How a referral actually travels: HL7 v2, secure messaging and the IHI.

A referral is the simplest thing in medicine: one clinician telling another, please see my patient. Getting that message from a GP's screen to a specialist's inbox, in Australia, turns out to be one of the more complicated journeys in health software, and more often than you would expect it still travels by fax.

It is worth following a single referral the whole way, because the layers it has to cross are exactly the layers your practice management software has to get right, and exactly where things quietly break. The message has to be in a format the other end can read, addressed to the right patient and the right provider, delivered down a secure channel that both ends can authenticate, to an address the sender actually knows. Each of those is a separate piece of national plumbing, and a referral is only as reliable as its weakest hop.

Start with the thing it is replacing: the fax

For all the money spent on digital health, the fax is still the default, and any digital path has to beat it on the one thing it does well: it always arrives. A 2018 study of Australian general practice put a number on the habit, finding that 92% of GPs and registrars named the fax as their primary way of sending letters to other health professionals.¹ That is not nostalgia; it is because a fax is addressed by a number anyone can find and lands as a page anyone can read, with none of the layers below to get wrong.

The cost is that a faxed referral is a flat image: it cannot be filed against the right patient automatically, searched, or actioned without someone re-typing it. The system has been trying to move past this for years, and in 2018 the Australian Digital Health Agency and the Royal Australian College of General Practitioners agreed national secure-messaging standards as a deliberate step toward retiring the fax.² To understand why that took national coordination, you have to look at what the digital alternative is actually made of.

A referral is a structured message, not a letter

The first layer is the language the message is written in, and in Australia that language is overwhelmingly HL7 version 2. HL7 v2 is the workhorse standard that carries clinical data between systems, widely used across the country including for the transmission of pathology results from laboratories to providers.³ It is not a document format so much as a grammar for clinical events, and a referral is one specific event within it.

A referral travels as the HL7 v2 REF (patient referral) message, which can carry a patient history summary together with the relevant diagnostic reports, and is acknowledged back to the sender so the GP's system knows it was received.⁴ When results come back the other way, they ride a different message, the ORU, which bundles the atomic results in discrete fields together with the formatted, human-readable report the way the lab intended it to look.⁵ The difference from a fax is the whole point: because the data is structured, the receiving system can file the referral against the patient, surface the values, and act on them, rather than leaving a clinician to read an image and re-key it.

Before it can be sent, everyone needs a number

A structured message is only useful if the receiving system can be certain who it is about, which is the job of Australia's healthcare identifiers. Every Australian with a Medicare or Department of Veterans' Affairs card has been allocated an Individual Healthcare Identifier (IHI), a unique 16-digit number that names the patient for healthcare purposes.⁶ Providers have their own equivalents: an HPI-I for an individual clinician and an HPI-O for a healthcare organisation such as a practice, hospital or pathology lab.⁷

These numbers are not a convenience; they are a national scheme established by the Healthcare Identifiers Act 2010, which sets up a service operator to assign and maintain them.⁸ The reason they matter to a referral is precision. A message keyed to the right IHI and the right HPI-O lands against exactly the right patient and the right provider, instead of being matched at the far end by name and date of birth and a measure of hope, which is how records get crossed in the first place.

Knowing where to send it is its own problem

Even with a well-formed message and the right numbers, the sender still faces a deceptively hard question: what is the recipient's electronic address, and is it still current? Clinicians move, practices change software, and an address that worked last year may now route into a void. Keeping those details accurate across every system is the address-book problem, and it is why provider directories exist.

The current answer is Provider Connect Australia, an Agency initiative launched in 2023 that lets a provider organisation maintain a single master copy of its business details and push updates automatically to its connected partners, including secure-messaging providers and service directories, so the address a sender resolves stays current.⁹ Underneath it sits the older machinery a sender uses to look up the delivery address for a given HPI-O, and an emerging FHIR-based provider directory doing the same job in a newer form. The mechanism varies; the requirement does not. If the directory is wrong, a perfectly valid referral is delivered, correctly, to nobody.

Getting it there securely, and proving who you are

With a message, identifiers and an address in hand, the referral finally needs a channel, and that channel is Secure Message Delivery. SMD is the Australian technical specification, published as ATS 5822, that defines how a clinical message is securely delivered from one provider to another.¹⁰ It is the part most people mean when they say "secure messaging", and it is the layer that replaces the fax line.

Because this is health information, both ends also have to prove who they are, and that is the role of NASH, the National Authentication Service for Health. A NASH certificate is a public-key credential, issued through Services Australia, that a provider organisation uses to authenticate itself and securely transact with national digital health systems.¹¹ It is the same credential that gates the Healthcare Identifiers Service, My Health Record and electronic prescribing, so it is not optional plumbing: without it, the far end has no basis to trust that the message is from who it claims to be.

Why your message used to bounce: the networks did not talk

For a long time, all of this had a dirty secret, which is that the secure-messaging networks did not interoperate. In practice a clinician could reliably send only to other providers who used the same secure-messaging software, so a GP on one product simply could not message a specialist on another, and the fax remained the only thing that reached everyone.¹² The standards existed; the willingness to make competing products honour them did not.

That is what the Agency's interoperability program set out to fix, and it is more recent than most people assume. After a proof of concept, the Agency partnered with 42 organisations to make secure messaging interoperable across 56 separate software products, with providers required to implement the new standards by 2020.¹² The single most useful thing to understand about Australian secure messaging is therefore this: cross-network delivery was a deliberate, coordinated fix achieved only in recent years, not a property the system always had. When you evaluate software, "we do secure messaging" and "we deliver to clinicians on other networks" are different claims.

Where it is heading: FHIR, slowly

The newest layer is FHIR, the web-based standard that is meant to make all of this cleaner, and Australia is building its national baseline through the AU Core Implementation Guide, which sets the minimum expectations on FHIR resources for conformance in local systems.¹³ It is genuinely the direction of travel, and worth designing toward.

But it is a direction, not a destination already reached. HL7 v2 still carries the overwhelming majority of real clinical messaging, and the Agency's own National Healthcare Interoperability Plan 2023-2028 frames FHIR as the target to build toward across years rather than the present state of the network.¹⁴ Any vendor telling you the country has "moved to FHIR" is selling the destination as if it were the road. Good software has to speak the standard that exists today and be ready for the one that is coming.

What to ask, when the plumbing is invisible

The whole chain is invisible right up until a referral silently fails to arrive, which is exactly why it is worth asking about before you buy rather than after you miss a result. The questions that actually separate clinical software are concrete: does it speak HL7 v2 properly, both the REF referral and the ORU result; does it use the Healthcare Identifiers Service so messages are keyed to the IHI and HPI-O rather than matched by name; does its secure messaging reach clinicians on other networks, not just its own; does it keep its directory details current; and does it hold a NASH certificate so the other end can trust it.

Trenthos builds Trenthos Vision to participate in this stack as a first-class citizen rather than an afterthought: HL7 v2 messaging, secure messaging across the Australian networks, the healthcare identifiers and NASH authentication, with those integrations progressing through the relevant conformance steps rather than described as finished. The honest summary is the one the journey makes obvious: a referral is only as good as its weakest hop, and the software worth choosing is the kind that has thought carefully about every one of them.