Practical Quality Control: the Experiences of a Public Health Laboratory

Abstract

In the 1930’s W.A. Shewhart pioneered the application of statistical principles to the quality control (QC) of production processes, eventually publishing the landmark book “Economic Control of Quality of Manufactured Products” (Shewhart, 1931). In this book, he states that a phenomenon is under control if its future variation can be predicted (within limits) based on previous experience. This is precisely the idea behind the control charts used in measurement processes—specifically, for chemical analysis. The International Organization for Standardization (ISO), in its standard ISO 9000 (ISO, 2005a), defines quality control as “the part of quality management focused on fulfilling quality requirements”. According to the standard, quality management also includes quality planning, quality assurance and quality improvement. The above definition is rather vague, because quality management systems based on the ISO 9000 family of standards can be applied to any kind of organization regardless of its field of activity, its size or whether it is from the public or private sectors. Testing laboratories typically distinguish between internal and external QC. In this context, the International Union of Pure and Applied Chemistry (IUPAC, 1998) gives a definition of internal QC that is well-suited to an analytical laboratory: “the set of procedures undertaken by laboratory staff for the continuous monitoring of operation and the results of measurements in order to decide whether results are reliable enough to be released”. Although the aforementioned document does not formally define external QC, it does mention that external control may be done by submitting blind samples to the measuring laboratory. This activity can be organized in the form of a collaborative test. The aim of these QC activities is to verify that the quality parameters of an analytical method ascertained in the method validation are maintained during its operational lifetime. Thus, method validation or revalidation tasks are periodic activities that end with a validation report, whereas QC activities are recurrent activities implemented in routine work. Apart from the use of fully validated methods, QC assumes the use of properly maintained, verified and calibrated equipment, reagents and consumables with the proper specifications; standards with well-established traceability; and qualified technicians working in suitable environmental conditions. However, fulfilling all these requirements is not enough to ensure the delivery of appropriate quality results over time: a laboratory’s capacity to produce technically correct results must be continuously monitored. Indeed, according to Thompson et al. (Thompson & Lowthian, 1993), QC is the only quality