On the compositing of samples for qualitative microbiological testing

The introduction of legislative microbiological criteria for foods [Official J L338 (2005) 1] has increased the exposure of enforcement and industrial laboratories to the need to test multiple food samples for organisms such as salmonellae. A consequence has been an increase in the number of organizations, both official and commercial, considering whether it is permissible to composite replicate sample units for the purposes of assessing compliance with the criteria. This note summarizes the statistical and practical aspects of compositing sample units for compliance testing. Provided that the method of choice is 'fit for purpose', then there is no statistical difference between testing, say, 30 x 25 g sample units or 3 x 250 g sample units. However, compositing of sample units should be done only when the method has been demonstrated unequivocally to be sufficiently sensitive to detect potentially lower numbers of target organism(s) in the quantity of composited sample under the conditions of test. Different approaches to compositing of samples are considered.