AOAC Germicidal Spray Test, Modified for Viruses

Originally developed in 1961, the AOAC Germicidal Spray Test is a standard method for evaluating the efficacy of liquid disinfectants on hard non-porous surfaces. This method is particularly appropriate for disinfectants which do not require dilution and are designed to be used with an aerosol or trigger spray. This method may be modified to allow for virucidal efficacy testing.

Summary of the AOAC Germicidal Spray Test Method, Modified for Viruses

• The first decision that should be made prior to performing this test is the number of carriers to be evaluated. There are two primary options:

1. Ten Individual Carriers: Each carrier receives a 0.01 ml inoculum over a 1″x1″ area.
2. One 10-in² Equivalent Carrier: One glass petri dish receives a 0.200 ml inoculum over its entire surface area.

• Viral stocks are thawed and diluted as appropriate, for a recoverable titer of 4.00 log₁₀ TCID₅₀ per carrier. 
• If the product is to be marketed as a “one step” cleaner/disinfectant, the test culture is supplemented with organic soil, usually fetal bovine serum.
• Test carriers are inoculated with test culture by evenly spreading it across the selected surface area(s), then dried.
• Appropriate plate recovery controls are harvested prior to test initiation to determine pre-test carrier microbial concentrations.
• The dry, contaminated test surfaces are then sprayed, individually, with test product.
• Contaminated test surfaces incubate in the disinfectant for a specified contact time.
• After the contact time has elapsed, neutralization of the test substance is performed and sterile cell scrapers are used to remove the viral films from the surface of each carrier. 
  
• Appropriate plate recovery controls are enumerated following test initiation to determine post-test carrier microbial concentrations.
• Dilutions of the harvested test substance-viral suspension are applied to permissive host cell monolayers, and incubated as required by the test virus (typically 7 days).

Strengths of the AOAC Germicidal Spray Test Method

• The AOAC Germicidal Spray Products Test, Modified for Viruses, will provide comparable data to bacteriacidal Spray Tests approved by the EPA for data submission in conjunction with registration of spray (aerosol/trigger-spray) disinfectants sold in the USA.
• The AOAC Germicidal Spray Products Test is currently acceptable for EPA data submission requirements in conjunction with registration of spray (aerosol/trigger-spray) disinfectants sold in the USA.

Weaknesses of the Test Method

• The AOAC Germicidal Spray Products Test, Modified for Viruses is subject to additional variability resulting from ambiguities of the official method. For example, the official test method does not address important parameters such as humidity level during drying of the glass test surfaces and standardization of the concentration of bacteria on the glass test surfaces.
• While certainly less removed from “real-life usage” than the AOAC Use-Dilution Test, the AOAC Germicidal Spray Products Test, Modified for Viruses is a somewhat unrealistic test model. Specifically, products undergoing testing are applied to the contaminated surfaces in large volumes relative to the amount of surface area, causing the product to “pool” on the test slides. If a consumer were to use the same volume of product per unit of surface area that is used in the test, then they would literally require gallons of product to disinfect their kitchen!
• Additionally, usage directions specifying 10 minute “wet dwell” times (the required dwell time for most products) simply aren’t practical and aren’t adhered to by the vast majority of consumers of spray disinfectant products.
• The AOAC Germicidal Spray Products Test, Modified for Viruses, may be removed as an acceptable method for EPA submission efficacy testing. 

Microchem Laboratory has put a great deal of time and thought into perfecting the way we execute the AOAC Germicidal Spray Test, Modified for Viruses. We take many measures to reduce test-to-test variability, and often recommend the method as a high-level screen for spray disinfectant products being considered for registration with the EPA or claim optimization in the United States.