HL7 v2 NK1-3 Field Guide: Relationship Codes and Common Mistakes
Two source systems can both send a technically valid NK1-3 value for "spouse" and still fail to match each other, because each one is drawing from its own locally customized code table.
NK1-3 is a coded field describing how the associated party relates to the patient — spouse, child, parent, guardian, employer, and so on. It uses a coded-element data type, so the value is really a code plus (optionally) the text it stands for, and many organizations extend or override the base code set with their own entries.
Valid code values
Example
SPO in this synthetic segment marks the contact as the patient's spouse.
Where this trips people up
Because NK1-3 is coded but the code table is only loosely standardized in practice, interfaces between two systems often carry an undocumented mapping table translating one vendor's relationship codes into another's. When that mapping is incomplete, unmapped codes either get dropped silently or default to "Other," quietly losing information a care team might actually need. Test suites should include relationship codes outside the sending system's common set to confirm the mapping fails loudly instead of silently.