# Proficiency testing

> Proficiency testing sends identical blind samples to many labs and compares their results with an assigned value, checking each lab's accuracy against its peers.

- Canonical: https://genetixbay.com/knowledge/glossary/proficiency-testing.html
- Also written: PT, proficiency test, PT scheme, interlaboratory comparison, round-robin testing, ring test
- Glossary category: Lab testing · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

In proficiency testing (PT), a provider prepares a homogeneous material, often [hemp](https://genetixbay.com/knowledge/glossary/hemp.md) or cannabis flower, an oil, or a spiked matrix, splits it and sends identical blind portions to participating labs. Each lab analyses it with its routine method. The provider compares the results with an assigned value and scores each lab, commonly with a z-score.

## In practice

A z-score between -2 and +2 is usually satisfactory; beyond ±3 is unsatisfactory and requires corrective action. [ISO/IEC 17025](https://genetixbay.com/knowledge/glossary/iso-17025.md) labs are expected to take part regularly. PT is one of the best tools against lab-to-lab bias: if one lab reads consistently high in PT rounds, that shows up. Some regulators publish PT outcomes or use them in audits. For consumers, asking a lab about its PT record is reasonable.

## Units and formula

z = (lab result - assigned value) / standard deviation for proficiency assessment. |z| ≤ 2 satisfactory, 2 < |z| < 3 questionable, |z| ≥ 3 unsatisfactory.

## Don't confuse with

- **Accreditation**: the overall assessment; PT is one input.
- **Split sample retest**: an informal check between two labs, useful but not scored.

## Related terms

[ISO/IEC 17025](https://genetixbay.com/knowledge/glossary/iso-17025.md), [Measurement uncertainty](https://genetixbay.com/knowledge/glossary/measurement-uncertainty.md), [Lab shopping](https://genetixbay.com/knowledge/glossary/lab-shopping.md), [Potency testing](https://genetixbay.com/knowledge/glossary/potency-testing.md), [Label inflation](https://genetixbay.com/knowledge/glossary/label-inflation.md)

## Read more

- [Buyer beware: potency labels, strain names and other claims that don't hold up](https://genetixbay.com/knowledge/basics/buyer-beware-label-claims.md)

## Sources

1. ISO/IEC 17025:2017. General requirements for the competence of testing and calibration laboratories. International Organization for Standardization, Geneva.