Capability of laboratories to determine core nutrients in foods: results of an international proficiency test

A proficiency test (PT) to assess the capabilities of laboratories to determine nutrients in a biscuit sample was carried out in September 2009. The need for such interlaboratory comparison arose from an increasing nutrition labeling requirements of different countries around the world. Forty-eight laboratories worldwide participated in this PT program for the determination of total lipids, saturated fat, trans-fat, protein, sugars, total dietary fiber (TDF), ash and sodium in food. This program was organized under the auspices of the Asia-Pacific Laboratory Accreditation Cooperation (APLAC). APLAC considers PT programs as one of the objective means of assessing the performance of the accredited laboratories. The program is one of the APLAC PT series whose primary purposes are to establish mutual agreement on the equivalence of the operation of APLAC member laboratories and to provide an opportunity to identify testing deficiency so that corrective actions can be taken, if necessary. The results of this program indicated that the consensus mean values estimated by robust statistics were in good agreement with the corresponding values obtained in the homogeneity tests except for trans-fat and TDF. The relative standard deviations of participant results for trans-fat and TDF were also found to be higher (19 and 24%, respectively) than the other test parameters. The relative standard deviations for other test parameters were less than 10%. One-half of the laboratories obtained satisfactory z-scores (| z | < 3) (\left| z \right| < 3) for all their reported results. Although participants were instructed to provide the measurement uncertainties of their reported results, only 27.5% of them had done so. Albeit many of the measurement uncertainties appear to be reasonable, some of them were found to vary widely, indicating that some laboratories still have difficulties in estimating measurement uncertainties. Although the overall measurement capability can be considered to be satisfactory, there is still room for improvement of analytical procedures.     

Evaluation of a proficiency test on cadmium and lead in herbal material using assigned reference values

This paper presents an international proficiency testing program (APLAC T065) on two trace elements, cadmium and lead, in an herbal sample, Herba Desmodii Styracifolii. The program was registered with a total of 109 laboratories from 42 countries. The assigned reference values of the analytes for performance assessment were provided by the organizers using an accurate gravimetric isotope dilution inductively coupled plasma-mass spectrometry technique. z-Score was used as the numerical indicator to interpret participants' competence. The between-laboratory variations for cadmium and lead were respectively 18.7% and 19.8% and the consensus values were found to be consistent with the assigned reference values. Twenty-two participants gave at least one unsatisfactory z-score, but the performance of the majority of participants on the analysis of cadmium and lead in herbal matrix was generally good when compared with the assigned reference values.     

Evaluation of the use of consensus values in proficiency testing programmes

Proficiency testing (PT) is an essential tool used by laboratory accreditation bodies to assess the competency of laboratories. Because of limited resources of PT providers or for other reasons, the assigned reference value used in the calculation of z-score values has usually been derived from some sort of consensus value obtained by central tendency estimators such as the arithmetic mean or robust mean. However, if the assigned reference value deviates significantly from the ‘true value’ of the analyte in the test material, laboratories’ performance will be evaluated incorrectly. This paper evaluates the use of consensus values in proficiency testing programmes using the Monte Carlo simulation technique. The results indicated that the deviation of the assigned value from the true value could be as large as 40%, depending on the parameters of the proficiency testing programmes under investigation such as sample homogeneity, number of participant laboratories, concentration level, method precision and laboratory bias. To study how these parameters affect the degree of discrepancy between the consensus value and the true value, a fractional factorial design was also applied. The findings indicate that the number of participating laboratories and the distribution of laboratory bias were the prime two factors affecting the deviation of the consensus value from the true value.     

Preparation of reference material for organochlorine pesticides in a herbal matrix

The development of reference material for four organochlorine pesticides, namely hexachlorobenzene and three isomers of hexachlorocyclohexane (alpha-hexachlorocyclohexane, beta-hexachlorocyclohexane and gamma-hexachlorocyclohexane), in a ginseng root sample is presented. Raw materials (Panax ginseng) were purchased from a local market and confirmed to contain certain levels of incurred organochlorine pesticide residues by a validated gas chromatography-mass selective detection method. A total of more than 300 bottles each containing 25 g of samples were prepared after the materials had been freeze-dried, milled and thoroughly mixed. The homogeneity and stability of samples from randomly selected bottles were verified and the reference values were characterized using a highly precise isotope dilution gas chromatography-mass spectrometry (ID-GCMS) method that was recently developed by our laboratory. The purity of standard organochlorine chemicals was determined against certified reference materials to establish the accuracy of the ID-GCMS analysis. The concentrations (+/- expanded uncertainty) of hexachlorobenzene, alpha-hexachlorocyclohexane, beta-hexachlorocyclohexane and gamma-hexachlorocyclohexane in the reference material were 0.198 +/- 0.015, 0.450 +/- 0.022, 0.213 +/- 0.011 and 0.370 +/- 0.032 mg kg(-1), respectively. A portion (70 bottles) of the samples was also used in a proficiency testing (PT) scheme for assessing the testing capabilities of field laboratories. The consensus mean values of the PT obtained from the 70 participants were on the same order but deviated by -2.7 to -14.1% from those of the assigned reference values. Because of the wide spread of participants' data (relative standard deviation ranging from 44 to 56%), the PT results were not included in the calculation of the assigned values of the reference materials. The materials served as suitable reference materials to ascertain the quality control and validation processes for the determination of organochlorine pesticides in herbal matrices.     

Application of the FAPAS protocol to a proficiency testing on Pb and Cd extracted from glazed ceramic

The proficiency-testing scheme (CNCA-07-06) concerning lead and cadmium determination extracted from glazed ceramic surfaces involved 43 Chinese laboratories as participants, and has been well implemented. All the test materials passed the statistical homogeneity tests, and were deemed suitable for distribution. Reference values were determined from the obtained results and the performance of participants was based on z-scores. As a result, the majority of participants obtained satisfactory z-scores and laboratories that need to revise their procedures were singled out. Overall, about 8.5% of all results submitted in this PT round were unsatisfactory in terms of z-scores (22 out of 258 total data sets submitted), while the percentage of doubtful z-scores was 4.6% (12 out of 258 total data sets). Besides, the factors that might be associated with unsatisfactory results were also evaluated and suggestions were made to help improve the performance of laboratories in this PT.     

Performance evaluation for proficiency testing with a limited number of participants

Proficiency testing (PT) is an essential tool for laboratories to assess their competency. Also, participation in PT has become one of the mandatory requirements for laboratory to seek accreditation according to ISO/IEC 17025. For this reason, the effectiveness of performance evaluation by PT scheme is of great concern for the participants and for accreditation bodies as well. In practice, owing to unavailability of other appropriate alternatives, PT scheme providers may have to choose using consensus values to evaluate the performance of participants. However, such consensus values approach was not recommended by relevant international guidelines for PT schemes with limited number of participants. With the use of Monte Carlo simulation technique, this study attempted to investigate the effectiveness of using consensus values for performance evaluation in PT schemes with limited number of participants. The simulation process was schemed according to the statistical model provided by ISO 5725-1 for laboratory measurement results, which covered components like method bias, laboratory bias, and measurement precision. The effectiveness of the consensus value approach was expressed as the percentage of participants in a simulation run could get the same evaluation result, either satisfactory or unsatisfactory, against the “true value.” The findings indicated that the number of participants, choice of consensus values, mass fraction of analyte, method bias, laboratory bias, and measurement repeatability of participating laboratories would all affect the effectiveness of the consensus value approach but at different extent. However, under certain circumstances, use of consensus value could still be considered as an acceptable approach for performance evaluation even the number of participants was limited. Some of the findings were further verified using real data from PT schemes where appropriate certified reference materials or reliable reference values were available.     

Certification of total arsenic, dimethylarsinic acid and arsenobetaine contents in a tuna fish powder (BCR-CRM 627)

Tuna fishes were collected in the Straits of Messina (Italy), were dissected and dorsal muscles minced, freeze-dried, ground and sieved. The obtained powder was stabilised by γ-irradiation and filled into brown glass bottles. The homogeneity and stability at +20?°C and +40?°C were verified with regards to the total arsenic, dimethylarsinic acid and arsenobetaine contents. Ten laboratories participated in the certification study. All participants had demonstrated beforehand their ability to produce accurate results for the As speciation in fish tissue. The certified values are: total arsenic (4.8 ± 0.3) mg/kg, arsenobetaine (52 ± 3) μmol/kg, dimethylarsinic acid (2.0 ± 0.3) μmol/kg. The material is available from the BCR since early 1998.     

纺织品中4-氨基偶氮苯的测定能力验证分析

“P00012-2011-T07纺织品中禁用偶氮染料的测定(测试项目:4-氨基偶氮苯)”能力验证严格按照能力验证计划运作要求实施,并采用稳健统计法对测试结果进行统计,测试样品指定值为54.8 mg/kg,标准化四分位距(NIQR)为6.8941 mg/kg.共有12个省市、自治区的61家实验室参加本次能力验证,采用Z比分数对各实验室结果进行评价,结果满意率为86.9％.     

Interlaboratory comparison for the determination of five residual organochlorine pesticides in ginseng root samples by gas chromatography

An interlaboratory comparison study for the determination of 5 residual organochlorine pesticides (hexachlorobenzene and 4 hexachlorocyclohexane isomers) in ginseng root was performed. This program [Asia Pacific Laboratory Accreditation Cooperation (APLAC) T049] was the first of its kind for an herbal matrix and involved the participation of 70 laboratories from 26 countries worldwide. Consensus mean values were computed statistically from the reported results, which were eventually used to assess the performance of individual laboratories in terms of the z-scores. The distribution of analytical data was found to be widespread, with standard deviation ranging from 43.9 to 55.9%, and the result patterns obtained were similar to those residue pesticide programs using other matrixes. Although the estimation of measurement uncertainty is a crucial requirement for all quantitative tests for laboratories that meet the requirements of International Organization for Standardization/International Electrotechnical Commisssion (ISO/IEC) 17025, some laboratories in this program had difficulties and showed unfamiliarity with respect to that quality criterion. It was recommended that laboratories review and rectify the situation promptly so that they would have a better understanding of measurement uncertainty or the test service provided.     

Certification of total arsenic, dimethylarsinic acid and arsenobetaine contents in a tuna fish powder (BCR-CRM 627)

Tuna fishes were collected in the Straits of Messina (Italy), were dissected and dorsal muscles minced, freeze-dried, ground and sieved. The obtained powder was stabilised by γ-irradiation and filled into brown glass bottles. The homogeneity and stability at +20 °C and +40 °C were verified with regards to the total arsenic, dimethylarsinic acid and arsenobetaine contents. Ten laboratories participated in the certification study. All participants had demonstrated beforehand their ability to produce accurate results for the As speciation in fish tissue. The certified values are: total arsenic (4.8 ± 0.3) mg/kg, arsenobetaine (52 ± 3) μmol/kg, dimethylarsinic acid (2.0 ± 0.3) μmol/kg. The material is available from the BCR since early 1998. Received: 31 March 1998 / Revised: 20 July 1998 / Accepted: 25 July 1998     

A review of proficiency testing scheme in Republic of Korea: in the field of water analysis

For regulatory purposes, it is often required to check the quality of environmental chemical analyses by routine proficiency testing (PT). In this study, the PT scheme of environmental laboratories in Republic of Korea has been reviewed. Especially, real water-based reference materials (RMs) were prepared and distributed for PT schemes. The consensus values from the PT were calculated by robust statistics with its standard uncertainties. The relative bias between the reference values and the consensus values was used as one of the indicators for the determination of the assigned values. The relationship between the performances of the laboratories, expressed by Z scores, and the concentration levels of the RMs were also revealed.     

The BIOREMA project—part 3: International interlaboratory comparison for bio-ethanol test methods

The main objective of the reference materials for biofuel specifications (BIOREMA) project is the development of two test materials (one bio-ethanol material and one biodiesel material) with well-established reference values. Of a series of three papers, this part describes the material preparation, homogeneity study, stability study, and characterisation of the bio-ethanol material. The test material thus obtained was used in an interlaboratory comparison (ILC) to assess current practices and comparability amongst laboratories providing bio-ethanol testing services. Only 13 participants provided data, resulting in a small dataset for evaluation. Further, it appeared that for a number of laboratories, there was not sufficient material for the determination of all requested parameters. In most cases, as far as the data permit, it can be concluded that the consensus values (based on participant’s results) are in good agreement with the reference or the BIOREMA values (obtained by NMIs participating in the project). For three parameters, namely ethanol content, water content, and density, there is good agreement between the reference and consensus values. For these parameters, the reproducibility standard deviation is close to, or even smaller than, the expanded uncertainty associated with the reference value. A number of parameters show very poor reproducibility, for example, pH_e, electrolytic conductivity, and acidity. The same applies to sodium and copper content, which are very low and therefore challenging parameters to measure accurately. The results of the ILC underpin the need for certified reference materials and demonstrate the requirement for more robust quality control to improve the precision and trueness of the results from testing laboratories.     

玩具油漆涂层中有机锡检测能力验证样品研制及应用分析

制备了用于可迁移有机锡检测能力验证的玩具油漆涂层样品,并开展了相关测定能力验证计划,采用迭代法、四分位法和敏感分析法3种不同统计方法对23家实验室数据进行统计分析,并对不满意结果的原因进行了分析。结果表明所研制样品的均匀性和稳定性良好,能满足能力验证要求。不同统计方法对实验室能力评定结果存在差异,但指定值差异很小,而能力评定标准差差异较大,运用卡方检验能选择出更合理的统计方法。     

Key metrological issues in proficiency testing––response to “Metrological compatibility-a key issue in further accreditation” by K. Heydorn

A discussion of proficiency testing (PT) topics started by Heydorn (Accred Qual Assur 15:643–645, 2010) is continued in the present paper. The role of PT in the accreditation of testing/analytical laboratories, the use of consensus values (average or weighted average, median, observed standard deviation, etc.) and a metrological background of PT schemes are discussed. It is shown that metrological traceability, comparability, and compatibility, as well as commutability of a reference material, are the key issues of any PT scheme that applies certified reference material as test items. Metrological compatibility of PT results in such schemes is a property demonstrating the closeness of the PT results to the certified value in comparison with the measurement uncertainty of their difference. The metrological background is especially important for the selection and use of PT schemes for a limited number of participants (fewer than 30) as detailed in IUPAC/CITAC Guide on the topic published in 2010 in Pure Appl Chem 82(5):1099–1135.     

The challenges and benefits of implementing the requirements of ISO/IEC 17043 by PT/EQA providers

ISO/IEC 17043 Conformity assessment??general requirements for proficiency testing is intended to replace previous international guides used to assess the competence of proficiency testing (PT) providers. It expands on the requirements of previous guides and is intended to accommodate PT providers of calibration laboratories and testing laboratories handling both qualitative and quantitative data. QMP-LS is an office-based external quality assessment provider for medical laboratories in Ontario, Canada and operates 46 different PT schemes for approximately 250 diagnostic tests. In 2010, these schemes were accredited to ISO/IEC 17043. Schemes included tests from the following disciplines: chemistry, hematology, microbiology, transfusion medicine, cytology, histology and genetics. Thirty of the schemes were qualitative. The challenges and benefits of implementing ISO/IEC 17043 are discussed, with particular emphasis on clauses addressing the following requirements: statistical design, determination of assigned value, homogeneity and stability testing, packaging, labeling and distribution, performance evaluation and subcontracting services.     

APLAC interpretation and guidance on the estimation of uncertainty of measurement in testing

The Asia Pacific Laboratory Accreditation Cooperation (APLAC) is the organization representing the community of bodies that accredit testing and calibration laboratories throughout the Asia Pacific region, including China and North America. APLAC members assess the competence of laboratories and grant them accreditation. They also provide mutual recognition of the equivalence of their accreditations, which facilitates international trade and acceptance by regulators. Harmonization of accreditation requirements is therefore vital to maintaining Mutual Recognition Arrangements. APLAC establishes management and technical criteria for accreditation and publishes guidance documents for accreditation bodies and laboratories.     

国际实验室间转基因产品检测能力验证的研究

以转基因大豆为例，介绍开展APLAC T034国际间实验室转基因检测能力验证的项目。对于样品量值(添加值)的设定进行试验，对样品的均匀性和稳定性进行检验，并对参试实验室的测试结果进行系统的评价和分析。影响转基因产品测试结果的关键因素是检测方法、基因组DNA提取的质量、PCR反应体系中模板DNA的加入量．以及是否使用标准物质作为质控对照和使用定量标准物质求出相关系数对定量检测结果进行校正。     

Development of a proficiency testing scheme for a limited number of participants in the field of natural water analysis

A proficiency testing (PT) scheme was developed for a limited number of analytical laboratories participating in the analysis of natural water in Israel. Three fit-for-intended-use reference materials (RMs) were prepared for a pilot PT from natural water: RM-blank and two in-house reference materials (IHRMs) with seven analytes fortified to achieve different levels of analyte concentrations. The measurands for the PT participants were the traceable spike values certified in the IHRMs, i.e., added mass concentrations of the analytes. The RM-blank and IHRMs were found to be homogeneous and stable over 11 days, the time necessary for the experimental part of the PT. The RMs were distributed to the PT participants as unknown test items similar to routine samples. The test results were evaluated using their deviations from the IHRM-certified values. Eight Israeli laboratories took part in the interlaboratory comparison. Individual laboratory performance and metrological compatibility of the PT results of the participants, as a local group of laboratories, were evaluated for every analyte.     

Development of a reference material for residues of chlorfluazuron and fluazuron in beef fat ACSL CRM 3

The certification by interlaboratory testing of a candidate reference material (ACSL CRM 3) for the concentration (mass fraction) of the pesticides chlorfluazuron (CFZ) and fluazuron (FZN) is described. The certification process was based on a two-stage nested design described in ISO Guide 35. Analysis of results reported by collaborating laboratories provided an assessment of the homogeneity of the RM. The certified values were chlorfluazuron -1.01 mg/kg and fluazuron -1.02 mg/kg. The upper and lower 95% confidence limits for CFZ and FZN were calculated to be: CFZ upper=1.05 mg/kg, lower=0.97 mg/kg; FZN upper=1.08 mg/kg, lower=0.95 mg/kg. A certificate was prepared according to guidelines set out in ISO Guide 31.     

Proficiency testing in food microbiology: experience from implementation of ISO/IEC 17043 and ISO/TS 22117

The proficiency testing (PT) scheme ??AQUA?? for food microbiology was organised by the Istituto Zooprofilattico Sperimentale delle Venezie (IZSVe) according to ISO/IEC 17043 and ISO/TS 22117. This paper describes the IZSVe experience on the application of the above-mentioned standards for the PTs, with focus on the Enterobacteriaceae enumeration one. Freeze-dried food matrices contaminated with American Type Culture Collection bacterial strains were used as test samples for each microbiological PT organised by IZSVe. The sample homogeneity and stability were verified prior to distribution to participants and throughout the PT, respectively. The participating laboratories analysed samples using their routine methods, and results were transmitted to IZSVe. Data and methods used by each participating laboratory were analysed in order to evaluate the laboratory performance. With reference to the Enterobacteriaceae PT, the test samples were homogeneous and stable. In addition, most laboratory results were obtained using equivalent test methods. Statistical approaches applied to analyse data generated from all participating laboratories revealed similar outcomes as no significant outlying count and only 5?% of unacceptable results were observed. Finally, the z-score, with the standard deviation that does not vary from round toround, was applied to compare and to evaluate the performance of each laboratory over time highlighting possible persistent trends over several rounds.