Interlaboratory evaluations of a standardized quantitative test method for determining the bactericidal and tuberculocidal efficacy of antimicrobial substances on hard non-porous surfaces

The development, validation, and use of new quantitative methodologies for testing the effectiveness of antimicrobial products are necessary to meet the regulatory challenges associated with an ever-changing marketplace, novel product claims, new infection control practices, and the emergence of new clinical pathogens. A series of four interlaboratory studies were conducted in a standardized manner on an interim quantitative method for testing liquid treatments against bacteria to assess its statistical performance. The Quantitative Method, a derivative of ASTM E2197, is designed to enumerate the number of viable microbes remaining on a test carrier following exposure to a liquid antimicrobial treatment; a log₁₀ reduction (LR) in viable bacteria is calculated based on the difference between the mean log₁₀ density values of the untreated control and treated carriers. The Quantitative Method uses 1 cm diameter disks (carriers) of brushed stainless steel as the material to represent a hard, non-porous surface. The LR value is used as the measure of product effectiveness, where higher LR values are indicative of greater microbial kill. The test microbes were Staphylococcus aureus, Pseudomonas aeruginosa, and Mycobacterium terrae. The liquid antimicrobial treatments used in these studies were highly relevant to those in the marketplace and provided a wide range of mean LR outcomes. The focus of the statistical assessment was on the repeatability of the LRs across experiments within a lab (Sᵣ) and the reproducibility of the LRs across labs (SR). Due to the additional sources of variability, the SR is expected to be higher than the variability within a laboratory (Sᵣ); this was observed in the studies reported here. Across the studies, the Sᵣ values for LR were small (i.e., less than 0.84), most notably for treatments generating high mean LRs (5 or above) where the Sᵣ was as small as 0.12. Overall, the SR values ranged from 0.227 to 1.217. Only three of the twenty-four treatment combinations over the study period resulted in SR values above 1.0 – the associated LRs for the three treatments ranged from 2.22 to 3.26. Antimicrobial treatments with a LR of 4.5 or higher exhibited SR of 0.561 or less. The statistical attributes reported here for the draft Quantitative Method when used to test P. aeruginosa, S. aureus, and M. terrae provide information for decision makers when considering the method as a candidate regulatory procedure. The data and statistical analyses contained in this report are historical in nature and provide useful baseline information for individuals conducting additional technical review of the method. Based on the data, the Quantitative Method displays a statistical profile consistent with other standard methods approved by standard-setting organizations where method performance data are available.