HL7 v2

HL7 v2 is the messaging standard that has quietly run hospital data exchange for decades. Created by Health Level Seven International, its pipe-and-hat delimited messages — triggered by real-world events like an admission, a lab result, or an order — move patient and administrative data between the disparate systems inside a hospital, and it remains one of the most widely deployed data standards in all of healthcare.

• Event-driven messages - Trigger events (ADT admissions, ORU results, ORM orders) generate structured messages between systems.
• Pipe-and-hat encoding - A compact delimited format using | and ^ separators that predates modern JSON and XML tooling.
• Segments and fields - Each message is built from ordered segments (MSH, PID, OBX) carrying strictly positioned fields.
• Ubiquitous but idiosyncratic - Nearly universal in hospital information systems, yet notoriously variable from one implementation to the next.

In practice, HL7 v2 is the standard most healthcare integration work still has to speak, even as newer web-friendly approaches gain ground — a huge share of live clinical traffic is v2 messages flowing over MLLP, not REST calls. When I look at healthcare APIs, HL7 v2 is usually the legacy substrate that a modern FHIR or REST facade is wrapping, which is why understanding it matters for anyone building Health Standards integrations or trying to make older clinical systems safely reachable by an API or an agent.