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.
The EPA also has a standardized test for towelettes (i.e., wipes).5 In the past several years, pre-saturated wipes have been found to be effective and are increasingly used in healthcare facilities.6 The “towelette” test is a modification of the AOAC Germicidal Spray Products Test for Disinfectants but instead of a contaminated surface being sprayed with disinfectant it is wiped with a disinfectant “towelette”. The EPA pre-saturated “towelette” disinfection test method is used to substantiate efficacy claims for disinfectant “towelettes”.5
The disinfectant “towelette” test method involves the application of the test organism to a glass surface prior to being treated by the disinfecting wipe in a standardized manner. Following the desired contact (e.g., 1 minute, 2 minutes), the carrier (i.e., glass slides) is placed in the growth medium for 48 hours to determine if all of the test organisms (e.g., at least 105 S. aureus or Pseudomonas aeruginosa) have been inactivated. During the contact time, the liquid delivered on the glass surface by the wipe is open to drying. Thus, the reduction of microorganisms is caused not only by the inactivation produced by the disinfectant but also a physical removal of microorganisms by the wipe. The EPA towelette test does not require the test surface to remain wet during the test time but it must be undisturbed (e.g., not touched or abraded).5 Thus, the treatment time for a wipe disinfectant consists of the wet time plus wiping as well as the undisturbed “dry” time. To “pass” a 60 carrier test with S. aureus, 59/60 surfaces wiped with the ”towelette” must show no detectable growth.
The Germicidal Spray Products Test for disinfectants is done similarly and is particularly appropriate for disinfectants which do not require dilution and are designed to be used with an aerosol or trigger spray.7,8 That is, the surface is inoculated, sprayed and after the treatment time has elapsed, the treated test surface is transferred to the growth medium for 48 hours to determine if the test organisms have been inactivated. Like the “towelette” test, to “pass” a 60 carrier test with S. aureus, 59/60 surfaces sprayed must show no detectable growth of the test bacteria. The treatment time for a disinfectant spray consists of the wet time plus the undisturbed “dry” time (if any) as there is no surface wipe involved.7,8
In summary, the issue of “contact time” and “treatment time” is complex as it is based on different EPA test methods used for liquid disinfectants versus a disinfectant “towelette” or “spray”, respectively. The registration test for liquid disinfectants is the AOAC Use-Dilution Test and the contact time should be the “wet” time.4 This simulates the contact time in the test tube with the inoculated carrier. The registration test for a disinfectant wipe and the spray are the EPA Disinfectant Towelette Test and Germicidal Spray test, respectively, and the label should be interpreted as the “treatment time”.5,7,8 For the wipe, the “treatment time” is the kill time and is equal to the combination of the physical removal and inactivation caused by the disinfectant regardless of the surface appearance (e.g., wet vs dry). For the spray, the “treatment time” is the kill time or the time for complete inactivation of the test bacteria caused by the disinfectant regardless of the surface appearance (e.g., wet or dry). Therefore, if a product is a liquid disinfectant (e.g., dilutable quaternary ammonium compound) and the label indicates an EPA registration label based on the Use-Dilution Test of 2 minutes, then the treated surface should remain wet for 2 minutes. In contrast, if a disinfectant wipe or a spray has an EPA registration time of 2 minutes, then the surface (i.e., wiped or sprayed) should be allowed to remain undisturbed for the EPA registration time of 2 minute (i.e., wet time is not relevant).
Infection preventionists, environmental service workers, nurses, regulators (e.g., state and federal), and accrediting agencies (e.g., The Joint Commission) surveying healthcare facilities should be aware of the different requirements for EPA registration of surface disinfectants registered by the Use Dilution method (liquids have a contact time and wet time) and those tested by the towelette and spray test (wipes have a treatment time and no wet time). To minimize the misinterpretation of these terms (i.e., contact time, treatment time) by surveyors, some hospitals employ a tool recognized by The Joint Commission, a risk assessment. This tool is generated by the Hospital Infection Control Committee or another medical/hospital staff committee and describes how the hospital minimizes infection transmission via the environment by the use of disinfectants.
- Rutala WA, Weber DJ. Selection of the ideal disinfectant. Infect Control Hosp Epidemiol. 35:855-865.
- Donskey CJ. Does improving surface cleaning and disinfection reduce healthcare-associated infections? Am J Infect Control. 2013:S12-S19.
- Rutala WA, Weber DJ. Surface disinfection: Treatment time (wipes and sprays) versus contact time (liquids). Infect Control Hosp Epidemiol 39:219-331
- Environmental Protection Agency. Standard Operating Procedure for AOAC Use Dilution Method for Testing Disinfectants. https://www.epa.gov/pesticide-analytical-methods/antimicrobial-testing-methods-procedures-mb-05-14
- Environmental Protection Agency. Standard Operating Procedure for Disinfectant Towelette Test. https://www.epa.gov/sites/production/files/2016-05/documents/mb-09-06.pdf
- Rutala WA, Gergen MF, Weber DJ. Efficacy of different cleaning and disinfection methods against Clostridium difficile spores: Importance of physical removal versus sporicidal inactivation. Infect Control Hosp Epidemiol. 33:1255-58.
- Environmental Protection Agency. Standard Operating Procedure for Germicidal Spray Products as Disinfectants. https://www.epa.gov/sites/production/files/2014-11/documents/mb-06-08.pdf
- Microchem Laboratory. http://microchemlab.com/test/aoac-germicidal-spray-products-test-aoac-96102
- Rutala WA. http://out.easycounter.com/external/disinfectionandsterilization.org