LabQMHub Editorial Team · ISO 15189:2022 Quality Management Series
Internal quality control tells a laboratory how consistent it is with itself, but consistency alone does not prove accuracy — a laboratory can run perfectly consistent internal controls while being systematically wrong compared to every other laboratory testing the same analyte. External quality assessment, commonly called EQA or proficiency testing (PT), closes this gap by comparing a laboratory’s performance against an independent, external reference.
ISO 15189:2022’s Clause 7.3.8 requires laboratories to participate in EQA/PT programs wherever available, and to have a defined approach when no formal program exists — recognizing that external, independent verification is an irreplaceable check that internal processes alone cannot provide.
Why External Verification Matters Beyond Internal Control
A systematic bias affecting an entire laboratory — a miscalibrated reference standard, a subtly incorrect procedure everyone has learned to perform the same wrong way — can remain completely invisible to internal quality control, since internal controls only measure a laboratory’s consistency with itself, not its accuracy relative to the broader world. EQA programs, where a laboratory tests blinded samples alongside hundreds or thousands of other participating laboratories, expose exactly this category of otherwise undetectable systematic error.
This external, independent perspective is why EQA/PT results carry particular weight during accreditation assessment: they represent evidence a laboratory cannot manufacture internally, no matter how rigorous its own quality control program might otherwise be.
Selecting and Enrolling in Appropriate EQA Programs
Laboratories should enroll in EQA programs covering every test in their menu wherever a suitable program exists, choosing programs appropriate to the specific method and clinical application used, since a poorly matched program may not meaningfully evaluate the laboratory’s actual testing process. Where no formal EQA program exists for a particular test, alternative approaches — split-sample comparison with another laboratory, or retesting of previously characterized samples — must be defined and documented.
Participation should never be treated as optional simply because a test is low-volume or the laboratory feels confident in its performance; EQA is precisely designed to catch the problems a confident, internally consistent laboratory would not otherwise discover on its own.
Responding to Unsatisfactory EQA Performance
An unsatisfactory EQA result requires the same disciplined investigation applied to any other identified nonconformity: root-cause analysis, evaluation of whether patient results reported around the same period may have been affected, and documented corrective action before the next EQA cycle — treating an unsatisfactory result as valuable, if uncomfortable, information rather than something to be minimized or explained away.
A pattern of repeated unsatisfactory performance for the same test, even if each individual instance was investigated and closed, deserves escalated attention — it may indicate a persistent, unresolved underlying issue rather than a series of unrelated one-off events, connecting to the trend-analysis principles covered later in this program’s quality indicator course.
Comparability of Results Across Methods and Sites
Beyond formal EQA, laboratories operating multiple instruments, methods, or sites for the same test must establish and periodically verify that results are comparable across all of them — a patient whose specimen is tested on one instrument should not receive a meaningfully different result than if it had been tested on another instrument within the same laboratory system.
This comparability verification is particularly important during instrument replacement, method changes, or multi-site consolidation, where an unnoticed systematic difference between platforms could create confusing, clinically significant discrepancies in a patient’s longitudinal results over time.
Frequently Asked Questions
What is the difference between internal quality control and external quality assessment?
Internal quality control measures a laboratory’s consistency with itself day to day; external quality assessment compares the laboratory’s performance against an independent reference and other participating laboratories.
What should a laboratory do if it has no EQA program available for a test?
It must define and document an alternative approach, such as split-sample comparison with another laboratory or periodic retesting of previously characterized samples.
How should a laboratory respond to an unsatisfactory EQA result?
Investigate the root cause, evaluate whether previously reported patient results may have been affected, and implement documented corrective action before the next EQA cycle.
Key Takeaways
- External quality assessment provides an independent check on accuracy that internal quality control, measuring only self-consistency, cannot provide.
- EQA/PT participation should cover every test in the laboratory’s menu, using programs appropriate to the specific method and clinical use.
- Where no formal EQA program exists, laboratories must define and document an alternative comparison approach.
- Unsatisfactory EQA results require root-cause investigation and documented corrective action, treated with the same rigor as any other nonconformity.
- A pattern of repeated unsatisfactory results for the same test warrants escalated attention beyond individual incident investigation.
- Result comparability across multiple instruments, methods, or sites must be established and periodically verified for the same test.
Conclusion
External quality assessment is the laboratory’s reality check — an independent confirmation that internal confidence in a test’s performance is actually justified by comparison against the broader testing community. It is one of the few quality mechanisms genuinely outside the laboratory’s own control, which is exactly what makes it valuable.
ISO 15189:2022’s EQA and comparability requirements ensure this external check is systematic and comprehensive, not limited to whichever tests happen to have convenient, readily available programs.
Continue to post-examination processes in the next course of this ISO 15189:2022 training series, covering how results are finalized and reported.
Source note: this article draws on “Quality Management in Clinical Laboratory Demystified” by Dr. Taleb Chalab Cham, ISO 15189:2022, and widely recognized clinical laboratory quality references including CLSI guidelines, CAP accreditation checklists, and WHO laboratory quality guidance.
