External Quality Assurance (EQA) – Detailed Study Notes

Context & Purpose of EQA

  • EQA = External Quality Assurance; final screencast topic for the lecture.
  • Rationale
    • Internal Quality Control (IQC) only compares a laboratory to itself over time ➜ cannot reveal inter-laboratory bias.
    • EQA validates laboratory performance against other laboratories using the same analyzer, reagent and analytical principle.
    • Core objective: detect systematic errors and ensure long-term accuracy of analytical methods.
  • Frequency
    • Not daily; usually fortnightly to monthly.
    • Complements daily IQC to achieve comprehensive quality assurance.

Placement in ISO 15189 Quality Management System

  • Quality policy consists of two complementary systems:
    • Internal QC (IQC) – daily monitoring of precision and accuracy within a run.
    • External QA (EQA) – periodic comparison of performance between labs.
  • Diagram (spoken description): EQA occupies the “analytical” branch under ISO 15189, sitting alongside IQC within the overall quality framework.

Types / Mechanisms of EQA

  1. Proficiency Testing (PT)
    • External provider sends unknown samples from a common pool.
    • Participating labs analyse, report results; provider collates and feeds back comparative data.
    • Simulated patient specimens ensure uniform challenge.
    • In Australia, the RCPA QAP (Royal College of Pathologists of Australasia – Quality Assurance Programs) supplies PT samples to nearly all pathology labs.
  2. Rechecking / Retesting
    • Previously read slides re-examined by a reference laboratory.
    • Previously analysed specimens re-measured, allowing direct result comparison.
  3. Site Visits / Inter-lab Exchange
    • Used when PT or rechecking is impractical.
    • Laboratories exchange samples to cross-check specialised tests lacking formal PT schemes.

Why EQA Matters

  • Mandatory for laboratory accreditation (ISO 15189, national schemes).
  • Supplies actionable information on analytical quality across sites.
  • Pinpoints systematic issues (kits, reagents, analysers, operator technique).
  • Highlights training needs → continuous improvement.
  • Serves as an ongoing performance metric within overall laboratory quality management.

Anatomy of an EQA Report (Example: Vitamin D$_3$)

  • Delivered by RCPA QAP; formats are standardised across analytes.
  • Visual Presentations
    • Histograms – distribution of participant results; lab’s value marked by a red triangle.
    • Youden Plots – cross-plots of low vs high concentration specimens; lab’s pair marked by ×.
    • Levey-Jennings (LJ)-style charts – time-series of results against target.
    • Linearity Plots – assess response across concentration range.
  • Allowable Performance Specification (APS)
    • Shown as white zones on graphs ➜ equivalent to 2SD IQC limits.
    • If result lies within APS → performance acceptable.
  • Result Summary Table consolidates numerical data, z-scores, SDI, comments.

Interpreting the Youden Plot

  • Coordinates: Low-concentration on X-axis; High-concentration on Y-axis.
  • Central square = both results within APS → acceptable.
  • Peer Review Box (inner rectangle)
    • Drawn using performance specs derived from peer group (same method/instrument) instead of target value.
    • Allows judgement of lab vs similar laboratories.
  • Additional classification layers inside plot:
    • By analytical principle, instrument and reagent.
    • Example shown: Vitamin D measured by Luminescence Immunoassay (LIA) on DiaSorin Liaison using DiaSorin reagents.

Recent Changes (2020 Report Redesign)

  • RCPA QAP updated layout to cleaner style (example: Total Bilirubin).
  • Core components (histograms, Youden, LJ, linearity) remain unchanged to preserve continuity.

Practical / Ethical / Accreditation Implications

  • Failure to participate or poor EQA performance can jeopardise ISO 15189 accreditation and patient safety.
  • Identified biases must prompt:
    • Immediate investigation (instrument maintenance, reagent lot checks).
    • Corrective actions (re-calibration, staff retraining).
  • Transparent sharing of EQA data fosters trust between laboratories and healthcare providers.

Connection to Previous Lectures & Foundational Principles

  • Builds on earlier discussion of Total Testing Process (TTP) error sources (pre-analytical, analytical, post-analytical).
  • Today’s focus: Analytical phase QC.
  • EQA complements IQC to cover both short-term precision and long-term accuracy/bias.

Key Take-Home Points

  • EQA provides external benchmark; IQC provides internal consistency.
  • Three main EQA formats: Proficiency Testing, Rechecking/Retesting, Site Visits/Exchange.
  • Reports use multiple visualisations (Histogram, Youden, LJ, Linearity).
  • APS ≈ 2SD limits; results inside white zone acceptable.
  • Youden plots simultaneously evaluate low & high controls; peer review box focuses on method-specific comparison.
  • Participation is compulsory for accreditation (ISO 15189, RCPA QAP).
  • Regular analysis of EQA outcomes drives quality improvement, staff education and patient safety.

Suggested Further Study / Practice

  • Review case studies on QC troubleshooting provided by lecturer.
  • Re-draw a Youden plot using sample data to solidify interpretation skills.
  • Cross-reference RCPA QAP performance specs with CLIA, Westgard rules for broader context.
  • Discuss corrective action pathways for common EQA failures during Collaborate Ultra session.