HL7

Health Level Seven International standards for healthcare information exchange and interoperability

Overview

HL7 (Health Level Seven) is a set of international standards for the exchange, integration, sharing, and retrieval of electronic health information. Developed by Health Level Seven International, a not-for-profit, ANSI-accredited standards developing organization, HL7 standards are used by healthcare organizations worldwide to enable interoperability between clinical and administrative systems.

The "Level Seven" in the name refers to the seventh layer (Application Layer) of the OSI communication model, indicating that HL7 focuses on the application-level exchange of clinical and administrative data. HL7 standards address the interfaces between systems that send or receive patient admissions, discharge, and transfer (ADT) data, queries, orders, results, clinical observations, billing records, and other healthcare information.

HL7 encompasses several distinct standards, the most important being HL7 Version 2 (v2), HL7 Version 3 (v3), Clinical Document Architecture (CDA), and Fast Healthcare Interoperability Resources (FHIR). Each represents a different generation and approach to healthcare data interchange.

History

HL7 International was founded in 1987 by a group of healthcare IT professionals who recognized the need for standardized communication between hospital information systems. The first version of the HL7 messaging standard (v1) was developed at a meeting held at the University of Virginia. Version 2.0 was published in 1988, and it quickly became the foundation for healthcare data exchange in the United States and beyond.

HL7 v2 evolved through numerous releases (2.1, 2.2, 2.3, 2.3.1, 2.4, 2.5, 2.5.1, 2.6, 2.7, 2.8, 2.9), each adding new message types and segments while maintaining backward compatibility. In the early 2000s, HL7 v3 was developed as a complete redesign based on a formal Reference Information Model (RIM), but its complexity limited adoption. The Clinical Document Architecture (CDA), derived from v3, achieved broader success for clinical document exchange.

In 2011, HL7 began developing FHIR (Fast Healthcare Interoperability Resources), which combines the best features of v2, v3, and CDA with modern web standards (REST, JSON, OAuth). FHIR has rapidly become the most significant healthcare interoperability standard, with the U.S. 21st Century Cures Act mandating FHIR-based APIs for patient data access.

Structure and Format

HL7 Version 2 (v2)

HL7 v2 uses a pipe-delimited text format. Messages are composed of segments, each on a new line, with fields separated by the pipe character (|). The first segment is always MSH (Message Header), which identifies the message type, sending/receiving applications, and encoding characters. 

MSH|^~\&|SENDING_APP|SENDING_FAC|RECEIVING_APP|RECEIVING_FAC|20230615120000||ADT^A01|MSG00001|P|2.5
EVN|A01|20230615120000
PID|1||12345678^^^HOSP^MR||DOE^JOHN^A||19800115|M|||123 MAIN ST^^ANYTOWN^CA^90210
PV1|1|I|4N^401^1^^^N||||1234567890^SMITH^ROBERT^J^^DR|||MED||||ADM|||1234567890^SMITH^ROBERT^J^^DR|IN||||||||||||||||||230615
NK1|1|DOE^JANE|SPO|456 MAIN ST^^ANYTOWN^CA^90210|5551234567

HL7 FHIR

FHIR uses RESTful APIs with resources represented in JSON or XML. Resources are modular, well-defined objects like Patient, Observation, Medication, and Encounter that can be read, created, updated, and searched via standard HTTP methods. 

{
  "resourceType": "Patient",
  "id": "12345",
  "name": [{
    "family": "Doe",
    "given": ["John", "A"]
  }],
  "gender": "male",
  "birthDate": "1980-01-15",
  "address": [{
    "line": ["123 Main St"],
    "city": "Anytown",
    "state": "CA",
    "postalCode": "90210"
  }]
}

Key Message Types and Resources

HL7 v2 Message Types

  • ADT - Admission, Discharge, Transfer (patient movement tracking)
  • ORM / OML - Order Message (lab tests, procedures, medications)
  • ORU - Observation Result (lab results, diagnostic reports)
  • SIU - Scheduling Information (appointment management)
  • DFT - Detailed Financial Transaction (charge posting)
  • MDM - Medical Document Management (clinical documents)
  • RDE - Pharmacy Dispense/Encode (medication dispensing)
  • BAR - Billing Account Record

FHIR Resources

  • Patient - Demographics and administrative information
  • Observation - Measurements and findings (vitals, lab results)
  • Condition - Diagnoses and problems
  • MedicationRequest - Medication prescriptions and orders
  • Encounter - Healthcare visit or interaction
  • DiagnosticReport - Diagnostic study results
  • AllergyIntolerance - Allergy and intolerance records
  • Immunization - Vaccination records

Use Cases

  • Hospital information systems: Patient registration, admissions, discharges, and transfers between HIS, EHR, and departmental systems
  • Laboratory information: Order entry for lab tests and return of results from laboratory analyzers and LIS to clinical systems
  • Pharmacy: Medication ordering, dispensing, and administration recording
  • Radiology: Imaging orders and diagnostic reports (often integrated with DICOM for images)
  • Patient portals: FHIR APIs enable patient-facing applications to access their health records
  • Health information exchanges: Regional and national HIEs use HL7 standards to share patient data across organizations
  • Public health reporting: Syndromic surveillance, immunization registries, and reportable condition notifications

Advantages

  • Healthcare-specific: Purpose-built for clinical and administrative healthcare workflows
  • Universal adoption: HL7 v2 is implemented in virtually every hospital and healthcare IT system worldwide
  • FHIR modernization: FHIR brings HL7 into the modern web era with REST APIs, JSON, and mobile-friendly design
  • Regulatory support: Government mandates (HIPAA, 21st Century Cures Act, EU regulations) drive adoption
  • Active community: Large international community of implementers, vendors, and standards developers

Related Standards

In the healthcare EDI landscape, HL7 focuses on clinical data exchange while ANSI X12 handles administrative and financial transactions (claims, eligibility, payments). The two standards are complementary: a hospital might use HL7 internally for clinical workflows and X12 for billing and insurance communication. For modern implementations, FHIR is increasingly used alongside or as a replacement for both HL7 v2 and X12 in certain scenarios. XML-based standards like CDA are part of the HL7 family and are used for clinical document exchange such as discharge summaries and continuity of care documents.