Proficiency testing for quality assurance of allergens methods.

Proficiency testing is an external quality control check, whereby the quality of an analytical result is checked against criteria that are set independently of the laboratory carrying out the analysis. Participants in a proficiency test are encouraged to use the method of their choice to determine the analyte in question. The collated results submitted by the participants are used to derive the best estimate of the 'true' level, or assigned value, of the analyte, as a consensus value of the whole data set. Generally, the data submitted will be normally distributed and from a single population, but if a data set is found to be multimodal, then the selection of one of the modes as the assigned value is possible where there is supporting data, typically methodology information. Unless there are independent grounds for preferring one mode over another, it is not possible to set an assigned value or calculate z-scores. However, the analysis of allergens has presented proficiency testers with a new challenge, since it has become apparent that quantitative results may be dependent on the brand of enzyme-linked immunosorbent assay kit used, the specific analyte targeted (e.g. total content or allergen protein content) and the limit of detection achievable. FAPAS has run more than 40 proficiency tests for allergen analysis over the past 7 years, during which time methods have been developed and improved and the requirements for determination of food ingredient allergens has increased. Two case studies are presented which highlight some of the issues around the use of allergen measurement methods.

Scoring in genetically modified organism proficiency tests based on log-transformed results.

The study considers data from 2 UK-based proficiency schemes and includes data from a total of 29 rounds and 43 test materials over a period of 3 years. The results from the 2 schemes are similar and reinforce each other. The amplification process used in quantitative polymerase chain reaction determinations predicts a mixture of normal, binomial, and lognormal distributions dominated by the latter 2. As predicted, the study results consistently follow a positively skewed distribution. Log-transformation prior to calculating z-scores is effective in establishing near-symmetric distributions that are sufficiently close to normal to justify interpretation on the basis of the normal distribution.

Acrylamide analysis: assessment of results from six rounds of Food Analysis Performance Assessment Scheme (FAPAS) proficiency testing.

Six proficiency tests have now been completed in an ongoing program of the UK Food Analysis Performance Assessment Scheme (FAPAS) for the analysis of acrylamide in a range of food matrixes. Homogeneous test material samples were requested by laboratories throughout the world, with 29 to 45 submitting results for each test. Results were analyzed by appropriate statistical procedures, and z-scores were awarded for reported values. In the absence of both legislation and collaborative trial data, the target standard deviation was derived from the Horwitz equation, although it is acknowledged that there is a need to establish a "fit for purpose" target standard deviation specifically for acrylamide analysis. Participants were encouraged to use the analytical method routinely used in their own laboratory and to provide details of their procedure. Close examination of the data submitted indicates that performance is generally acceptable in terms of accuracy. There is no significant difference between results submitted by gas chromatography and liquid chromatography (GC and LC) methods, and no method dependency on the use of internal standards or sample size. However, choice of extraction solvent may be important, with indications that plain water is an acceptable extraction method. There is evidence from the most recent test that direct (underivatized) GC methodology may present problems, but more data are required and this aspect will be monitored in the continuing proficiency testing program.

Testing for bias between the Kjeldahl and Dumas methods for the determination of nitrogen in meat mixtures, by using data from a designed interlaboratory experiment.

Bias between the Dumas and the Kjeldahl methods for the determination of protein nitrogen in food was studied by conducting an interlaboratory study involving 40 laboratories and 20 different test materials. Biases were found to be small and statistically significant only for the chicken test materials, where a bias of 0.020±0.004% m/m was detected.

A comparison of the Kjeldahl and Dumas methods for the determination of protein in foods, using data from a proficiency testing scheme.

Both the Kjeldahl and the Dumas methods for the determination of protein in foodstuffs are currently in use, but the empirical nitrogen factors used to convert the determined nitrogen content to protein content are based on the Kjeldahl method alone. Non-equivalence between the two methods could therefore result in some laboratories reporting an incorrect protein content. We report here a study using data accumulated over several years in the results of a proficiency testing scheme. On average the Dumas method provided results that were relatively higher by about 1.4% than the Kjeldahl method, but the difference between the methods depended on the type of foodstuff. The methodology of looking for bias between analytical methods is critically discussed.