Glossary
The "modular approach” versus the "global approach"
A typical representation of an analytical process is shown in Figure 1. After collection, a laboratory sample is homogenized and screening for the target analyte (e.g., microorganisms, DNA, protein, residues, etc.) is performed applying appropriate methods. The results are collected and interpreted according to the final aim (legal, research, development etc).
The method validation approach of an analytical process can be distinguished into two classes: (a) the “global approach”, wherein the process from sample to end-result is considered as a single unit, and (b) the “modular approach”, wherein the validation of the process is subdivided in validating each distinct step of the process independently. A valid analysis can then be obtained through the combination of validated methods for each distinct step with each other.
While the global approach is very useful in case of limited number of different matrices and/or targets to be covered, the modular approach has considerable advantages when matrices are variable but targets are similar or contain sufficiently similar elements to be analyzed together. The set of VTEC PCR methods is a good example of such a situation.
The area where the modular approach has been extensively documented and legally implemented is GMO traceability (1, 2, 3), where real-time PCR is by now the most commonly applied technology in the analytical step (5). At each step or module, only methods that fulfill particular performance criteria can be applied as it is essential that the output of a particular module does not influence/bias the outcome of any consecutive step (Figure 1). Therefore minimal performance criteria for each module have to be established. Technical parameters relevant to qualitative PCR methods, as applied in microbiology include PCR method specificity, the dynamic range, the PCR efficiency, and the limit of detection (LOD); these parameters have been selected as the ones to be included in the validation of the module “qualitative PCR”(4).
References
1. Community Reference Laboratory for GM Food and Feed (CRL-GMFF). 2009. Event-specific method for the detection of dried-killed bacterial biomass PT 73 derived from E. coli GM strain AG3139 using real-time PCR. (CRLVL04/08VP)
2. European Commission. 2003. Regulation (EC) No 1829/2003 of the European Parliament and of the Council on genetically modified food and feed.
3. European Commission. 2003. Regulation (EC) No 1830/2003 of the European Parliament and of the Council concerning the traceability and labeling of genetically modified organisms and the traceability of food and feed products produced from genetically modified organisms and amending Directive 2001/18/EC.
4. Jasson, V., L. Jacxsens, P. Luning, A. Rajkovic, and M. Uyttendaele. 2010. Alternative microbial methods: an overview and selection criteria. Food Microbiol. 27:710-730.
5. Perelle, S, F. Dilasser, J. Grout, and P. Fach. 2005. Detection of Escherichia coli serogroup O103 by real-time polymerase chain reaction. J. Appl. Microbiol.98:1162-1168.