Expert View

By Dr. George Anastasopoulos

Technical & Intl. Business Development Mgr., PJLA

I recently received the following question from a calibration laboratory: “We would like to request clarification regarding the distinction between interlaboratory and intralaboratory programs in the context of ISO/IEC 17025 compliance. Specifically, we seek confirmation on whether both types of programs are considered acceptable evidence of ongoing competence for the purposes of maintaining accreditation.”
 

First, I would like to note that this question is also valid for a testing laboratory. So my reply is applicable to both Testing and Calibration laboratories:

An interlaboratory program, often referred to as an interlaboratory comparison (ILC), involves two or more independent laboratories measuring the same or similar items under predetermined conditions and comparing the results. 

Its primary purpose is to evaluate comparability between laboratories. It can be performed in a more formal process through proficiency testing (PT) organized by an external provider. 

ILC’s are performed according an interlaboratory protocol, a documented set of standardized instructions defining how an interlaboratory comparison or proficiency testing exercise will be conducted, including the purpose, scope, participant requirements, sample handling, test methods, data reporting, statistical analysis, confidentiality, and reporting procedures, ensuring all participants follow consistent conditions for valid and comparable results.

In interlaboratory comparisons, statistical z-scores or performance ratings are used to benchmark a laboratory’s results against those of other participants, providing an objective measure of performance and identifying deviations from the assigned reference value. They are numerical indicators to show how a laboratory’s result compares to the overall group. A z-score tells you how many “standard deviations” your result is from the accepted value. 

Usually when:

• |z| ≤ 2 → Satisfactory result (within normal variation)
• 2 < |z| < 3 → Questionable result (possible issue to investigate)
• |z| ≥ 3 → Unsatisfactory result (significant deviation)

In contrast, an intralaboratory program, or intralaboratory comparison, is carried out within a single laboratory, where different analysts, instruments, or methods are used to measure the same or similar items under controlled conditions. This is intended to verify internal consistency and assess the repeatability and reproducibility of results within that laboratory. 

In intralaboratory comparisons, internal control charts are graphical tools used to monitor measurement results over time, helping detect trends, shifts, or out-of-control conditions within the same laboratory. Repeatability data measures the variation when the same operator, equipment, and method are used under identical conditions over a short time, while reproducibility data measures the variation when different operators, equipment, or conditions within the same lab perform the same test. 

Together, these tools provide ongoing evidence of internal consistency, method stability, and staff competence.

For accreditation purposes, particularly under ISO/IEC 17025, the distinction matters because the two approaches address different but complementary aspects of competence. 

Interlaboratory comparisons provide objective evidence of performance against external peers, help detect systematic bias, and are often required when formal PT is available, as referenced in ISO/IEC 17025:2017 clause 7.7.2 and ILAC P9:01/2024 “ILAC Policy for Proficiency Testing and/or Interlaboratory comparisons other than Proficiency Testing”. 

Intralaboratory comparisons, on the other hand, serve as an ongoing internal quality control tool, supporting the monitoring of methods, personnel, and equipment performance in accordance with clause 7.7.1. 

Accreditation bodies expect laboratories to demonstrate both types of evidence—interlaboratory programs to confirm traceability and external comparability, and intralaboratory programs to prove active internal quality assurance practices. 

Using only intralaboratory checks risks masking biases, while relying solely on interlaboratory PT may overlook day-to-day operational issues. 

A balanced approach ensures continuous verification of competence, strengthens measurement assurance, and reduces the likelihood of nonconformities during accreditation assessments.

I hope this helps!

Disclaimer: The above recommendations are offered solely as examples of good practice and do not constitute formal advice or assurance for achieving or maintaining accreditation.

About the Author

Dr. George Anastasopoulos is the Technical and International Business Development Manager at Perry Johnson Laboratory Accreditation, Inc.(PJLA), a leading U.S.-based accreditation body operating globally across multiple technical and scientific sectors. He also serves as General Secretary of the International Personnel Certification Association (IPC).

A Mechanical Engineer with an MSc and PhD in Applied Mechanics from Northwestern University, Dr. Anastasopoulos is an active member of several international standards committees, including ISO/TC 176, ISO/CASCO, and ASTM, contributing to the development of ISO 9001 and ISO/IEC 17025. He is also a regular participant in IAF and ILAC global accreditation activities.

He has received the EOQ Presidential Georges Borel Award for his international achievements in promoting quality worldwide and has published extensively in journals and conferences. As a keynote speaker and project leader, he has contributed to numerous initiatives in conformity assessment, management systems, and quality assurance across the U.S., EU, and beyond. He can be contacted at ganast@pjlabs.com