Efficacy Tests and Standards

As the understanding of antimicrobial materials has improved, regulators and working groups such as the US Environmental Protection Agency (EPA) and the Organisation for Economic Co-operation and Development (OECD) have recognised that there are no standardized methods for the determination of efficacy under conditions typical of the environment under which a material is expected to be employed. This is especially the case with regard to indoor surfaces and touch points.

The OECD Working Group under the Inter-Organisation Programme for the Sound Management of Chemicals (IOMC) represents the interests of a number of countries and regions. It has proposed a tiered system approach¹ with Tier One test methods providing proof of principle and Tier Two methods more closely representing in-use environmental conditions.

Tier One testing

Tier One tests are adequately covered by standards such as BS EN ISO 22196: Measurement of antibacterial activity on plastics and other non-porous surfaces. This establishes the basic effectiveness of a material under warm, damp conditions: 35^oC and >90% humidity over a period of 24 hours. This, and the Japanese Industrial Standard on which it is predicated (JIS Z 2801), is the basis on which most early technical and marketing claims were made and often persist in promotional literature. This is now seen as inappropriate, or possibly dangerous, in that efficacy under the test conditions has been shown² not to persist under typical indoor settings where humidity is often 50% and temperatures only 22 – 25^oC.

Tier Two testing

Tier Two tests are still in the committee stage of various Standards Organisations and the OECD recommends a pragmatic, best practice approach to test protocols while new standards are under development. OECD also recommends allowing the inoculum to dry and significantly reduced test times. The development of appropriate Tier Two test standards will enable better  comparisons and the wider implementation of these technologies.

Antimicrobial Copper testing

Through dialogue with the EPA, a set of three Tier Two methods was established based upon existing tests for liquid disinfectants designed to be applied to hard surfaces. In particular, pre-drying of inocula on test coupons and a 2 hour assessment period were recommended.  Its development included extensive discussions with experts in the infection control community, the Association for Professionals in Infection Control and Epidemiology (APIC) and the American Society for Healthcare Environmental Services (ASHES). These test methods incorporated the more recent OECD recommendations regarding test conditions and times.  See EPA Tests and Approval for details of the three EPA tests.

The table below compares the conditions of the Tier Two  'Efficacy as a Sanitizer' EPA test with the Tier One JIS Z 2801 (ISO 22196)

Table comparing conditions of antimicrobial efficacy tests for hard surfaces

Efficacy test conditions	Antimicrobial Copper	Silver-containing coatings
Protocol	EPA test protocol	Japanese Industrial Standard JIS Z 2801 (ISO 22196)
Time	2 hours	24 hours
Exposure	Open to atmosphere	Plastic film to retain moisture
Drying time	10 - 20 mins	Remains wet
Temperature	Room temperature (~20°C / 68°F)	Close to body temperature (~35°C / 95°F)
Relative humidity	Ambient (~50%)	>90%
Inoculum level	High (>10 million CFU)	Lower (500,000 – 1 mi CFU)

Table comparing conditions of EPA Efficacy as a Self-Sanitizer test with JIS Z 2801 (ISO 22196)

Industry best practice antimicrobial efficacy test

The EPA Efficacy as a Self-Sanitizer Tier Two (2-hour kill) test has been developed into a detailed protocol by AlControl Laboratories to make it accessible to all those looking for an appropriate test for hard surfaces in the absence of a published standard. See the Find Services page for labs familiar with this test protocol.

The protocol is provided in the downloads below along with the certificates of analysis for 13 commonly available approved copper alloys. The test certificates include the alloy ISO designation, test results (including controls) and a statement confirming the test has been passed. The alloys are all those listed in Publication 214:  Antimicrobial Copper Alloys – Guidance on Selection.

Further test development

In order to better understand the broad spectrum efficacy and mechanisms at play, developments of the Tier Two tests have been used. 

Instead of using the very high microbe count in the inoculum (as originally specified in the liquid disinfectant test), progressively reduced numbers of MRSA bacteria were put onto a set of test samples³.  This showed that when more real-world contamination levels were used (10³ CFU/cm²), copper alloys were able to kill the microbes in 15 minutes rather than the 2 hour test period required by the EPA methods.

 

Click to enlarge

At the University of Southampton, Environmental Health Unit, work  has shown that by reducing the inoculum volume from 20 µl to just 2 µl the drying phase was reduced to a minimum whilst still maintaining excellent repeatability.  This small droplet size closely approximates to a human touch contamination event.  Perhaps surprisingly, this showed that copper alloys worked significantly faster (10 million bacteria killed in less than 10 minutes) under these drier deposition conditions than the EPA method⁴.  Logically, using a real-world microbe inoculum, kill times would be even faster.

 

Click to enlarge

Further test development

Standards bodies and regulators are working on developing new standards based on test methods that reflect in use conditions.