It is important to guarantee that results expressing the GM content are reliable, comparable and fulfil the requirements of existing EU legislation. The use of different measurement units to express a GM content, the appearance of new analytical methods that do not require a calibrant and the composite EU legislation on GMOs have triggered the need for a document to clarify how to obtain reliable and comparable results.
For this guidance document, past and current EU legislations have been reviewed with a special emphasis on what is meant by 'GM percentage' in the different legal texts. The metrological traceability of measurement results and the currently available guidance are explained and summarised. The particular case of botanical impurities and the genetic constitution of GM seeds are described and illustrated to better understand the complexity hidden behind this type of analysis. An overview of the different analytical methods based on DNA measurements and used for the expression of quantitative GM content results is provided, including the use of new techniques based on digital PCR (dPCR).
A measuring system that allows for comparing results by making them traceable to the same reference system has been elaborated in detail. Needs and tools are described and a solution has been proposed to convert results expressing GM content to the required measurement unit, whenever this is needed.
By following these recommendations, results obtained in GM copy number per haploid genome equivalent (cp/HGE) by dPCR can be converted into mass fraction percentage and compared to the results obtained by quantitative PCR (qPCR) either with a calibrant certified for its GM mass fraction or with a calibrant certified for its GM purity.
The general principle is to relate a measurement result to a GM quantity embedded in a specified certified reference material (CRM) either directly or via one single conversion factor (CF) per event. This conversion factor and its related uncertainty need to be determined precisely for each CRM batch, preferably on the pure GM CRM (100 %), using, for example, dPCR. The estimated uncertainty associated with this conversion factor must be integrated into the measurement uncertainty of the final results expressed in GM mass fraction.
CF are currently not yet established for most CRMs. CF values have been recently reported in a few pioneer dPCR studies. However, such proof of concept studies remain incomplete. Therefore, to avoid a gap between new technologies and current EU regulation, the working group recommends to launch a dedicated study to determine CF values. Such a study should involve a limited number of competent laboratories with a proven experience in dPCR. The study could be coordinated by the EURL-GMFF.