Fhir Developer Skill

FHIR Developer Skill

Quick Reference

HTTP Status Codes

Required Fields by Resource (FHIR R4)

Required vs Optional Fields (CRITICAL)

Only validate fields with cardinality starting with "1" as required.

Common mistake: Making subject or period required on Encounter. They are 0..1 (optional).

Value Sets (Enum Values)

Invalid enum values must return 422 Unprocessable Entity.

Patient.gender

male | female | other | unknown

Observation.status

registered | preliminary | final | amended | corrected | cancelled | entered-in-error | unknown

Encounter.status

planned | arrived | triaged | in-progress | onleave | finished | cancelled | entered-in-error | unknown

Encounter.class (Common Codes)

Condition.clinicalStatus

active | recurrence | relapse | inactive | remission | resolved

Condition.verificationStatus

unconfirmed | provisional | differential | confirmed | refuted | entered-in-error

MedicationRequest.status

active | on-hold | cancelled | completed | entered-in-error | stopped | draft | unknown

MedicationRequest.intent

proposal | plan | order | original-order | reflex-order | filler-order | instance-order | option

Bundle.type

document | message | transaction | transaction-response | batch | batch-response | history | searchset | collection

Validation Pattern

Python/FastAPI:

from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

def operation_outcome(severity: str, code: str, diagnostics: str):
    return {
        "resourceType": "OperationOutcome",
        "issue": [{"severity": severity, "code": code, "diagnostics": diagnostics}]
    }

VALID_OBS_STATUS = {"registered", "preliminary", "final", "amended",
                    "corrected", "cancelled", "entered-in-error", "unknown"}

@app.post("/Observation", status_code=201)
async def create_observation(data: dict):
    if not data.get("status"):
        return JSONResponse(status_code=422, content=operation_outcome(
            "error", "required", "Observation.status is required"
        ), media_type="application/fhir+json")

    if data["status"] not in VALID_OBS_STATUS:
        return JSONResponse(status_code=422, content=operation_outcome(
            "error", "value", f"Invalid status '{data['status']}'"
        ), media_type="application/fhir+json")
    # ... create resource

TypeScript/Express:

const VALID_OBS_STATUS = new Set(['registered', 'preliminary', 'final', 'amended',
  'corrected', 'cancelled', 'entered-in-error', 'unknown']);

app.post('/Observation', (req, res) => {
  if (!req.body.status) {
    return res.status(422).contentType('application/fhir+json')
      .json(operationOutcome('error', 'required', 'Observation.status is required'));
  }
  if (!VALID_OBS_STATUS.has(req.body.status)) {
    return res.status(422).contentType('application/fhir+json')
      .json(operationOutcome('error', 'value', `Invalid status '${req.body.status}'`));
  }
  // ... create resource
});

Pydantic v2 Models (use Literal, not const=True):

from typing import Literal
from pydantic import BaseModel

class Patient(BaseModel):
    resourceType: Literal["Patient"] = "Patient"
    id: str | None = None
    gender: Literal["male", "female", "other", "unknown"] | None = None

Coding Systems (URLs)

Common LOINC Codes (Vital Signs)

Data Type Patterns

Coding (direct) vs CodeableConcept (wrapped)

Coding - Used by Encounter.class:

{"system": "http://terminology.hl7.org/CodeSystem/v3-ActCode", "code": "AMB"}

CodeableConcept - Used by Observation.code, Condition.code:

{"coding": [{"system": "http://loinc.org", "code": "8480-6"}], "text": "Systolic BP"}

Reference

{"reference": "Patient/123", "display": "John Smith"}

Identifier

{"system": "http://hospital.example.org/mrn", "value": "12345"}

Common Mistakes

Resource Structures

For complete JSON examples of all resources, see [references/resource-examples.md](references/resource-examples.md).

Quick reference for error responses:

{
  "resourceType": "OperationOutcome",
  "issue": [{"severity": "error", "code": "not-found", "diagnostics": "Patient/123 not found"}]
}

RESTful Endpoints

POST   /[ResourceType]              # Create (returns 201 + Location header)
GET    /[ResourceType]/[id]         # Read
PUT    /[ResourceType]/[id]         # Update
DELETE /[ResourceType]/[id]         # Delete (returns 204)
GET    /[ResourceType]?param=value  # Search (returns Bundle)
GET    /metadata                    # CapabilityStatement
POST   /                            # Bundle transaction/batch

Conditional Operations

If-Match (optimistic locking):

• Client sends: If-Match: W/"1"
• Mismatch returns 412 Precondition Failed

If-None-Exist (conditional create):

• Client sends: If-None-Exist: identifier=http://mrn|12345
• Match exists: return existing (200)
• No match: create new (201)