Understanding HL7 v2 AL1-5: Allergy Reaction in Practice
AL1-5 is where allergy data gets messy on purpose, because reactions are described in whatever words the clinician typed. Testing it with a single clean value hides most of the parsing problems it actually causes.
AL1-5 describes the reaction associated with the allergy, such as a symptom or clinical effect. It is a repeating field, so a single allergy can list multiple reactions, and in practice it is populated with a mix of short codes and loosely structured free text.
What this field contains
Example
AL1-5 lists two reactions for this penicillin allergy: hives and anaphylaxis, separated by a component delimiter in this example.
Where this trips people up
The delimiter question is the biggest source of bugs here. Some source systems repeat AL1-5 using the HL7 repetition separator, others cram multiple reactions into one string using a component or subcomponent separator, and some just write a long free-text sentence with no structure at all. A parser tested against only one of these patterns will drop or garble reactions the first time it meets a different one. It is also worth testing wildly inconsistent capitalization and spelling, since this field is rarely normalized against a controlled vocabulary before it hits your interface.