Intro to HL7 messages

Jeff Simons Decena
3 min readAug 5, 2022

Many developers new to health informatics, just like me a few years back, have so many questions about what is HL7 and HL7 messages. When someone looks at the message, it’s just like any pipe delimited string. But what is exactly is an HL7 message?

HL7 in long form is read as Health Level 7.

It is created by Health Level Seven International (HL7) which is a non-profit ANSI-accredited standards development organization dedicated to providing standards and solutions that empower global health data interoperability. Level 7 refers to the seventh level of the Open Systems Interconnection (OSI) — the application level.​

In short, it is a Health Application.

Now that we know what is HL7 means, we go back to our question, what is the HL7 message?

HL7 message refers to a set instructions of international standards for the transfer of clinical and administrative data between software applications used by various healthcare providers.

Above is a sample of an HL7 message. It is a set of instructions in pipe delimited format that is being exchanged by various healthcare providers or software vendors.

HL7 message is somewhat like a pigeon hole where each hole has a meaning.

In the above example, we can see the first row:


The first row is the MESSAGE HEADER which tells the service provider what type of message they are receiving.

When you count the 9th place of the segment, it will tell you the message type (ADT) and event type (AO1) where ADT^A01 — ADT refers to Patient Admission.

ADT is actually an acronym for Admit, Discharge and Transfer and there are many types of this event. See below:

Jeff Simons Decena

Senior Fullstack Engineer, React Native developer and HL7 Health Informatics professional