Development of the Logical Observation Identifier Names and Codes (LOINC) vocabulary

Abstract

The LOINC (Logical Observation Identifier Names and Codes) vocabulary is a set of more than 10,000 names and codes developed for use as observation identifiers in standardized messages exchanged between clinical computer systems. The goal of the study was to create universal names and codes for clinical observations that could be used by all clinical information systems. The LOINC names are structured to facilitate rapid matching, either automated or manual, between local vocabularies and the universal LOINC codes. If LOINC codes are used in clinical messages, each system participating in data exchange needs to match its local vocabulary to the standard vocabulary only once. This will reduce both the time and cost of implementing standardized interfaces. The history of the development of the LOINC vocabulary and the methodology used in its creation are described.

Figure 1

An HL7 unsolicited observation result (ORU) message. The message is composed of segments (MSH, EVN, PID, PV1, ORC, OBR, OBX), and each segment contains fields that are separated by a vertical bar (|). The MSH segment contains information that identifies the kind of message that follows. The PID and PV1 segments identify a patient and a particular visit to which the message pertains. The ORC and OBR segments provide information about who ordered an observation and the common context in which the observation was made. The repeating OBX segments represent individual observations (results or measurements). The structure of HL7 messages represents a simple semantic data model.

Figure 2

Use of a coded identifier (ID) field in the MSH segment of the HL7 message. Field 9 of the MSH segment is used to indicate the type of message that follows. Message Type is an ID field, meaning that its value must come from an HL7 table, in this case table 0076.

Figure 3

Use of a coded element (CE) field in an OBX segment of the HL7 message. Field 4 of the OBX segment contains the Observation Identifier (what was observed) and is of type CE (Coded Element). Each CE type field consists of six parts, but only three parts are shown in this example. The first part of a CE field is the code, the second part is the description (meaning or text) of the code, and the third part is the coding scheme from which the code was selected. This example shows a LOINC coding (LN) being used as the observation identifier.

Figure 4

Variable names and value names as used in a series of HL7 OBX segments. The kind of observation is indicated using a variable name. The value of the observation can be any of the allowed HL7 types, including NM (numeric) or CE (coded element). For numeric observations, the value of the observation is number, whereas for coded observations the actual value of the observation is indicated by a code. Possible value codes could come from other vocabularies, such as SNOMED, Read Codes, or the UMLS Metathesaurus. In this example, all codes are local, as indicated by the L following each code description.

Figure 5

Institution codes used as variable names in an HL7 message. As shown by the L following the description of the code, each site is using its own local coding scheme for variable names (observation identifiers). Each site has chosen a different code to represent a serum sodium concentration measurement. The purpose of creating LOINC codes is so that all sites can use universal codes for the names of observations.

Figure 6

Using other parts of an HL7 message to carry important information. Since order priority is sent as part of the OBR segment (in this case STAT), there is no need to make a name for “STAT Hematocrit” within the LOINC vocabulary.

Figure 7

Variable-style names versus value-style names in HL7 OBX segments. The “variable” representation treats the concept as a variable (or field) that has a binary value, e.g., HLA B27 Antigen = Present. This style is usually used when there is a panel of such variables, each of which will be tested for and reported as present or absent. In the case of “value” representation, the variable is a more open-ended one and the value of the field is the antigen that was found, e.g., Antigen Found = HLA B27. Value representation is often used when testing is being done to establish the HLA type of a person, and only the types of the antigens found will be reported. The use of the variable- and value-type styles of results reporting is common in blood bank testing and microbiology cultures as well as HLA typing.

Figure 8

Use of atomic (post-coordinated) codes versus a molecular code (pre-coordinated) in HL7 messages. Some systems send the location of a given measurement as a separate observation in the message, whereas other systems express this information as part of a single name.