Infection prevention and control in ultrasound - best practice recommendations from the European Society of Radiology Ultrasound Working Group

A recent publication has shown that bacterial contamination of ultrasound (US) transducers is significantly higher than contamination of public toilet seats and bus poles [1]. Once any surface is contaminated, pathogens can survive a prolonged period of time [2, 3]. Even in proven cases of infection transmission, the exact route may remain unclear and insufficiently decontaminated needle guides as well as post-procedures follow-up US examinations without high level decontamination merit consideration [4, 5]. This highlights the need for thorough standardised decontamination protocols.

National guidance and legislation regulating decontamination procedures vary throughout Europe. European guidance on interventional US procedures has already been published stressing amongst other issues the importance of good hygiene [6]. However, detailed European guidance on transducer decontamination, choice of US gel and transducer covers is lacking. There remains a wide range of practice amongst European US practitioners, as shown in a survey of the European Society of Radiology (ESR) US Working Group (WG) [7].

Radiologists from the ESR US WG, together with expert microbiologists, formed a multi-disciplinary group who as a first step undertook the survey mentioned above [7]. This identified considerable variations in practice and apparent confusion as to what is best practice.

A detailed literature review of available US-specific evidence was carried out with a variety of PubMed searches of publications from 1990 to 2017, including international and national surveys and guidelines, observational studies, case reports and opinion pieces. In the absence of research systematically addressing the specifities of US procedures and their respective environments and the presence of such a heterogeneous evidence base, it was not possible to grade the evidence or to indicate the strength of existing guidelines. Therefore, the decision was taken to formulate the best practice recommendations hereby presented.

These recommendations have been derived from reviewing the evidence base as obtained above and applying key principles of prevention of cross-infection in the healthcare setting where there is no published specific evidence derived from the US environment. Subsequently, these consensus recommendations were discussed and agreed by the WG members who undertook this task, stressing that they need to be incorporated into local guidelines and must be compliant with respective national legislation. Transducers can be vectors of infection transmission with most serious outbreaks relating to endoscopic US procedures [8‐11]. Risk evaluations have been attempted but subsequently disputed; hence, the exact risks will remain uncertain [12‐14].

Evidence shows that adequate protocols combined with staff training can achieve efficient disinfection [1]. It is the hope of the ESR US WG that publication of these recommendations, despite their limitations in terms of the evidence base, will raise awareness and improve the training of all US practitioners, thus ultimately contributing to improvements in patient safety.

In principle, once any surface has been colonised, pathogens can survive for periods of time longer than many might expect [2]. This applies in particular to synthetic materials including US transducer surfaces and other parts of the US equipment. Survival times on dry inert surfaces of bacteria such as Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA) can be several months or longer, and that of viruses such as hepatitis A, herpes simplex virus (HSV) and rotaviruses several weeks (Table 1). Even fungi such as Candida albicans can survive up to 120 days. In addition, post-contamination survival will be even longer with co-existent organic material such as skin cells or body fluids, providing a protective nidus for microbes which even disinfectants may not fully penetrate.

Ultrasound examinations and procedures carry different risks depending on the likelihood of exposure to the normal bacterial flora of patients (as discussed below), contact with body fluids and the degree of invasiveness of the US procedure. Adopting a modified Spaulding classification¹ [15], procedures can be classified into:

Non-critical: non-invasive, contact of US transducer with intact skin only, requiring low level disinfection.

or

Critical: invasive, such as US-guided punctures or injections, contact of the US transducer with mucous membranes and body fluids, or a combination of both.

The reasoning for these recommendations is as follows:

US-assisted invasive procedures range from minimal invasive fine needle aspirations to endoscopic and intra-operative use of US. When assessing the risk of transmission of infective agents, all these procedures have in common the breaching of the intact skin or mucous membranes. Taking acupuncture as an example of a minimally invasive procedure: fine acupuncture needles have been demonstrated to carry viral material after treatment of hepatitis C-positive patients [16]. In cases of directly US-assisted punctures, contact of the transducer with infected materials cannot be excluded. Consequently any US-assisted invasive procedure or any procedure potentially causing micro-trauma to the skin or mucous membranes has to be categorised as critical.

The category “semi-critical” as detailed in the Spaulding classification describes devices that are in contact with intact mucous membranes of non-sterile body sites such as the vagina. Because the integrity of these mucous membranes cannot be taken for granted and procedure-associated micro-trauma can never be excluded, this category has been omitted. The generally accepted recommendations for disinfection are similar to those for critical procedures, i.e., transducers require high level disinfection (HLD) or sterilisation.

Elimination of all microbes including bacterial and fungal spores. This is usually achieved through autoclaving (using steam under high pressure) or exposing instruments to high temperatures; thus it is not suitable for US transducers. Current methods of sterilisation do not inactivate prions.

Chemical sterilisation by exposing medical devices to chemical agents such as peracetic acid, hypochloric acid, etc., is possible. Nevertheless, is it not considered to be fully equivalent to heat/steam sterilisation and chemicals may cause transducer surface damage. Furthermore, most of the agents used for chemical sterilisation are likely to pose a health hazard to both patients and staff through direct skin contact or inhalation.

Worldwide there are an increasing number of infection prevention surveys and guidance documents available, most recently from the Australasian Society for Ultrasound in Medicine and the Australasian College for Infection Prevention and Control [20, 21]. Detailed Scottish guidance was published in 2016 [22‐24], and subsequently adapted for Ireland [25]. Welsh guidance is available from 2014 but mainly focuses on endoscope decontamination [26]. References to hygiene can be found in 2016 Society and College of Radiographers and British Medical Ultrasound Society Guidelines for Professional Ultrasound Practice [27]. UK results of the large World Federation for Ultrasound in Medicine and Biology (WFUMB) survey were published in 2016 [28].

In Germany, Merz et al., like others, favours automated systems for high level disinfection, in particular devices using hydrogen peroxide (Trophon® EPR), now approved by the US Food and Drug Administration [29, 30]. Another important aspect of automated systems is the standardised and reproducible decontamination process thus avoiding operator-associated errors or variations. Ultraviolet (UV) light is less effective in eradicating microbes in comparison to hydrogen peroxide [31]. However, comparing different methods of decontamination is outside the scope of this publication. A publication by Rutala last year emphasises the need for HLD of all semi-critical and critical devices, already detailed in the original American guidance from 2008 [32, 33]. The American Institute of Ultrasound in Medicine (AIUM) also formulated guidance in 2014 [34] and French guidance and a survey were recently published [35, 36]. As previously mentioned, guidance from the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) in the interventional US setting is available [6], but no recent European Directive relating to this topic could be found.

Published evidence highlights contamination risks of US transducers, even with the use of transducer covers, and after LLD. Although published cases of proven infection transmission through US are limited at present, this should not induce complacency as the true rates are unknown. Many practitioners do not seem to adhere to or be aware of basic infection prevention procedures as shown in several recent surveys. This emphasises the need for new and improved standards.

With publication of these best practice recommendations the ESR US WG aims to stress:

US practitioners cannot always know which patients carry transmissible pathogens or who may be susceptible to acquiring infections; hence, high standards of infection prevention and control will ensure that all patients are kept safe.

These recommendations should be reviewed and locally adapted in accordance with respective national guidance, where available, to suit the respective clinical environment. Three flow charts (see Appendix) are included with this set of recommendations to assist US practitioners and others in achieving best practice.

Published evidence is limited and partly dated, therefore publication of evidence-based guidance at present is challenging, but needed. We hope that more emerging evidence will make this possible in the near future through the conduct of audits, surveys and even clinical trials where possible.

The ESR US WG is already in discussions with the European Coordination Committee of the Radiological Electromedical and Healthcare IT Industry (COCIR) and the ESR Patient Advisory Group. A joint session with discussion was organised at the European Congress of Radiology in March 2017 and both groups were consulted in the process of writing these recommendations. It would be desirable for manufacturers of US equipment, transducer covers, US gel and cleaning/disinfection consumables/equipment to collaborate further with US practitioners and patient representatives to improve standards. Additional research studies on the susceptibility of pathogens to practicable, low/non-toxic and affordable disinfection measures are needed, and how these can be best applied in the context of US.

We recognise that implementing thorough US decontamination protocols will necessitate an initial investment and increasing ongoing consumable costs as well as additional staff training. However, we believe that the implementation of clear recommendations will reassure patients, and contribute to the quality of their care.