Preventing Infection

Hand disinfection: Effectiveness is worth the wait

Experts are currently discussing what amount of hand disinfectant should be used, and how long it should be left on for hygienic hand disinfection. According to a new study, the length of time the disinfectant is rubbed in determines its effectiveness. The size of the hands, or the amount used, however, are less significant.

In its 2009 hand hygiene guideline, the World Health Organization (WHO) called for improved methods for evaluating the effectiveness of alcohol-based hand disinfectant. These should more accurately reflect actual use – with short exposure times and small amounts. Currently, experts are discussing what the ideal quantity is – for example, a handful – or how long the required minimum application time is – whether 15 or 30 seconds.

Various recent studies have concluded that the size of the hands and the product’s drying time (application time until the hands are dry) are important parameters, but their influence on the effectiveness is still unclear. The test method for hygienic hand disinfection EN 1500 requires that the test product must be rubbed in for a certain amount of time and that the sampling takes place immediately afterwards, so that the hands are often still wet at this time. This is a fundamental limitation in these studies, because the test method does not adequately reflect clinical practice, and the influence of the product’s drying time on efficacy cannot be adequately assessed.

A research group led by Assistant Professor Miranda Suchomel from the Institute of Hygiene and Applied Immunology at the Medical University of Vienna in Austria has slightly modified the EN 1500 test method for her latest study. Here the test product was rubbed in until the hands were dry.

Test groups with hands of different sizes

The scientists selected their 15-person test group in such a way that each of five people had “small”, “medium” or “large” hands. The researchers defined hands with a surface area of less than 375 cm² as “small” hands, hands with a surface area of 375 to 424 cm² were “medium”, and “large” hands were larger than 424 cm². In three consecutive test series. the subjects rubbed 1, 2, or 3 milliliters (ml) of isopropanol (60%) into their hands, which were contaminated with Escherichia coli, until they were dry.

The researchers recorded the drying time for each participant. They then evaluated the efficacy data gathered according to the EN 1500 method, putting it in relation to the application time and the tested amount. In a second step, the scientists investigated the influence of hand size on drying time and effectiveness.

The longer the application time, the fewer bacteria

The evaluation by the Suchomel group showed that the drying time has the greatest influence on the effectiveness of the tested alcohol. According to the study’s authors, an additional bacterial reduction of 0.29 log10 can be recorded for every ten seconds of additional application time.

log10 levels

Germ reduction is often indicated in log steps. A log10 level means a 90% reduction in the number of germs. Only 10% of the original population (100%) survived. A log level describes the reduction by one power of ten each.
In the present case, a germ reduction of an additional 49% per ten seconds of additional application time is achieved.

Other factors are the amount of alcohol used and the size of the hands, but these are not decisive.

Another surprising result of the study is that the size of the hands does not seem to be the sole factor influencing the drying time. Other parameters, such as skin moisture, skin roughness and density of hair growth, also seem to play a role.

Greater amounts are not necessarily more effective

The application time is decisive for the effectiveness of alcohol-based hand disinfectants, while the size of the hands or a specific amount of disinfectant, are not decisive criteria. In a practical example, 2 ml on small hands can be more effective than 3 ml on large hands. The results also mean that an adequate and effective dose of hand disinfectant is very individual.

The authors of the study conclude that the application time until dry (contact time) should be the determining factor for application recommendations for hand disinfectants. It remains to be investigated which would be the optimal recommendation for the application period. This should be balanced between efficacy requirements and user acceptance, so as to disrupt the workflow in patient treatment as little as possible.

Further Information

Suchomel M, Leslie RA, Parker AE, Macinga DR: How long is enough? Identification of product dry-time as a primary driver of alcohol-based hand rub efficacy. Antimicrob Resist Infect Control (ARIC) 2018 May

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