CQL Syntax Notes

Learning notes for Clinical Quality Language

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What is CQL?

Clinical Quality Language (CQL) is a domain-specific language for expressing clinical knowledge. It’s used in:

• Clinical Decision Support (CDS)
• Electronic Clinical Quality Measures (eCQM)
• FHIR Clinical Reasoning

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Basic Syntax

Library Declaration

library DrugRepurposingLogic version '1.0.0'

using FHIR version '4.0.1'

include FHIRHelpers version '4.0.1'

context Patient

Data Retrieval

// Get all active medications
define "Active Medications":
  [MedicationRequest: status = 'active']

// Get specific drug
define "Metformin Orders":
  [MedicationRequest] M
    where M.medication.coding.code = 'DB00331'

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Value Sets and Code Systems

Code System Definition

codesystem "DrugBank": 'https://go.drugbank.com'
codesystem "SNOMED": 'http://snomed.info/sct'

Value Set Definition

valueset "Diabetes Medications": 'http://example.org/ValueSet/diabetes-meds'

Code Definition

code "Metformin": 'DB00331' from "DrugBank"
code "Type 2 Diabetes": '44054006' from "SNOMED"

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Expressions

Simple Expressions

// Boolean
define "Has Diabetes":
  exists([Condition: "Type 2 Diabetes"])

// Numeric
define "Age in Years":
  AgeInYears()

// String
define "Patient Name":
  Patient.name.first().given.first() + ' ' + Patient.name.first().family

Conditional Logic

define "Diabetes Risk Level":
  case
    when "Has Diabetes" then 'High'
    when "Age in Years" > 45 then 'Medium'
    else 'Low'
  end

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Queries

Basic Query

define "Recent Medications":
  [MedicationRequest] M
    where M.authoredOn during Interval[@2024-01-01, @2024-12-31]

With Relationships

define "Medications with Conditions":
  [MedicationRequest] M
    with [Condition] C
      such that M.reasonReference.reference = 'Condition/' + C.id

Sorting and Limiting

define "Last 5 Medications":
  (Last 5 from [MedicationRequest] M
    sort by authoredOn desc)

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Functions

Built-in Functions

// Date functions
define "Days Since Last Medication":
  days between "Last Medication Date" and Today()

// String functions
define "Drug Name Upper":
  Upper("Drug Name")

// List functions
define "Medication Count":
  Count("Active Medications")

Custom Functions

define function GetEvidenceLevel(score Decimal):
  case
    when score >= 0.9999 then 'L1'
    when score >= 0.999 then 'L2'
    when score >= 0.99 then 'L3'
    when score >= 0.9 then 'L4'
    else 'L5'
  end

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Drug Repurposing Example

Complete Library

library DrugRepurposingCDS version '1.0.0'

using FHIR version '4.0.1'
include FHIRHelpers version '4.0.1'

codesystem "DrugBank": 'https://go.drugbank.com'

context Patient

// Get patient's current medications
define "Current Medications":
  [MedicationRequest: status = 'active']

// Get DrugBank IDs
define "DrugBank IDs":
  "Current Medications" M
    return M.medication.coding.where(system = 'https://go.drugbank.com').code

// Check if any medication has repurposing candidates
define "Has Repurposing Candidates":
  exists("DrugBank IDs")

// Recommendation text
define "Recommendation":
  if "Has Repurposing Candidates"
  then 'Review EuTxGNN for potential repurposing candidates for current medications.'
  else 'No medications available for repurposing analysis.'

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Testing CQL

CQL Testing Framework

// Test case
define "Test Has Metformin":
  exists([MedicationRequest] M
    where M.medication.coding.code = 'DB00331')

Tools

• CQL Playground
• Bonnie - eCQM testing
• CQL Execution Engine

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Integration with EuTxGNN

Potential Use Cases

1. CDS Alert: Alert when patient takes drug with L1/L2 repurposing evidence
2. Quality Measure: Track patients who might benefit from repurposed drugs
3. Research Cohort: Identify patients for clinical trials

Example CDS Rule

define "Show Repurposing Alert":
  "Has Repurposing Candidates" and
  not exists("Recent Repurposing Review")

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Resources

• CQL Specification
• CQL Author’s Guide
• FHIR Clinical Reasoning
• CQFramework GitHub