Why Proficiency Testing on top of internal QC

Internal QC (control samples, replicates) can be all green while the lab is **still producing

wrong results. That's why the standard also requires external** checking: comparison with

other laboratories.

Internal QC's blind spot

Think of a clock that runs 5 minutes fast:

• You look at it every day and it "agrees with itself" — stable, repeatable, "in control".
• Only when you compare it against someone else's clock do you discover it shows the wrong time.

Internal QC checks stability (precision/repeatability around your own value). It cannot detect

a systematic error (bias) present in ALL your measurements — because it's in your control

sample too.

Internal QC (ISO/IEC 17025 §7.7.1)

• Who judges: you, with your own materials (control samples, replicates, blanks).
• Question: "Am I stable and in statistical control?"
• Frequency: continuously — every work batch.
• Screens: Control Samples, Control-Chart Limits, the replicate RPD check.

Proficiency Testing (ISO/IEC 17025 §7.7.2)

• Who judges: an independent provider (per ISO/IEC 17043) or an interlaboratory comparison:

all labs analyse the same unknown sample and are compared.

• Question: "Do I agree with the rest of the world?" — it detects the bias internal QC can't see.
• Frequency: planned (typically 1–2 times/year per method); the accreditation body (ESYD)

reviews the participation plan.

• Scoring: usually a z-score: |z| ≤ 2 satisfactory · 2 < |z| < 3 questionable

(investigate) · |z| ≥ 3 unsatisfactory (corrective action required).

• Screen: Quality Control → Proficiency Testing.

At a glance

The practical rule

Internal QC = do I agree with myself? Proficiency Testing = do I agree with everyone else?

The first protects you every day; the second protects you from being consistently wrong.