# Health Level 7

Health Level Seven, abbreviated to HL7, is a range of global standards for the transfer of clinical and administrative health data between applications with the aim to improve patient outcomes and health system performance. The HL7 standards focus on the application layer, which is "layer 7" in the Open Systems Interconnection model. The standards are produced by Health Level Seven International, an international standards organization, and are adopted by other standards issuing bodies such as American National Standards Institute and International Organization for Standardization. There are a range of primary standards that are commonly used across the industry, as well as secondary standards which are less frequently adopted.

## Purpose

Health organizations typically have many different computer systems used to process different patient administration or clinical tasks, such as billing, medication management, patient tracking, and documentation. All of these systems should communicate, or "interface", with each other when they receive new information or when they wish to retrieve information. HL7 International specifies a number of flexible standards, guidelines, and methodologies by which these healthcare systems can communicate with each other. The standards allow for easier 'interoperability' of healthcare data as it is shared and processed uniformly and consistently by the different systems. This allows clinical and non-clinical data to be shared more easily, theoretically improving patient care and health system performance.

## Primary standards

HL7 International considers the following standards to be its primary standards – those standards that are most commonly used and implemented:
* Version 2.x Messaging Standard – an interoperability specification for health and medical transactions
* Version 3 Messaging Standard – an interoperability specification for health and medical transactions
* Clinical Document Architecture (CDA) – an exchange model for clinical documents, based on HL7 Version 3
* Continuity of Care Document (CCD) – a US specification for the exchange of medical summaries, based on CDA.
* Structured Product Labeling (SPL) – the published information that accompanies a medicine, based on HL7 Version 3
* Clinical Context Object Workgroup (CCOW) – an interoperability specification for the visual integration of user applications
Other HL7 standards/methodologies include:
* Fast Healthcare Interoperability Resources (FHIR) – a standard for the exchange of resources
* Arden Syntax – a grammar for representing medical conditions and recommendations as a Medical Logic Module (MLM)
* Claims Attachments – a Standard Healthcare Attachment to augment another healthcare transaction
* Functional Specification of Electronic Health Record (EHR) and Personal Health Record (PHR) systems – a standardized description of health and medical functions sought for or available in such software applications
* GELLO – a standard expression language used for clinical decision support

### HL7 Version 2

The HL7 version 2 standard (also known as Pipehat) has the aim to support hospital workflows. It was originally created in 1989.
HL7 version 2 defines a series of electronic messages to support administrative, logistical, financial as well as clinical processes. Since 1987 the standard has been updated regularly, resulting in more than ten iterations. The v2.x standards are backward compatible, meaning a message based on version 2.3 will be understood by an application that supports version 2.6.
HL7 v2.x messages use a non-XML encoding syntax based on segments (lines) and one-character delimiters. Segments have composites (fields) separated by the composite delimiter. A composite can have sub-composites (components) separated by the sub-composite delimiter, and sub-composites can have sub-sub-composites (subcomponents) separated by the sub-sub-composite delimiter. The default delimiters are carriage return for the segment separator, vertical bar or pipe (|) for the field separator, caret (^) for the component separator, ampersand (&) for the subcomponent separator, and number sign (#) for the default truncation separator. The tilde (~) is the default repetition separator. Each segment starts with a 3-character string that identifies the segment type. Each segment of the message contains one specific category of information. Every message has MSH as its first segment, which includes a field that identifies the message type. The message type determines the expected segment types in the message. The segment types used in a particular message type are specified by the segment grammar notation used in the HL7 standards.
The following is an example of an admission message. MSH is the header segment, PID the Patient Identity, PV1 is the Patient Visit information, etc. The 5th field in the PID segment is the patient's name, in the order, family name, given name, second name (or their initials), suffix, etc. Depending on the HL7 V2.x standard version, more fields are available in the segment for additional patient information.
HL7 v2.x has allowed for the interoperability between the plethora of digital health systems, from Patient Administration Systems, to Electronic Health Records, and specialised Laboratory and Radiology Information Systems. Currently, the HL7 v2.x messaging standard is supported by every major health informatics vendor in the United States.

### HL7 Version 3

The HL7 version 3 standard has the aim to support all healthcare workflows. Development of version 3 started around 1995, resulting in an initial standard publication in 2005. The v3 standard, as opposed to version 2, is based on a formal methodology (the HDF) and object-oriented principles.
RIM - ISO/HL7 21731
The Reference Information Model (RIM) is the cornerstone of the HL7 Version 3 development process and an essential part of the HL7 V3 development methodology. RIM expresses the data content needed in a specific clinical or administrative context and provides an explicit representation of the semantic and lexical connections that exist between the information carried in the fields of HL7 messages.
HL7 Development Framework - ISO/HL7 27931
The HL7 Version 3 Development Framework (HDF) is a continuously evolving process that seeks to develop specifications that facilitate interoperability between healthcare systems. The HL7 RIM, vocabulary specifications, and model-driven process of analysis and design combine to make HL7 Version 3 one methodology for the development of consensus-based standards for healthcare information systems interoperability. The HDF is the most current edition of the HL7 V3 development methodology.
The HDF not only documents messaging, but also the processes, tools, actors, rules, and artifacts relevant to the development of all HL7 standard specifications. Eventually, the HDF will encompass all of the HL7 standard specifications, including any new standards resulting from the analysis of electronic health record architectures and requirements.
HL7 specifications draw upon codes and vocabularies from a variety of sources. The V3 vocabulary work ensures that the systems implementing HL7 specifications have an unambiguous understanding of the code sources and code value domains they are using.
V3 Messaging
The HL7 version 3 messaging standard defines a series of Secure Text messages (called interactions) to support all healthcare workflows.
HL7 v3 messages are based on an XML encoding syntax, as shown in this example:$^{:2.2.1}$

### Clinical Document Architecture

The HL7 Clinical Document Architecture (CDA) is an XML-based markup standard intended to specify the encoding, structure and semantics of clinical documents for exchange. The standard was jointly published with ISO as ISO/HL7 27932.

### Continuity of Care Document

The Continuity of Care Document framework is a US-specific standard for the exchange of medical summaries, based on the Clinical Document Architecture standard.

### Structured Product Labeling

Structured Product Labeling describes the published information that accompanies a medicine, based on HL7 Version 3.

### Clinical Context Object Workgroup

CCOW, or "Clinical Context Object Workgroup," is a standard protocol designed to enable disparate applications to share user context and patient context in real-time, and at the user-interface level. CCOW implementations typically require a CCOW vault system to manage user security between applications.

## Other standards and methods
### Fast Healthcare Interoperability Resources (FHIR)

Fast Healthcare Interoperability Resources is a modern interoperability specification from HL7 International designed to be easier to implement, more open, and more extensible than HL7 versions 2.x or 3.x. It leverages a modern web-based suite of API technology, including a HTTP-based RESTful protocol, HTML and Cascading Style Sheets for user interface integration, a choice of JSON or XML for data representation, OAuth for authorization and Atom for query results. The main purpose of the FHIR standard is to ensure interoperability between different computer systems. It defines the data format and protocol for exchanging medical information, regardless of how it is stored in these systems.

### Services Aware Interoperability Framework

The HL7 Services-Aware Enterprise Architecture Framework (SAIF) provides consistency between all HL7 artifacts, and enables a standardized approach to Enterprise Architecture (EA) development and implementation, and a way to measure the consistency.
SAIF is a way of thinking about producing specifications that explicitly describe the governance, conformance, compliance, and behavioral semantics that are needed to achieve computable semantic working interoperability. The intended information transmission technology might use a messaging, document exchange, or service approach.
SAIF is the framework that is required to rationalize interoperability of other standards. SAIF is an architecture for achieving interoperability, but it is not a whole-solution design for enterprise architecture management.

### Arden syntax

The Arden syntax is a language for encoding medical knowledge. HL7 International adopted and oversees the standard beginning with Arden syntax 2.0. These Medical Logic Modules (MLMs) are used in the clinical setting as they can contain sufficient knowledge to make single medical decisions. They can produce alerts, diagnoses, and interpretations along with quality assurance function and administrative support. An MLM must run on a computer that meets the minimum system requirements and has the correct program installed. Then, the MLM can give advice for when and where it is needed.

### Clinical Quality Language

Clinical Quality Language (CQL) is a ANSI certified clinically focused high-level expression language standard curated by Health Level 7. It is designated for clinical knowledge sharing in the domains of electronic clinical quality measurement and clinical decision support.
Clinical quality language is being used for a variety of clinical applications including WHO SMART guidelines where it is used for encoding decision logic and performance indicators. The Centers for Medicare & Medicaid Services adopted CQL for clinical quality measure specifications since 2019.
CQL allows modular and flexible expression of logic and is both human-readable and machine processable.
An implementation of CQL was open sourced and published by the National Committee for Quality Assurance in 2023 with the aim of encouraging adoption of the language.

### MLLP

A large portion of HL7 messaging is transported by Minimal Lower Layer Protocol (MLLP), also known as Lower Layer Protocol (LLP) or Minimum Layer Protocol (MLP). For transmitting via TCP/IP, header and trailer characters are added to the message to identify the beginning and ending of the message because TCP/IP is a continuous stream of bytes. Hybrid Lower Layer Protocol (HLLP) is a variation of MLLP that includes a checksum to help verify message integrity. Amongst other software vendors, MLLP is supported by Microsoft, Oracle, Cleo.
MLLP contains no inherent security or encryption but relies on lower layer protocols such as Transport Layer Security (TLS) or IPsec for safeguarding Protected health information outside of a secure network.

### Functional EHR and PHR specifications

Functional specifications for an electronic health record.

## Message details
### The OBR segment

An OBR Segment carries information about an exam, diagnostic study/observation. It is a required segment in an ORM (order message) or an ORU (Observation Result) message.