by William A. Rutala, PhD, MPH,1 and David J. Weber, MD, MPH1, 2
In 2014, we published a paper on the “Selection of the Ideal Disinfectants”.1 Disinfectant selection (i.e., disinfectant product) is one of the two essential components for effective disinfection. The other component, the practice, is the thorough application of the disinfectant such that the disinfectant contacts all contaminated surfaces. In addition, there should be proper training of hospital staff (especially environmental services and nursing) and use of an EPA-registered disinfectant active against the concerning microbes (e.g., sporicidal agent for Clostridium difficile). The combination of “product” and “practice” results in effective surface disinfection and leads to the reduction of patient risk and improved patient outcomes.1, 2 One of five key considerations in the selection of an ideal disinfectant is commonly referred to as the “contact time”.1 The purpose of this paper, which was recently published, is to provide a better understanding of “contact time” for liquids and “treatment time” for disinfectant wipes and sprays.3
The method that has been used for over 60 years for EPA registration of liquid and dilutable liquid disinfectants for hard surfaces is the Association of Official Analytical Chemists (AOAC) Use-Dilution Method. In brief, this is performed by immersing stainless steel carriers in bacteria (e.g. Staphylococcus aureus), treating them with the disinfectant for a prescribed time, and then placing the carriers in growth media to determine if there are any surviving bacteria.4 With this test, all microbes on a carrier (e.g., ≥1,000,000 S. aureus on a stainless steel carrier) must be inactivated to result in a negative carrier. If ≥1 organism survives, the carrier is positive and you must have nearly all carriers negative (e.g., performance standard for S. aureus is 0-3 positive carriers out of 60) to support EPA registration. This method is a qualitative test (i.e., pass/fail based on any bacterial growth) rather than a quantitative method (e.g., determines the log10 reduction such as 5-log10 reduction in 1 minute). Thus, this Use Dilution test measures the ability of the disinfectant to inactivate the test organism (normally about 106 test organisms per carrier) in the disinfectant in a measured time.