Proficiency-testing analysis

Figure 3. Model for proficiency test analysis and reporting strategy...

Assuming an analytical error of 2 % which is based on the analysis of the results of a number of proficiency tests and collaborative trials (Daas and Miller 1998), a maximum uncertainty of 0.24% would imply a 0.1% probability of rejecting a good result. Given that the uncertainty of a content to be assigned is below a predetermined value, the results of the collaborative trial are acceptable otherwise it is recommended to repeat the trial in whole or in part. 

Fox A (2000) AOAC International Offers Leadership in Proficiency Testing. Food Testing and Analysis, 5(6) 7-g. 

There are two main types of proficiency testing scheme. First, there are those set up to assess the competence of a group of laboratories to undertake a very specific analysis, e.g. lead in blood or the number of asbestos fibres in air collected on membrane filters. Secondly, there are those schemes used to evaluate the performance of laboratories across a certain sector for a particular type of analysis. Because of the wide range of possible analyte/matrix combinations it is not practicable to assess the performance of laboratories when analysing all the possible sample types. Instead, a representative cross-section of analyses is chosen (e.g. determination of different pesticide residues in a range of foodstuffs or the determination of trace levels of metals in water samples). 

In some sectors, particularly clinical analysis, proficiency testing is referred to as External Quality Assessment (EQA). 

There is no experimentally established optimum frequency for the distribution of samples. The minimum frequency is about four rounds per year. Tests that are less frequent than this are probably ineffective in reinforcing the need for maintaining quality standards or for following up marginally poor performance. A frequency of one round per month for any particular type of analysis is the maximum that is likely to be effective. Postal circulation of samples and results would usually impose a minimum of two weeks for a round to be completed and it is possible that over-frequent rounds have the effect of discouraging some laboratories from conducting their own routine quality control. The cost of proficiency testing schemes in terms of analysts time, cost of materials and interruptions to other work has also to be considered. 

Thus, as for the European Union, the requirements are based on accreditation, proficiency testing, the use of validated methods of analysis and, in addition, the formal requirement to use internal quality control procedures which comply with the Harmonised Guidelines. Although the EU and Codex Alimentarius Commission refer to different sets of accreditation standards, the ISO/IEC Guide 25 1990 and EN 45000 series of standards are similar in intent. It is only through these measures that international trade will be facilitated and the requirements to allow mutual recognition to be fulfilled will be achieved. 

As shown above, these include a laboratory to be third-party assessed to international accreditation standards, to demonstrate that it is in statistical control by using appropriate internal quality control procedures, to participate in proficiency testing schemes which provide an objective means of assessing and documenting the reliability of the data it is producing and to use methods of analysis that are fit-for-purpose . These requirements are summarised below and then described in greater detail later in this chapter. 

The list has been constructed on the premise that contractors will use methods of analysis that are appropriate and accredited by a third party (normally UKAS), participate in and achieve satisfactory results in proficiency testing schemes and use formal internal quality control procedures. In addition, Parts B and C are made available to the potential contractors so that they are aware of what other demands will be made of them and can build the costs of providing the information into their bids. 

Verification is confirmation by examination and provision of objective evidence that specified requirements have been fulfilled. ISO 9000 2005 Proficiency testing is a periodic assessment of the performance of individual laboratories and groups of laboratories that is achieved by the distribution by an independent testing body of typical materials for unsupervised analysis by the participants. [lUPAC Orange Book]... 

We also can take suitable data from a proficiency test (PT). In this case the laboratory must have participated in the PT successfully. We also have to consider, if the PT covered all relevant uncertainty components and steps of analysis. This includes e.g. if the matrix of the PT sample was similar to routine samples. If this is the case, we again calculate U from 2 Sr. 

Koch M, Metzger, JW (2001) Definition of assigned values for proficiency tests in water analysis. Accred. (Jual. Assur. 6, 181-185... 

Two UK proficiency testing schemes are the Food Analysis Performance Assessment Scheme (PAPAS), which replaced NAMAS in 2000, which is arranged by the Central Science Laboratory, with details at http //ptg.csl.gov.uk/fapas.cfm... 

CX/MAS 02/12 (2002), Codex Alimentarius Commission, Codex Committee on Methods of Analysis and Sampling (FAO/WHO), Validation of methods through the use of results from proficiency testing schemes, agenda item 8c of the 24th session, Budapest, Hungary, Nov. 18-22, 2002. 

The method of analysis is either prescribed, when this is regulated by law, for example, or the choice is left to the participant. In the latter case the method is reported with the result. In keeping with the aims of a proficiency testing scheme, the method used should be the routine method employed by the laboratory, and not some enhanced protocol designed to improve performance in the scheme. 

In a farsighted move in 1989, the European Union laboratory IRMM started a series of interlaboratory comparisons to provide objective evidence for the degree of equivalence and the quality of chemical measurements by comparing a participant s measurement results with external certified reference values (IRMM 2006). At the time most proficiency testing schemes used consensus results for the mean and standard deviation to derive z scores. With the IMEP-1 analysis of lithium in serum, the world was alerted to the problem of lack of accuracy in analytical measurements. The data of the first IMEP-1 trial are replotted in figure 5.6 notice that the apparent outlier was the only laboratory to come close to the assigned value. 

Recognizing that heading off a potential problem is usually better (cheaper and less embarrassing) than coping with a nonconformance, the standard requires the laboratory to engage in preventive actions. This is defined as a proactive process to identify opportunities for improvement. The use of trend and risk analysis of proficiency testing data (see chapter 5) is mentioned as a possible preventive action. 

Some important assays commonly used in biochemical genetics laboratories do not provide quantitative data (e.g. MPS-EP, qualitative urinary organic acid analysis, AA-TLC). In addition, all successful investigations depend heavily upon selection of the correct analytes to measure and the appropriate interpretation of the quantitative or qualitative results in their clinical context. These challenges suggest a requirement for external quality assessment or proficiency testing schemes that can inform participants about their performance in these areas when compared with other centres. 

Among the elements of quality control in mycotoxin analysis, proficiency tests, control materials (reference materials and certified reference materials), traceability in spiking, and recovery checks have been demonstrated to be particularly relevant. 

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