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Demystifying FHIR Systems

· 5 min read
Rahul Agarwal
Rahul Agarwal
Medplum Core Team

One of the main sources of confusion when starting an implementation is with FHIR system strings.

This field is ubiquitous across FHIR elements, but many developers who are new to healthcare don't understand its purpose or how to set it properly. They are used in even the most basic implementations, and even the sample data we provide for prototyping has many system identifiers.

So today, we're going to delve into system strings to understand what they're for and how to use them!

System strings are commonly found on two distinct element types:

Identifiers​

A common occurrence in healthcare is that the same entity (patient, practitioner, device, etc.) is present in many different systems, each assigning their own unique ID. With FHIR, we can neatly keep track of all these unique IDs using the identifier field.

To avert any name collisions, each Identifier has an associated system string, which acts as a namespace for the identifier. This namespace is typically an absolute URL to ensure its global uniqueness.

Let's look at an example. Say we have two patients, Alice and Bob, who have both visited Hospital 1 and Hospital 2. They have the following medical record numbers:

AliceBob
Hospital 112345
Hospital 298760
Hospital 198760
Hospital 212345

Simply searching for the patient with record number "12345" would cause confusion.

GET [base]/Patient?identifier=12345

The system string is our guiding light here. It allows us to clarify which identifier comes from each hosptial.

AliceBob
{
"resourceType": "Patient",
"name": [{"given": ["Alice"]}],
"identifier": [
  // MRN - Hospital 1
  {
    "system": "http://hospital-1.org",
    "value": "12345"
  },
  // MRN - Hospital 2
  {
    "system": "http://hospital-2.org",
    "value": "98760"
  }
]
}
{
"resourceType": "Patient",
"name": [{"given": ["Bob"]}],
"identifier": [
  // MRN - Hospital 1
  {
    "system": "http://hospital-1.org",
    "value": "98760"
  },
  // MRN - Hospital 2
  {
    "system": "http://hospital-2.org",
    "value": "12345"
  }
]
}

Now if we add the system string to our search, we can do a targeted query for Bob.

GET [base]/Patient?identifier=http://hospital-2.org|12345

See our search guide for more information about searching with system strings.

CodeableConcepts​

Healthcare thrives on codes. Labs, medications, billing - they all have alphanumeric code systems. These standardized codes help healthcare actors communicate, reduce ambiguity, and streamline interoperability. You may have heard of some of these codes, like CPT for "procedure codes" or ICD-10 "diagnosis codes".

In an ideal world, there would be one universal code system for any application. But real-life healthcare is more complicated.

Let's take medications as an example. There are at least four common coding systems used to identify medications (for a deeper dive, check out our guide on medication codes).

This is where CodeableConcepts come in handy. They anticipate that the same concept (e.g. drug) might have different representations (aka codes) in different systems.

The example below shows how Tylenol would be represented in RxNorm and NDC. Here, the system string lets us know which code system we're using.

{
  text: 'Tylenol 325 MG Oral Tablet';
  coding: [
    // RxNorm
    {
      system: 'http://www.nlm.nih.gov/research/umls/rxnorm',
      code: '209387',
    },
    // NDC
    {
      system: 'http://hl7.org/fhir/sid/ndc',
      code: '50580045850',
    },
  ];
}

However, not all CodeableConcepts map to a standard system. For example, assume that you are using the Communcation.category field to organize messages based on product lines. Since product lines are specific to your company, there won't be a standard code system available. In these cases, you will develop in-house, or local , codes.

Best Practices for System Strings​

So now that we understand Identifier and CodeableConcepts better, we can talk about how to write good system strings.

Identifiers​

For Identifiers, the strategy is simple: each system string should correspond 1:1 with the source system. For instance, a patient ID from a particular hospital should have a system string like https://hospitalname.org/patientId.

CodeableConcepts​

When it comes to CodeableConcepts, it gets a bit more complex. Whenever possible, you should use standardized code systems to avoid reinventing the wheel and promote good data hygiene. The FHIR community has defined standard system strings for these code systems.

Some commonly used code systems:

DomainCode Systemsystem string
Procedure Names. Provider roles.SNOMEDhttp://snomed.info/sct
Clinical ObservationsLOINChttp://loinc.org
BillingCPThttp://www.ama-assn.org/go/cpt
ICD-10http://hl7.org/fhir/sid/icd-10
HCPCShttp://terminology.hl7.org/CodeSystem/HCPCS
MedicationsRxNormhttp://www.nlm.nih.gov/research/umls/rxnorm
NDChttp://hl7.org/fhir/sid/ndc

For local codes, **the system string should reflect the degree of consensus **you want to enforce across your organization.

A system string like https://my-healthcare-company.org/productLine could indicate a company-wide standard for product lines, while https://my-healthcare-company.org/messaging/productLine could refer to a standard specific only used within the messaging function.

Conclusion​

System strings are your go-to tool for successful healthcare data management. By keeping them clean and consistent, you'll save yourself a lot of confusion and time.

See Also​