Why participate in proficiency testing

Laboratory measurements are usually carried out under a management system, usually based on ISO/IEC 17025, which specifies the use of validated methods, (certified) reference materials, control charts, etc. These provisions contribute substantially to measurement quality, but in practice one has to consider that the performance of validated methods may vary over time: (certified) reference materials may not be readily available or suitable for the specific test; control charts may not disclose bias, etc. PT is a complementary tool in that it provides an external and independent assessment of the validity of the measurement results. As such it assesses the entire quality system.

Thus, participation in PT can be very useful for laboratories, irrespective of its outcome. An unsatisfactory performance in PT aids in the identification and correction of problems in the measurement process. Such corrections might, for example, include additional operator training, modification of methods or equipment and improvement of data processing. Satisfactory performance in PT gives third parties confidence in a laboratory's ability to perform a given measurement. These parties include (potential) customers, regulatory authorities and accreditation bodies.

Regular participation over successive rounds has even greater benefits as it enables the monitoring of trends in performance over time, which can lead to sustained credibility and improvement, irrespective of whether or not the laboratory needs to participate for recognition or accreditation purposes.

Participants are normally expected to use the measurement method of their choice, which should be consistent with the routine procedure used in their laboratory. If a laboratory deliberately deviates from routine practice while measuring PT items (e.g. by using the best analyst, performing multiple analyses etc.), much of the added value of PT will be lost.