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Dear Editor, The European Union Reference Laboratory for Genetically Modified Food and Feed (EURL GMFF) is organizing regularly, in line with its mandate under Regulation (EU) 2017/625, Proficiency Testing rounds (PTs) primarily for EU National Reference Laboratories (NRLs) appointed in the frame of official controls for the determination of GMOs in different food and feed commodities. For more than a decade, this EURL/NRLs network is implementing fit-for-purpose PCR methods, validated by the EURL GMFF with participation of selected NRLs, is providing common analytical guidance and training, and is sharing harmonized analytical procedures with the whole European Network of GMO Laboratories (ENGL) [1]. Hence, Broothaerts et al. [2] have considered a population of competent and experienced laboratories capable of delivering reliable results for each PT round, regardless of the specifically addressed PCR target. Consequently, it had been expected that the collected results were normally distributed, which constituted the null hypothesis in the study. In the Letter to the Editor, the authors justify the application of a log transformation to GMO PT data from the Fapas GM PT scheme with how a PCR method is generating the results, i.e., with the “sequential multiplication of small numbers of genomes with responses converted to quantities of GMO material via a log-linear calibration.” However, the numbers of genomic targets multiplied during PCR are generally in the order of hundred thousands for the taxon-specific target and over 100 for the GM target, which should not be considered “small numbers”. Furthermore, the PCR amplification step itself is only part of the analytical procedure, which starts with sub-sampling, DNA extraction, total DNA quantification, and testing for the absence of PCR inhibitors in the extracted DNA. Among others, the contribution from partial inhibition to the variation of results reported by participants can be significant, particularly when dealing with complex food or feed matrices. Overall, there are linear and non-linear concentration dependencies involved in a whole GMO quantification process so that the contribution from the exponential amplification of targets to the data distribution should not be overstressed. The rational for suggesting the statistical design to be applied in the PTs organized by the EURL GMFF has been described extensively in the paper [2]. Furthermore, the authors would like to draw the attention of the Editor to the following issues:

analytical and bioanalytical chemistry/analytical and bioanalytical chemistry

Response to Letter to the Editor regarding: “Log transformation of proficiency testing data on the content of genetically modified organisms in food and feed samples: is it justified?”