Purpose: To determine if multi-organism/multi-application reference materials could be produced and certified according to ISO Guide 34 with adequate stability, precision and performance to be used as received for commonly performed selective media methods in food safety laboratories.
Methods: E. coli (NCTC9001), K. aerogenes (NCTC8167), S. aureus (NCTC6571) and C. albicans (NCPF3255) were combined and preserved by dehydration at activity levels targeted to produce countable colonies at approximately the mid-point of 7 AOAC approved rehydratable film test methods( Methods 990.12, 991.14, 2001.05, 997.02, 2003.11). Approximately 1000 individual samples were prepared from the formulation lot. Twenty samples were selected at random for internal certification by the listed methods. Twenty ISO Guide 17025 accredited laboratories were chosen for a 3rd party inter-laboratory proficiency testing study using the listed methods. Archived samples were tested for stability as well as alternative method performance. Concurrently, single organism controls were prepared and validated similarly. Food matrix impacts were further investigated.
Results: Results of internal certification and inter-laboratory proficiency testing were statistically evaluated to determine value assignments and overall performance. Expanded uncertainties of each value assignment were determined as per ISO Guide 34 and were calculated to be about +/-10%. The %RSD of the inter-laboratory proficiency testing data with n = 20 was less than 8.8%. Mixing organisms into a single sample showed no negative effect on performance relative to single organism formulations. Stability of the CRM exceeds 8 months with testing continuing.
Significance: The performance data indicates that stable, fit for use multi-organism/multi-applications CRMs can be developed and produced.