Based on the IHE XDS.b profile, we defined four architecture models that would be feasible for an HIE network in South Korea (Figure 1). The models depend on the location of each registry and repository, focusing on security issues at the physical locations where health information will be shared.

The Integrated Registry - Repository model is a central architecture in which all shared clinical documents (the repository) and the metadata (the registry) for searching them are stored in a central cloud server. The One Registry - Multiple Repositories model is a distributed architecture in which the repository is located within each healthcare organization or in its Electronic Health Record (EHR) vendor's server, with a registry for linking documents between locations maintained in a central server. The Federated Registry - Multiple Repositories model supports information exchange through communication between the regional registries. Finally, the Peer-to-Peer model is based on direct exchange between healthcare organizations using socket communication or secure email.

In this study, we focused on developing a common HIE platform that would support the Integrated Registry - Repository and the One Registry - Multiple Repositories models. We considered the two architecture models because, at the time of our study, there was no requirement to integrate other registries and there were not even other regional registries to connect to make a federated registry in South Korea. In addition, we pursued sharing of patients' care record summaries among healthcare organizations; therefore, we did not consider the Peer-to-Peer model. However, the architecture models would depend on the EHR infrastructure, the healthcare environment, and the cultural environment.

Based on the IHE XDS.b profile, we derived common HIE functions and services for each actor, including a document repository, document registry, and document consumer. Based on the service-oriented architecture (SOA), we developed a common HIE platform that provides these functions and services to minimize the effort and difficulty in implementing the IHE XDS.b profile, CDA standards, and common terminology set.

The common HIE platform supports open application program interfaces (APIs) for implementing the document registry, document repository, document consumer, and master patient index. The document registry receives the metadata from clinical documents generated by healthcare organizations, stores the metadata, and supports searches for the documents. The document repository stores clinical documents, requests indexing in the document registry, and provides the documents when they are requested by other healthcare organizations. The master patient index (MPI) maintains patient identifiers and patient consent information. In addition to providing the services needed for sharing documents, we also included supportive HIE services that allow the standardization of local terminologies to international terminologies, the management of the master data of participating healthcare organizations and departments, and the regulation of transaction logs.

To design the common HIE platform, we derived atomic services based on the IHE XDS.b profile and then designed atomic services and derived the functional service requirements of the HIE applications, enabling the definition of a composite service by the combination of multiple atomic services. For healthcare organizations and EHR vendors to easily utilize the platform, we developed these functions and services in the form of open APIs, as shown in Figure 2.

We developed a number of services for CDA documents with open APIs, for example, CDA Viewer, which displays CDA documents in readable format by applying the XSLT style sheet. The 'CRS and Referral Generate' API supports the automatic generation of CDA documents based on the corresponding values of data elements that were extracted from the EHR system. The terminology mapping can be used to generate CDA entries by automatically mapping local terms to structured coded terms.

As a standard for clinical documents to be exchanged among healthcare organizations, we adopted HL7 CDA Release 2 (R2) Level 3. Based on CDA R2 and some of the IHE CDA content modules, we defined the data elements and templates for the referral and care record summaries to support collaborative care.

In this study, the referral summary was defined as a type of document created to support collaborative care by referring patients from clinics or hospitals to specialists. It is used for referrals and replies between two healthcare organizations. The care record summary (CRS) was defined as a document that summarizes hospital visits and hospitalizations; it is automatically created and stored for the purpose of the continuity of patient care. A CRS contains a patient's relevant health history for one or more encounters, and it is intended to share a core set of the patient's medical records between healthcare providers. The HL7 Care Record Summary [8], Continuity of Care Document [9], and Consolidated CDA [10] were referenced in the process of designing the CRS.

Table 1 shows the major CDA data items in the headers and bodies of two types of documents, including coded entries with standard terminology for ensuring semantic interoperability.

Regarding the data items for problems, medications, and test results, we created coded entries for each data item with the international terminologies. We used the International Classification of Disease (ICD-10; translated, and known as KCD-6 in Korea) and Logical Observation Identifiers Names and Codes (LOINC) for the diagnoses and the examination names, respectively. For medications, we allowed for the use of multiple code sets, including the Korean Electronic Data Interchange (EDI) drug codes used for insurance claims and the international Anatomical Therapeutic Chemical (ATC) codes. In South Korea, the Korea Health and Welfare Information Service [11] has developed and published the Korea Standard Terminology of Medicine (KOSTOM), which is the integrated system of terminologies practically used by and collected from the actual health and medical fields; their terms were mapped to the international terminology, such as ICD and LOINC. Therefore, we also adopted KOSTOM in addition to ICD and LOINC for the secondary use of data for both domestic and international purposes.

Because most hospitals and clinics do not use LOINC codes for tests, manual mapping from the local test codes to the LOINC codes was necessary for creating the CDA entries. Thus, we included a terminology mapping service and tool in the common HIE platform.

For privacy and security reasons, only patients who agreed to participate in HIE using an informed consent were initiated to share their medical records. The consent stated the purpose of HIE and the scope of data items for sharing. In the HIE system, a scanned copy of the patient's consent was transferred and registered to the registry server in the form of binary data of the <StructuredBody> element of a CDA document. Once a patient's consent was registered in the HIE system, the patient's universal unique identifier was created.

In South Korea, there is no unique patient identifier that can be used only for medical purposes. Although all Koreans have their own Resident Registration Numbers (RRNs), there have been increasing issues with the use of RRNs for HIE because of privacy and security concerns.

Because most hospitals and clinics have their own local patient identifiers, we created and assigned universal unique identifiers (UUIDs) to each patient who agreed to participate in the HIE using a MPI based on the IHE Patient Identifier Cross-referencing (PIX) and Patient Demographics Query (PDQ) profiles.

We used HL7 V3 ADT messaging to transmit each hospital's local patient identifier to the MPI system, enabling searches for patients and their UUIDs. Using the MPI system, we managed patient demographic information, including UUID, name, birth date, and address.

To support the implementation of a secure HIE network, we provided the required security guidelines after investigating the current security conditions and the solutions that were being used at the participating healthcare organizations. The security guidelines, in essence, comprised both technical and administrative security. Technical security considered medical information (e.g., life cycles of data) and storage (e.g., passwords and access control). Administrative security focused on integrated management (e.g., organizational policies, backup and restore, and documentation) and auditing (e.g., audit record policies and reports).

We considered security with regard to accessing devices, the network, the platform, and the content necessary for implementing a nationwide HIE network using our common platform.

As part of a nationwide initiative, our first phase was to implement an HIE network for WC hospitals and their collaborating hospitals and clinics because industrially injured workers typically require long-term collaborative care, and their medical costs are covered by industrial accident insurance; we found that HIE outcomes can be used to evaluate insurance reimbursement to promote HIE participation.

In our pilot implementation of the network, two WC hospitals, one in Incheon and the other in Ansan, participated. The hospitals are located in metropolitan areas and have approximately 500 beds, and there were six collaborating clinics near the WC hospitals. The two hospitals had recently adopted an EHR system developed by a domestic EHR vendor. The six participating clinics used two different EHR systems. The WC hospitals are public healthcare organizations that provide rehabilitation for workers, but their medical service is not limited to injured workers.

Our participating WC hospitals and clinics had different IT infrastructures and environments. The WC hospitals had their own network infrastructures and managed their EHR systems through an IDC center of Korea COMWEL. However, the clinics' EHR systems were connected through a broadband network, indicating the weakness of security.

Therefore, as shown in Figure 3, we chose an hybrid architecture of, physically, the Integrated Registry - Repository model and, logically, the One Registry - Multiple Repositories model. Using the common HIE platform, we implemented the HIE registry, repository, and MPI system components in Korea COMWEL's IDC center, called the HIE IDC center; for each EHR vendor in the center, there was one registry with distributing repositories. Both WC hospitals and the clinics communicated with the HIE IDC center through their gateway servers and encrypted network connections. The gateway server for the clinics, located on a DMZ network, provided open APIs and bridged service between the healthcare organizations.

Our security guidelines required that each doctor's PC have a vaccine program, a PC firewall, and a data loss prevention solution. Regarding network security, a virtual private network (VPN), a secure sockets layer (SSL) protocol, an intrusion prevention system (IPS), a Web firewall, and server dualization were applied to prevent hacking and system errors. Server security was reinforced by the installation of a secure OS, a server vaccine program, a database encryption solution, and a storage backup solution.

The common HIE platform was used to support the transfer of referral summaries from a clinic to a WC hospital and vice versa and the sharing of patients' care record summaries among the participating healthcare organizations. Figure 4 shows the workflow and scenarios implemented in our HIE network for injured workers.

Regarding the transfer of referral summaries, when a patient visits a clinic near his or her workplace and the patient needs to be transferred to a WC hospital, a doctor creates a referral summary using the clinic's EHR system. The referral summary is automatically generated in CDA format and stored in the repository, and its metadata are entered into the registry. The WC hospital will be automatically notified of the referral. After the referred hospital confirms the referral and checks the CDA document, a specialist views the referral summary during the patient's visit. The doctor can then also send the reply documents to a clinic on completion of the patient's treatment through the HIE network (Figure 4A).

To share care record summaries, a CDA CRS is automatically generated in the event of a patient's hospital visit and discharge, and it is stored and entered into the repository and the registry. When the patient requires emergency treatment or receives treatment from multiple hospitals and clinics, the CRS can be used to share that patient's health information among the facilities, supporting continuity of care (Figure 4B).

Since the HIE system should be incorporated into the organization's workflow and the doctor's workflow in order be successful [12], HIE service scenarios were developed by analyzing intra and interorganizational workflows of patient's care.

To accelerate the interoperability between different EHR systems, we adopted international standards, such as HL7 CDA and HIE profiles, and we developed a common HIE platform that supports the implementation of the standards, thus enabling participation in the HIE network and connection between EHR systems. We packaged together the components of the common HIE platform that were required for the registry, repository, and MPI servers. In addition to the HIE system packages, we packaged useful tools that support CDA content generation, standard terminology management, and master data management.

In our HIE network, we installed the registry server and MPI server packages in a Korea COMWEL IDC center and built repository servers for each hospital and clinic using the repository package. These packages allowed the system to address changes in management authority and physical server location because they are simple to install and remove.

To easily deploy the common HIE platform in other healthcare organizations, the RESTFUL-based open API service, testing tools, and manuals will be provided to vendors and developers. After the pilot implementation and operation of a regional HIE network for two WC hospitals and their collaborating clinics is finished, the network will be expanded to other WC hospitals and healthcare organizations stage by stage.