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Health Level 7 (HL7)

Health Layer 7 (HL7) is a clinical interface standard for the exchange of information between medical applications. HL7 is also the name of the standard organisation that developed the published the protocols. The organisation was formed in March 1987 in the United States as a committee of healthcare providers, software vendors and consultants to develop standards of electronic interchange of financial and administrative information between different clinical and supporting systems. It was hoped that this would at least greatly reduce the need for custom interface programming and maintenance.

The ‘7’ refers to the seventh (application) layer of the International Standards Organisation’s (ISO) Open Source Interconnection (OSI) Reference Model for networks. The OSI model is used for moving data from one software application to another located on different computer. The model consists of seven layers, from the highest to the lowest: physical, link, network, transport, session, presentation and application. The lowest two (layer and link) are handled by hardware, while the rest are handled by software.


It is application layer, along with its protocols for file transfer, that is forms the focus of HL7. The issues that are addressed by HL7 include the data to be exchanged, the time it takes for the exchange to occur and the application specific error communication the applications, and this is done through its framework. HL7 converts data into messages, the format of which is defined by a set of HL7’s encoding rules.

The message usually consists of fields, which represent data such as patient’s name, address or age, which are then grouped into segments that could represent groupings such as patient identification, allergy or next of kin.

The messages are then sent whenever a trigger event occurs or are bunched up and sent once. Trigger events could include patient admission or discharge or just a general query about a patient.

When the message is sent out, the receiving system validates the message, and then returns a message to the sending system indicating whether the message was successfully received, that it contained all the required fields or that it could be interpreted by the receiving system.

The HL7 has not always been as it is now. The version 1.0 draft was first released in October 1987 and was followed by a series of version 2s until 2001 when 2.4 was accepted as an ANSI standard. However since 1996, HL7 has been working on a version 3.0. The latest version would address areas such as the lack of support for new technologies such as Object-oriented technologies, XML (eXtensible Markup Language) and Web technologies.

Version 3.0 would encode messages using XML and would also address security issues not addressed by previous versions, bringing in line with the directives spelt out in the Health Information Portability and Accountability Act (HIPAA).

The use of HL7 as a standard in clinical software development is growing worldwide as many countries have now established affiliates of the organisation (United Kingdom, Australia, Finland, Japan, Germany, the Netherlands, New Zealand, South Africa and India). In the United Kingdom, the National Health Services has adapted HL7 as the standard to transmit electronic medical records in the GP2GP initiative (The transfer of patient records between general practices).



International Medical Informatics Association
American Medical Informatics Association
UK Health Informatics Society
Healthcare Information and Management Systems Society

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Last Updated: 10 August 2006.

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