External quality assessment
External quality assessment (ex q a) is the challenge of the effectiveness of a laboratory's quality management system. EQA typically refers specifically to the challenge of quality systems in a medical laboratory. The term external refers to the fact that an organizer outside of the laboratory's organisation provides a statement of quality to the laboratory.
Typically, an EQA scheme consists of several rounds per year. In each round several (or many) participants receive test items, which are also called samples. The EQA organizer is also called the provider. The organizer ensures that the test items are sufficiently similar and homogeneous, typically according to ISO 13528. Often the test items' properties and analyte concentrations are known to the organizer, but not disclosed to participants before the final report. However, there are also many schemes where the organizer doesn't know the sample composition. The participants' results are then compared to check if any participant had a bias towards e.g. higher values, or an unexpected imprecision. At the end of each round, the EQA organizer sends out reports and/or certificates to the participating laboratories.
Laboratories are often accredited for ISO 17025 and ISO 15189 compliance. Internationally, the International Laboratory Accreditation Cooperation establishes global recognition of these standards. Locally, conformance with ISO standards is checked by accreditation bodies. These often mandate, but always encourage laboratories to participate in EQA schemes.
Typically, EQA scheme organizers are accredited according to the ISO 17043 standard.
EQA or proficiency testing is different from quality assurance which, in a laboratory setting, is the total process whereby the quality of laboratory results can be guaranteed. This is because relying solely on the lab-internal quality management can create a false sense of safety.
For a laboratory, gaining and keeping the ISO 17025 accreditation status is of high commercial importance. For this reason, laboratory employees might treat EQA test items differently. However, PT/EQA schemes for laboratories aim to provide the laboratory management with an insight into the quality of the routine work of their laboratories. The following qualifying factors apply:
- PT/EQA results will only provide an effective insight into routine results if the PT samples are treated in the same way as routine samples.
- If PT/EQA samples are treated differently from routine samples then the PT results may be excellent but nothing will be learnt about the quality of the routine service.
There are several ways in which PT samples may be given ‘special’ treatment. They may be handled by more experienced staff than those who examine typical routine samples, subjected to more rigorous checking procedures than normal, or results and information from other participants (collusion) may be sought before reporting. These practices must be discouraged by laboratory management. If the scheme organizers suspect collusion then the laboratories concerned will be contacted.
To help to prevent malpractice and in order to gain maximum benefit from PT, management are advised to deal with situations where results for PT samples are incorrect in a sensitive manner. Problems may result from general failures in the quality system rather than from errors by individual staff. If incorrect PT results are not handled with sensitivity, staff may become defensive and will make more effort with PT samples in future to avoid further criticism. It is essential to involve staff closely in the process of quality system development. A positive approach to PT will help to reassure staff.
Statistics & scoring
In most EQA schemes, laboratories receive scores for their results. The most popular score is the Z-score, also called standard deviation index (SDI). The score is given per analyte and per test item.
PT/EQA is of paramount importance in food, water and medical testing where the impact of incorrect results can impact on people’s health. Many countries have organisations with a government remit to protect people’s health such as the Health Protection Agency in the United Kingdom.
Other very important sectors are mining: Ore value estimation is very important commercially, therefore its reliance is usually proven by laboratories with their participation in EQA schemes.