FHIRPath

FHIRPath uses a dot-notation syntax to traverse the hierarchical structure of FHIR resources. It allows developers to navigate from a root resource into nested elements, backbone elements, and data types.

• Navigate to Patient.name.given to extract first names
• Traverse collections using Condition.stage.summary
• Access deeply nested extensions with .extension.where(url='...').value

The language treats all paths as collections, meaning a single path can return zero, one, or multiple values depending on cardinality.

FHIRPath is the standard language for defining formal constraints (invariants) within FHIR profiles. These constraints enforce business rules that go beyond simple cardinality and terminology bindings.

• Define rules like start <= end on a Period element
• Enforce co-occurrence: condition.exists() implies severity.exists()
• Validate cross-element logic within a single resource

Invariants are expressed in the constraint element of a profile's ElementDefinition, providing machine-readable validation rules.

FHIRPath provides powerful functions for filtering and transforming collections of elements. The where() and select() functions enable SQL-like querying directly within FHIR expressions.

• Filter by criteria: Patient.identifier.where(type.coding.code = 'MR')
• Project specific properties: Observation.component.select(code)
• Combine with exists(), all(), and count() for cardinality checks

These functions are critical for slicing discriminators and search parameter expressions.

FHIRPath includes operators tailored to FHIR data types, enabling precise manipulation of clinical and administrative data without custom parsing logic.

• String operators: startsWith(), contains(), matches(regex)
• Quantity math: (value + 10) > 50 'mg' for dose comparisons
• Date arithmetic: today() - birthDate > 18 years for age checks
• Coding equality: code.memberOf('http://snomed.info/sct?fhir_vs=isa/404684003')

This type-awareness makes FHIRPath uniquely suited for healthcare data validation.

FHIRPath is used to define custom search parameters in FHIR profiles and implementation guides. The expression specifies exactly which element a search parameter indexes.

• Define a search on Patient.extension.where(url='...').value.ofType(CodeableConcept)
• Enable composite parameters by combining multiple paths
• Support chained and reverse chained searches through path expressions

This allows implementers to expose domain-specific query capabilities beyond the base FHIR search parameters.

FHIRPath expressions serve as discriminators for slicing, the mechanism that differentiates between repeated elements in a profile. The discriminator path tells validators which element value distinguishes one slice from another.

• Slice Observation.component by code using path code
• Differentiate Patient.identifier slices by system and type
• Support multi-key slicing with compound discriminator paths

This is fundamental to profiling patterns like US Core Blood Pressure, where systolic and diastolic components are distinct slices.

A conformance checking tool that executes FHIRPath constraints against resource instances. The validator evaluates every invariant expression to determine if a resource is structurally and semantically valid.

• Constraint evaluation: Runs all constraint elements with FHIRPath expressions
• Terminology binding: Validates codes against bound ValueSets
• Profile conformance: Checks cardinality, slicing, and extension rules
• Error reporting: Returns OperationOutcome with line/column details
• Integration: Embedded in HAPI FHIR, Firely .NET SDK, and validator CLI

FHIRPath is the expression language used to define custom search parameters. The expression element specifies which resource elements are indexed for querying.

• Simple path: Patient.name indexes all name parts
• Complex expression: Encounter.participant.where(type.coding.code='ATND')
• Chaining: Enables searches like GET /Observation?patient.name=Smith
• Composite parameters: Combine two paths for paired queries
• Compartment definitions: Use FHIRPath to define resource membership in patient compartments

While StructureMap uses its own mapping language, FHIRPath expressions are embedded within expression elements to source data from input resources during transformations.

• Source extraction: source.extension('http://hl7.org/fhir/us/core/StructureDefinition/us-core-race')
• Conditional logic: FHIRPath where() and select() for filtering
• Type handling: as(CodeableConcept) for safe type casting
• Relationship: StructureMap handles the transformation logic; FHIRPath handles data access
• Alternative: FHIR Mapping Language uses a different syntax for the same purpose

FHIRPath provides functions that invoke the FHIR Terminology Service for code system operations directly within expressions.

• memberOf('http://snomed.info/sct?fhir_vs=isa/404684003'): Tests code membership in a ValueSet
• subsumes(code): Checks if a concept subsumes another in the hierarchy
• conformsTo('http://hl7.org/fhir/StructureDefinition/Patient'): Validates against a profile
• resolve(): Follows references to retrieve full resources
• extension(url): Accesses specific extensions by canonical URL