Proficiency-testing test sample

Provided the sample matrix and analyte concentration are appropriate, matrix Certified Reference Materials (CRMs) can make ideal proficiency testing samples. The assigned value is the certified value given on the certificate accompanying the CRM. The certificate will also give an uncertainty estimate for the certified value, and the use of CRMs allows the traceability of analytical data to be established. However, matrix CRM availability is limited and the materials are often expensive. Hence, Certified Reference Materials are seldom used as PT samples. 

In this case, the assigned value is obtained from data produced by a number of expert laboratories who have analysed the proficiency testing sample by using a recognized reference method. The laboratories must be able to demonstrate their... 

Little concrete information is available on the proportion of private biomonitoring laboratories that have attained CLIA certification, but CLIA regulations impose substantial penalties on noncomplying laboratories (CDC 2004). Because proficiency test samples and standard reference materials for analytes not typically measured may be unavailable (CMS 2005), comparability of private-laboratory results with reference ranges developed elsewhere may be open to question. 

After arrival, checking, and coding, the proficiency test samples and corresponding matrix blanks are usually divided into portions to allow application of multiple sample preparation methods for the different analytical techniques prior to analysis (9). 

Handling and preparation of authentic/proficiency test samples is preferably performed in a laboratory room dedicated to this purpose. The key advantage of a separate room is limitation of the possibilities of sample contamination. In previous tests, several participants have experienced cross-contamination, often with the consequence of false positive identifications and subsequent failure of the test. 

Parallel to the preparation of proficiency test samples, some laboratories do prepare quality control samples by the same methods, to check... 

Method 3, however, poses an additional challenge to the participants if no reference standard is available because it implies that the participants must have the chemical synthesized. Laboratories that are capable of synthesizing CWC-related compounds, in general, show a high competence in the analysis of authentic/proficiency test samples. Commercially available Scheduled compounds, alcohols, and thioalcohols are listed in Annex 2. 

The analysis strategy described shows what a tremendous amount of work the analysis of proficiency test samples actually holds. This example is just one approach that could possibly lead to successful completion of the analysis within the short timeline of the test. In practice, there may, of course, be other approaches leading to a similar result. 

EN17 Daggett, S.G., Greenberg, N. and Peters, C. (1991). Examples of non-physio-logical interferences in external quality control (proficiency test) samples that affect accuracy assessment. Clin. Chem. 37, 959, Abstr. 234. 

L/S ratio results show considerable differences among laboratories. For proficiency testing samples LM-09 and LM-10 from the CAP, the reported CVs were 26% (at mean L/S = 4,46) and 27% (at 2.31), respectively. ... 

College of American Pathologists proficiency test samples, Human Serum NIST SRMs Na Acidify, spike with Mg [N/MT] Determine Na with ICP-MSr from Na/ Mg ratio in a manner similar to that used in IDMS [ICP-MS] [N/MT-ICP- MS[ Long and Vetter (2002)... 

The proof of method applicability has been achieved by analysing proficiency test samples separately validated by reference methods. See the typical reconstructed compound specific mass chromatograms in Figure 4.145. 

To further assess performance, analysis of six proficiency test samples were analysed.Results of the analysis of the six soil samples were in agreement with the published reference values (Figure 5). 

BRM-2 highlighted the distinction between primary (certified) and secondary (e.g. check samples for proficiency testing) RMs. 

Magnitude of the response caused by a certain amount of analyte Alternatively, certified reference materials, samples analyzed with reference methods or proficiency test materials may be apphed... 

False-positive results with bDNA have been observed with proficiency testing specimens for HTV-1 in the College of American Pathologists HIV-1 viral load survey and HCV in the viral quality control program administered by the Netherlands Red Cross. The reason for the false-positive results with these proficiency testing specimens is not known but may be sample matrix effects. The extent to which this problem occurs with clinical samples has not been determined. However, both the HIV-1 and HCV bDNA assays were designed to have a false-positive rate of 5%. 

The principle of proficiency testing schemes consists in analyzing one or more samples sent to the laboratories by an external body. The analytical results returned to the organizer are evaluated in comparison to the assigned value(s) of the sample(s). 

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). 

Figure 7.1 Sample distribution schemes for proficiency testing schemes (a) single sample distribution (b) split-sample testing.

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. 

All proficiency testing schemes should have a statistical protocol which states clearly how the data will be processed and how laboratory performance will be evaluated. This protocol should also describe how the assigned value for any parameter in a test sample is estimated. This is an important consideration, as the performance of individual laboratories is gauged by comparison with the assigned value. 

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