeine|Codeine Phosphate|Codeine|P|30|MG|Component example#2

The SNF Clinical Formulation (SCD) has form:

CUI|MetaID|ShortName|Component1/Component2/...|OrderableDoseForm|Notes

For example, the two SCDCs above can be combined to form the following:

C0654321|ACETAMINOPHEN 325 MG / CODEINE 30 MG Oral Capsule|C0111111/C0123456|Oral Tablet|CF example

The SAB for RxNorm is RXNORM, and the term types are SCDC and SCD, respectively.