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HL7 Reference

Free reference guide: HL7 Reference

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About HL7 Reference

This HL7 Reference is a searchable cheat sheet for healthcare interoperability standards. It covers HL7 v2 message structure including MSH (message header with field delimiters), PID (patient identification), ADT events (A01 admit through A40 merge), ORM/OBR/OBX (order and result segments), ORU result reporting, ACK acknowledgments, and MLLP transport protocol.

The reference also includes FHIR R4 resources (Patient, Observation, Condition, MedicationRequest, Bundle) with JSON examples, FHIR search parameters and modifiers, CapabilityStatement, SMART on FHIR OAuth2 authentication, and open-source FHIR server implementations (HAPI FHIR, Microsoft FHIR Server, Google Healthcare API).

Medical terminology code systems (LOINC for lab observations, SNOMED CT for clinical findings, RxNorm for drug terminology), document standards (CDA R2, C-CDA), and IHE integration profiles (PIX, PDQ, XDS) are included for healthcare IT developers, integration engineers, EHR vendors, and health informatics professionals.

Key Features

  • HL7 v2 message structure with pipe-delimited segments, field encoding characters, and CR separators
  • Complete MSH, PID, OBR, OBX segment field reference with positions and data types
  • ADT event types (A01-A40) for admit, discharge, transfer, register, update, and patient merge
  • FHIR R4 resource examples: Patient, Observation, Condition, MedicationRequest, and Bundle in JSON
  • FHIR search parameters, modifiers (:exact, :contains, :missing), chaining, and _include patterns
  • SMART on FHIR OAuth2 authentication flow with scopes and EHR/standalone launch sequences
  • Medical code systems: LOINC 6-axis structure, SNOMED CT hierarchy, RxNorm concept types
  • CDA R2 document architecture, IHE profiles (PIX, PDQ, XDS), and HL7 v2 vs FHIR comparison

Frequently Asked Questions

What is the HL7 v2 message structure?

HL7 v2 messages consist of segments separated by carriage returns (0x0D). Each segment contains fields delimited by pipe (|), with sub-delimiters for components (^), repetition (~), escape (\), and sub-components (&). The MSH segment defines these encoding characters and includes sending/receiving application, message type (e.g., ADT^A01), control ID, and version.

What FHIR resources are covered in this reference?

The reference covers core FHIR R4 resources: Patient (demographics, identifiers), Observation (lab results with LOINC codes, reference ranges), Condition (diagnoses with ICD-10 codes), MedicationRequest (prescriptions with RxNorm codes and dosage), Bundle (searchset, batch, transaction collections), and CapabilityStatement (server capabilities metadata).

How do ADT messages work in HL7 v2?

ADT (Admit/Discharge/Transfer) messages track patient movements. Key events: A01 (admit/inpatient), A02 (transfer), A03 (discharge), A04 (register/outpatient), A08 (update patient info), A28 (add new patient), A40 (merge patients). Each ADT message includes MSH, EVN (event), PID (patient), and PV1 (visit) segments.

How does FHIR search work?

FHIR search uses HTTP GET with query parameters: /Patient?name=Hong for basic search, modifiers like :exact and :contains for string matching, date comparisons with ge/le prefixes, _include for related resources, chaining (subject.name=Hong), and _count for pagination. The server CapabilityStatement at /metadata lists supported search parameters.

What is SMART on FHIR?

SMART on FHIR enables third-party app integration with EHR systems using OAuth2 authentication. The flow includes app registration, authorization code request with scopes (patient/Patient.read, openid, fhirUser), access token acquisition, and FHIR API calls. It supports both EHR launch (initiated from within the EHR) and standalone launch (client-initiated).

What medical code systems does this reference cover?

LOINC (Logical Observation Identifiers) uses a 6-axis system for lab codes (e.g., WBC = 6690-2). SNOMED CT provides hierarchical clinical terminology for diagnoses, procedures, and body structures. RxNorm standardizes drug names with concept types: IN (ingredient), BN (brand), SCD (semantic clinical drug), and SBD (semantic branded drug).

What is the difference between HL7 v2 and FHIR?

HL7 v2 (since 1987) uses pipe-delimited text over TCP/MLLP, is message-based with event triggers, and remains the most widely deployed standard. FHIR (since 2014) uses JSON/XML over RESTful HTTP, is resource-based with CRUD operations, and is more mobile/web-friendly with a lower learning curve. v2 dominates existing systems while FHIR is growing in new integrations.

Is this HL7 reference free to use?

Yes, this HL7 cheat sheet is completely free with no account required. All v2 segment references, FHIR resource examples, code system guides, and integration profiles are searchable and filterable by category in your browser. It is part of liminfo.com's collection of free healthcare IT reference tools.