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Disinfectant testing using a modified use-dilution method: collaborative study
E C Cole1, W A Rutala, G P Samsa
1University of North Carolina School of Medicine, Department of Medicine, Chapel Hill 27599.
Abstract:
An initial collaborative study of the AOAC use-dilution method (UDM), used for bactericidal disinfectant efficacy testing, demonstrated extreme variability of test results among the 18 laboratories testing identical hospital disinfectants. In an effort to improve the method, 32 changes were made by the UDM Task Force. These changes represented improvements in quality assurance practices and elimination of method variability; however, the basic framework of the method was retained. A second collaborative trial was conducted to determine if the interlaboratory variability of test results could be reduced to an acceptable level using the modified UDM. Twelve of the original 18 laboratories participated in the second study. Each laboratory processed 60 penicylinders (P) for each of the 6 randomly selected, federally registered disinfectants and 3 test organisms (Staphylococcus aureus, Salmonella choleraesuis, Pseudomonas aeruginosa). The number of positive penicylinders (greater than 1 positive P/60 replicates = failure) for the 6 products when P. aeruginosa was used as the challenge organism ranged 1-30, 0-36, 0-15, 0-5, 0-3, and 0-60 for the 3 quaternaries and 3 phenolics, respectively. The results of the variance components analysis for P. aeruginosa and the other 2 organisms showed that the variance components for laboratories were not significantly reduced for any organism in this study. Such interlaboratory variability of results questions the use of the original or the modified UDM for registration purposes.
Insights
The AOAC use-dilution method (UDM) for disinfectant testing showed significant variability among laboratories even after modifications. This interlaboratory variability questions the UDM's reliability for disinfectant registration purposes.
Area of Science:
- Microbiology
- Infectious Diseases
- Public Health
Background:
- The AOAC use-dilution method (UDM) is a standard for testing bactericidal disinfectant efficacy.
- An initial collaborative study revealed extreme variability in UDM results across multiple laboratories.
- This variability raised concerns about the method's reliability for disinfectant registration.
Purpose of the Study:
- To assess if modifications to the UDM could reduce interlaboratory variability in disinfectant efficacy testing.
- To determine if the revised UDM is suitable for regulatory purposes.
Main Methods:
- A second collaborative trial involved 12 laboratories testing 6 disinfectants against 3 standard organisms (Staphylococcus aureus, Salmonella choleraesuis, Pseudomonas aeruginosa).
- Laboratories processed 60 replicates per disinfectant and organism using the modified UDM.
- Variance components analysis was used to quantify interlaboratory variability.
Main Results:
- Significant interlaboratory variability persisted for all tested organisms, including Pseudomonas aeruginosa.
- The modified UDM did not significantly reduce the variance attributed to different laboratories.
- Results showed a wide range of positive penicylinders across laboratories for the tested disinfectants.
Conclusions:
- The modifications made to the UDM did not adequately reduce interlaboratory variability.
- The continued variability questions the suitability of both the original and modified UDM for disinfectant registration.
- Further improvements are needed to ensure consistent and reliable disinfectant efficacy testing.