The standard is not the problem

Most interoperability conversations start and end with a standard: HL7 v2, or FHIR, or occasionally an argument about which. That framing is comfortable because it makes the problem sound technical and bounded. In practice, two systems can both be fully FHIR-conformant and still fail to exchange anything useful. Conformance says the message will parse. It does not say the receiving system will understand what the sender meant.

Identity is the first real obstacle

Before any clinical data can move, both systems must agree on who the patient is. In a UK setting that usually means the NHS number, which sounds settled until you meet the records where it is absent, wrong, or attached to a demographic set that no longer matches. Every interoperability project spends more time on patient matching than its plan allowed for. Budget for it explicitly rather than discovering it during testing.

Coded data is where meaning gets lost

A message can carry a diagnosis code correctly and still mislead. Local code sets drift from national ones. Free-text qualifiers carry clinical meaning the code does not. A system that maps an unrecognised code to a default value is quietly producing wrong data at scale. Mapping tables need clinical review, not just technical validation, and they need reviewing again when either system is upgraded.

Someone has to own the interface after go-live

Interfaces break. An upstream system is upgraded, a field changes length, a code set is extended. If nobody owns monitoring, the first sign is a clinician noticing results have stopped arriving — which may be days later. Define ownership, alerting and a response route before go-live, and test that the alerting works by deliberately breaking something in a test environment.

What to ask a supplier

Ask which specific standards and versions are supported, not whether the system is interoperable. Ask what happens to an unrecognised code. Ask how interface failures are detected and who is alerted. Ask to see the interface specification before signing, not after. A supplier who cannot answer these concretely is telling you something useful.