Proficiency Testing: Requirements and Best Practices

External quality assessment requirements under ISO 15189, CAP, and CBAHI — selecting appropriate PT programs, responding to unsatisfactory results, and using PT data proactively for performance monitoring.

External quality assessment — most commonly delivered through proficiency testing (PT) programs — is a mandatory component of accreditation under ISO 15189, CAP, CBAHI, and essentially every major laboratory accreditation framework. It is also one of the most powerful tools available to laboratory leadership for detecting systematic problems that internal quality controls may miss.

What is proficiency testing?

Proficiency testing involves a laboratory analyzing samples provided by an external PT program organizer, submitting results, and having those results compared against peer laboratories or defined reference values. The comparison reveals how the laboratory's results align with the broader laboratory community — identifying systematic biases, precision problems, or measurement failures that internal QC may not detect.

Regulatory requirements

Under ISO 15189, laboratories must participate in external quality assessment for all examinations performed where programs are available. Results must be reviewed, compared to acceptability criteria, and acted upon when outside acceptable limits. CAP has specific PT requirements tied to laboratory specialty, with defined surveys and submission schedules. For laboratories operating under CBAHI or NABL, analogous programs apply.

Selecting appropriate PT programs

Laboratories should select PT programs that cover all examinations performed for which programs exist, use matrices that match the laboratory's actual specimen types, and have peer groups appropriate to the laboratory's methods and instrument platforms. Where no formal PT program exists for a particular examination, the laboratory must demonstrate equivalence through inter-laboratory comparison with another accredited facility or split-sample testing with a reference laboratory.

Responding to unsatisfactory results

An unsatisfactory PT result is not automatically a major quality failure — but it is a required trigger for investigation. The laboratory must review the failure, determine the likely cause, implement corrective action if warranted, and document the entire process. If a systematic cause is identified, the laboratory must assess whether patient results produced during the affected period could be impacted. Common causes include transcription errors, specimen matrix effects, reagent lot issues, calibration drift, and method-specific interferences.

Using PT data proactively

Beyond its compliance function, PT data is genuinely useful for performance monitoring. Peer comparison statistics — Standard Deviation Index (SDI), Coefficient of Variation Ratio (CVR) — provide context for the laboratory's performance relative to its peer group. Laboratories that systematically review their PT performance over time, rather than treating each survey as a binary pass/fail event, extract substantially more value from the exercise.

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