Guidance for the in-house preparation of quality control materials (QCMs)

First edition   2014-08-15

    Reference materials (RMs) are widely used in measurement laboratories for a variety of purposes and it is important to recognize that the material most appropriate for a particular application should be used. Certified reference materials (CRMs), i.e. those which have property values and associated uncertainties assigned by metrologically valid procedures are primarily used for method validation and calibrations providing metrological traceability.

    The preparation of reference materials for metrological quality control (i.e. control of the quality of measurements not products) is an important activity which provides materials suitable for the day-today demonstration that a particular (part of a) measurement system is under statistical control. Such materials do not require characterization by metrologically valid procedures, and can be prepared “inhouse”,i.e. by laboratory staff familiar with their behaviour, to fulfil specific quality control requirements.

    Reference materials which are sufficiently homogeneous and stable are necessary for metrological quality control purposes, such as demonstrating a measurement system is under statistical control, performs as expected and provides reliable results; where the trueness of the measurement result is not critical. Different industries use various terminologies to describe such materials (e.g. in-house reference materials, quality control materials, check samples, etc.). For the purposes of this Guide, the term “Quality Control Materials” (QCMs) will be used to simplify repeated citation. While CRMs are produced by established reference material producers and are commercially available,

QCMs are often prepared by a laboratory for its own internal use. Frequently, QCMs are characterized only for a limited scope (a limited number of property values) and for specific laboratory applications.

    The rationale for preparing quality control materials can be one or a combination of the following factors:

    — to have an RM representing as closely as possible routine samples, suitable for quality control;

    — to have a suitable day-to-day RM to complement a commercially available CRM;

    — no suitable CRM exists;

    — the application does not require a material having the full characteristics of a CRM (e.g. traceability and uncertainty for specified property values).

    QCMs are RMs and as such the requirements of ISO Guide 34[1] for the production of RMs apply. However, if the material is only used in-house by the preparing laboratory, some requirements (e.g. for transport stability) can be relaxed. The preparation of a QCM is related to that of a CRM and those preparing QCMs may wish to consult ISO Guides 34[1] and 35[2] for further guidance. Where appropriate, this Guide will refer to relevant parts of these Guides.

   It is recognized that the aim of many laboratories requiring QCMs is to minimize the time and effort needed to prepare the materials. To this end, many laboratories use samples of real products for which there is a body of analytical data available. A number of case studies are included as annexes of this guidance document to provide examples of how such data may be processed to confirm fitness for purpose of the materials.