Reproducibility problem with a proposed standard method to measure disinfection efficacy
A collaborative study was carried out in four laboratories to determine the reproducibility of a proposed ASTM International standard method for quantitatively evaluating the efficacy of disinfectants on hard, non-porous surfaces against bacteria. The method, known as the Quantitative Method, has also been suggested as a future regulatory standard for the United States and internationally. The multi-lab study was carried out using Pseudomonas aeruginosa and an alkyl dimethyl benzyl ammonium chloride antimicrobial product diluted in hard water. Results of the study showed acceptable repeatability in log10 reductions within each laboratory, but unacceptable reproducibility across laboratories despite careful analyst training and standardization of test conditions. A follow-up study ruled out analyst-to-analyst differences as the cause of the poor reproducibility. As it currently exists, the Quantitative Method is not sufficiently reproducible. Ruggedness testing to assess the sensitivity of the method to small changes in operational factors is recommended.