How Do You Avoid Identifier Collisions in FHIR? Lessons from Real-World Integrations

Fast Healthcare Interoperability Resources, better known as FHIR, is the healthcare data standard that enables secure and consistent exchange of clinical information. By defining a set of common resources, FHIR ensures that when a resource is labeled as a patient, you can trust that it truly represents a patient record.

Still, FHIR’s flexibility is both a strength and a risk. Without careful implementation, identifier handling can quickly become a source of data integrity issues. One of the most common pitfalls? Identifier collisions.

The Problem: When Identifiers Aren’t Unique

On a recent project standing up a FHIR instance, we ran into a major issue: values in the identifier fields weren’t unique. This happened because the source systems used sequential primary keys as identifiers.

The result? Querying for a patient with an identifier value of 123 returned multiple patients, all with the same identifier value. Clearly, this was not acceptable. Fortunately, we caught it early in testing.

Root Cause: FHIR’s Identifier Flexibility

FHIR’s Identifier datatype is extremely powerful but easy to misuse. By design, the identifiers array does not enforce uniqueness. This means you can unintentionally create collisions unless identifiers are scoped properly.

The FHIR Identifier structure includes two key fields:

• • system – The namespace or URI that defines the scope of the identifier
  • value – The actual identifier

From the FHIR specification:

“The system is an absolute URI that defines a set of identifiers (i.e., how the value is made unique).”

In other words: the system makes the value unique. If you only query on the value, you’re leaving the door open for collisions.

The Fix: Always Scope Identifiers

The solution was straightforward: ensure every identifier includes a system field, and structure all queries using both system and value.

If your data doesn’t include a system field, that’s a red flag. It may require a broader system audit to ensure identifiers are being generated and tracked correctly.

For our project, all identifiers were properly scoped; we simply needed to adjust our queries to include the system parameter.

Lessons Learned: Best Practices for FHIR Identifiers

Here are key takeaways to avoid running into identifier collisions in your FHIR implementations:

• • Always include a system for each resource identifier
  • Structure queries using both system + value, not value alone
  • Validate FHIR resources to confirm identifiers are scoped correctly
  • Never assume identifier values are globally unique across systems
  • Consider logging or alerting when identifiers are missing a system

Conclusion: Getting FHIR Right

FHIR is a powerful standard, but its flexibility can lead to unintended pitfalls. Proper identifier handling is essential to ensure accurate queries and reliable data exchange.

If you’re implementing or auditing a FHIR integration:

• • Treat identifiers as system-scoped, never standalone
  • Validate incoming data for completeness
  • Monitor for potential collisions early in your workflows

Have you encountered identifier challenges in your FHIR projects? Share your lessons learned, or reach out to RBA for help with secure, scalable FHIR integrations.

Reference: FHIR Identifier Datatype Documentation