Interlaboratory studies proficiency testing

Characteristic Collaborative/Interlaboratory (Method Performance) studies Proficiency-Testing Schemes... 

Proficiency testing is a special type of laboratory performance studies defined as study of laboratory performance by means of ongoing interlaboratory test comparisons (ISO Guide 33 [1989] ISO/REMCO N 280 [1993] IUPAC [1993] Prichard et al. [2001]). Proficiency testing is an essential part of external quality assessment schemes and performance checks. 

The previous chapters of this book have discussed the many activities which laboratories undertake to help ensure the quality of the analytical results that are produced. There are many aspects of quality assurance and quality control that analysts carry out on a day-to-day basis to help them produce reliable results. Control charts are used to monitor method performance and identify when problems have arisen, and Certified Reference Materials are used to evaluate any bias in the results produced. These activities are sometimes referred to as internal quality control (IQC). In addition to all of these activities, it is extremely useful for laboratories to obtain an independent check of their performance and to be able to compare their performance with that of other laboratories carrying out similar types of analyses. This is achieved by taking part in interlaboratory studies. There are two main types of interlaboratory studies, namely proficiency testing (PT) schemes and collaborative studies (also known as collaborative trials). 

This chapter has considered two of the types of interlaboratory comparison exercise in which your laboratory may participate. It is important to remember that proficiency testing schemes and collaborative studies have different aims. The former is a test of the performance of the laboratory, whereas the latter is used to evaluate the performance of a particular analytical method. Laboratories should participate in proficiency testing schemes (where an appropriate scheme is available) as this provides an independent check of the laboratory s performance. This chapter has described the key features of proficiency testing schemes and explained how the results from participation in a scheme should be interpreted. 

Any or all of these conditions can be varied. To provide some guidance, intralaboratory reproducibility is used to express changes only within a laboratory, and interlaboratory reproducibility" is used to refer to the changes that occur between laboratories, for example in proficiency testing, interlaboratory method validation studies, and the like. Interlaboratory reproducibility is usually two to three times the repeatability. 

If data are normally distributed, the mean and standard deviation are the best description possible of the data. Modern analytical chemistry is often automated to the extent that data are not individually scrutinized, and parameters of the data are simply calculated with a hope that the assumption of normality is valid. Unfortunately, the odd bad apple, or outlier, can spoil the calculations. Data, even without errors, may be more or less normal but with more extreme values than would be expected. These are known has heavy-tailed distributions, and the values at the extremes are called outliers. In interlaboratory studies designed to assess proficiency, the data often have outliers, which cannot be rejected out of hand. It would be a misrepresentation for a proficiency testing body to announce that all its laboratories give results within 2 standard deviations (except the ones that were excluded from the calculations). 

Maier, E A., Quevauviller, Ph, and Griepink, (1993), Interlaboratory studies as a tool for many purposes proficiency testing, learning exercises, quality control and certification of matrix materials. Analytica Chimica Acta, 283 (1), 590-99. 

Residue-testing laboratories might also need to review their sample preparation processes and consider modifying or eliminating tissue homogenization prior to residue extraction. Suppliers of proficiency-testing services should also question whether certain drugs are appropriate to include in such studies. For example, liver spiked with sulfaquinoxaline, sulfadiazine, or sulfamerazine is not suitable for preparation of spiked interlaboratory check samples or reference materials. 

LEDOUX M and hall s, Proficiency testing of eight French laboratories in using the AOAC mouse bioassay for paralytic shellfish poisoning interlaboratory collaborative study , J. AOAC Int 2000 83 305-10. 

Attempts to correlate analytical performance with other seemingly indicative laboratory characteristics, such as participation in proficiency testing schemes, regular use of certified RMs, number of years of experience and number of samples analysed per year were all equally unsuccessful. Therefore, in the absence of any simple and obvious means of identifying and preselecting only reliable laboratories as participants in certification studies, an investigation was undertaken of the validity of adopting the consensus mean (after outlier elimination) from an interlaboratory study as a certified value. 

Laboratory performance study An interlaboratory study that consists of one or more measurements by a group of laboratories on one or more homogeneous, stable, test samples by the method selected or routinely used by each laboratory. The reported results are compared with the consensus value. Note, this is very similar to a Proficiency Testing scheme (see below). 

A Proficiency Test (PT) is defined as the study of laboratory performance by means of ongoing interlaboratory test comparisons . It is also known as an external quality assessment scheme, external laboratory performance check or external quality assurance (EQA). There are many such schemes run by independent external bodies for different analytes in a variety of matrices. Evidence in published papers shows that the performance of analytical laboratories improves as a result of participating in Proficiency Testing schemes and the betw een-laboratory precision can improve, sometimes dramatically. This is especially true in the early years of participation. 

Consequently, it seems clear that a value obtained through simple interlaboratory studies or proficiency tests should never be accepted as certified. Such studies do not... 

Needs for interlaboratory studies Method development and validation Proficiency testing... 

Values of I z I over 2 should lead the concerned laboratory to investigate rapidly for the reasons of this weak performance before the next trial of the interlaboratory study starts. An example of z-score is shown in the section 12.5.6. Figures 12.3a-c show graphs of z-scores obtained in proficiency testing schemes carried out within the QUASIMEME (see section 12.5.6) and MICROBATH (bathing water microbiology method development study) frameworks. 

In the field of QA/QC the network will check regularly on the availability of methods for high-priority substances and their validation status, and initiate, whenever necessary, the organisation of interlaboratory studies for specific emerging substances. QA/QC information, such as available reference materials, proficiency testing schemes and training programmes for laboratories will be circulated. 

For interlaboratory studies leading either to decisions which could affect the professional status of the participants e.g. proficiency testing within accredita-... 

A full description of the methods is usually not required for proficiency tests or for standardization purposes it is, however, an essential element when the performance of a laboratory or a method has to be evaluated, e.g. in the frame of step-by-step projects. The BCR has elaborated reporting forms for the description of various types of analytical methods for the certification of inorganic and organic parameters (see Appendix), which may be used as a basis to prepare forms for all types of interlaboratory studies. The questions are intended to remind the participants which important parameters are to be taken into consideration (possibly affecting the quality of the final result). These forms can also be used in technical discussions with other participants when differences in the results are noticed. Finally, they may constitute the basis for the method description in the certification reports. 

Starink R J Visser R G Audier M, Results of Proficiency Test. Phthalates in PVC. Institute for Interlaboratory Studies, Dordrecht, the Netherlands, April 2002. 

This approach corresponds to interlaboratory studies organized on a voluntary basis for specific purposes (e.g., proficiency testing). In this case, the selected laboratories and methods may be very diverse and not necessarily prepared of submitting results aimed at establishing assigned values to a RM. The... 

Principles and Characteristics According to the lUPAC definition, an interlaboratory study is one in which several laboratories measure a quantity in one or more identical portions of homogeneous materials under documented conditions, the results of which are compiled into a single report. Three types of interlaboratory studies are distinguished, namely method-performance, laboratory-performance or material-certification studies. The aim of method-performance or collaborative studies is to assess the performance characteristics of a specific method. In laboratory-performance or proficiency studies a homogeneous test material is analysed of which the true concentrations are known or have been assigned in some way. The participants apply whatever... 

NARL studies are designed to test the proficiency of laboratories by using test samples that resemble real world samples. Homogeneity of the test samples is established at a level that is sufficient for the purpose of the study taking into consideration the degree of interlaboratory variability appropriate for the particular analysis. 

Accuracy The closeness of agreement between a test result and the accepted reference value. Interlaboratory comparison (ILC) The organization, performance, and evaluation of tests on the same sample by two or more laboratories in accordance with predetermined conditions to determine testing performance. According to purpose, they can be classified as collaborative studies or proficiency studies. 

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