FHIR R4 Symptoms Implementation Guide, published by HL7 International / Clinical Interoperability Council. This guide is not an authorized publication; it is the continuous build for version 1.0.0-ballot built by the FHIR (HL7® FHIR® Standard) CI Build. This version is based on the current content of https://github.com/HL7/fhir-symptoms-ig/ and changes regularly. See the Directory of published versions

Symptom/Diagnosis Relationship

Page standards status: Informative

Relationship Between Symptoms and Diagnoses

In clinical documentation, symptoms represent the subjective evidence of a health state, typically experienced and reported by the patient (e.g., “fatigue,” “chest pain,” “nausea”).
In contrast, diagnoses represent the clinical interpretation or determination of an underlying condition that explains one or more symptoms or findings (e.g., “pneumonia,” “myocardial infarction,” “anemia”).

Within FHIR, these two types of information are represented by distinct but complementary resources:

Clinical Concept	FHIR Resource	Typical Coding Systems	Description
Symptom	Observation	SNOMED CT, LOINC	Captures the presence, absence, severity, or characteristics of a symptom as reported by a patient or observed by a clinician.
Diagnosis	Condition	SNOMED CT, ICD-10, ICD-11	Represents the clinician’s diagnostic conclusion or disease identification that accounts for one or more symptoms and findings.

Both resource types may coexist within a patient record and are often linked to support clinical reasoning, quality measurement, and decision support use cases.

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Linking Symptoms and Diagnoses in FHIR

FHIR provides multiple mechanisms to establish relationships between Observations representing symptoms and Conditions representing diagnoses.

Condition.evidence.detail

The primary mechanism for associating a Condition with supporting evidence is the Condition.evidence.detail element.
This allows a Condition to reference one or more Observations (symptoms, test results, or other findings) that informed the diagnostic conclusion.

Observation associatedSymptomOrCondition extension

When documenting a symptom Observation, the Observation associated Symptom or Condition extension element can link that symptom to a known or suspected Condition that it relates to.

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Conceptual Model

Patient Experience → Symptom (Observation) → Clinical Interpretation → Diagnosis (Condition)

• Symptoms (Observations) provide the evidentiary basis for diagnostic reasoning.
• Diagnoses (Conditions) represent the resultant clinical interpretation, often referencing the Observations that support them.

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Implementation Considerations

• Traceability: Use Condition.evidence.detail to maintain a traceable, computable link between diagnostic conclusions and their supporting findings.
• Symptom Clustering: Multiple symptom Observations may be grouped using Observation.hasMember, or captured together via a Questionnaire/QuestionnaireResponse or an Observation panel.
• Temporal Context: Include effective[x] elements on Observations and Conditions to accurately reflect timing and sequence.
• Provenance & Certainty: Capture provenance (who recorded the symptom) and the certainty/verification status of the Condition (e.g., Condition.verificationStatus) to clarify whether a diagnosis is provisional, confirmed, or refuted.
• Terminology Alignment: Prefer SNOMED CT for clinical concepts (symptoms and conditions) to enable semantic linkage. Use LOINC for structured instruments or patient-reported measures where appropriate. Use ICD-10/ICD-11 for billing/classification needs while preserving clinical codes for interoperability.
• Multiplicity: A single Observation (symptom) may support multiple Conditions; similarly, a single Condition may be supported by multiple Observations.

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Summary Table

Aspect	Symptom (Observation)	Diagnosis (Condition)
Clinical Nature	Manifestation experienced or observed	Underlying cause or interpretation
Primary FHIR Resource	Observation	Condition
Relationship Element	Observation.extension:associatedSymptomOrCondition	Condition.evidence.detail
Typical Codes	SNOMED CT (e.g., Pain in chest), LOINC (e.g., Severity of nausea)	SNOMED CT, ICD-10 (e.g., Pneumonia, Migraine)
Example Relationship	Symptom supports one or more Conditions	Condition references multiple supporting Observations

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Guidance Summary

Best Practice: Use Condition.evidence.detail to link Conditions to the Observations (Symptoms) that support them. Optionally use Observation.extension:assocaitedSymptomOrConditoin to indicate the reverse relationship. Ensure consistent coding, provenance, and temporal alignment to support computable reasoning and interoperability.