Ultrasound and the risk of nosocomial cross infection

Hospital-acquired infection is on the increase globally. In the UK it costs the national health service approximately £1 billion per year1. Medical equipment, including ultrasound machines and transducers, may act as both a source and a vector of nosocomial infection2, 3. Under certain unfavorable circumstances ultrasound gel can become contaminated with a variety of pathogenic agents4-6. Several studies have confirmed the transmission of bacteria and viruses from a patient's skin to ultrasound equipment, the most significant organisms including Staphylococcus aureus (including methicillin-resistant strains, i.e. MRSA), Pseudomonas, Acinetobacter species, Candida albicans, hepatitis (B and C), human immunodeficiency virus (HIV) and herpes2, 7. The infection risk associated with ultrasound transducers differs according to the nature of the examination for which they are used, and can be classified, according to the system devised by Spaulding 40 years ago, as being non-critical, semi critical or critical8, 9. Non-critical probes are those that come into contact with intact skin, as occurs during abdominal, breast and small-parts examinations. Semi-critical probes come into contact with a mucosal membrane, as occurs during transesophageal, transvaginal and transrectal ultrasonography, or diseased/breached skin. Critical probes come into contact with blood and/or other body cavity fluids or internal tissues, as occurs during intraoperative ultrasonography. Transducers used for ‘semi-critical’ ultrasound examinations need high-level disinfection9. The transducer should only be used after application of a suitable sterile or aseptic cover. Latex condoms are recommended as probe covers for transvaginal and transrectal ultrasonography as they have been found to be superior to commercially available probe covers, in terms of both the cover being breached and leakage: these affect 1.7% of examinations performed with condoms compared with 8.3% of those performed with probe covers12. However, latex allergy, or at least our awareness of it, is on the increase, and an alternative non-latex cover should always be available. The allergic patient should ideally be examined first on the list, as this limits potential exposure from prior patients and procedures, and consideration should be given to other potential sources of latex in the examination room, such as the examination couch and leg supports, which should be isolated and covered accordingly. The ultrasound cover must be disposed of after completion of the examination and treated as clinical waste in accordance with local guidelines. The transducer must then be cleaned and disinfected. The disinfecting agent should have antibacterial and antiviral properties and be compatible with the material from which the transducer is made. Whilst most ultrasound manufacturers recommend the use of a 2% glutaraldehyde solution for the disinfection of contaminated transvaginal transducers, concerns have been raised as to whether this agent might be detrimental to the transducer itself and have toxic effects on human gametes and embryos9. An alternative procedure for disinfecting endocavitary transducers involves mechanical removal of the gel from the probe before cleaning the transducer with a dry tissue and then with a 70% isopropyl alcohol wipe13. This is similar to the procedure recommended for non-critical instruments. It should be noted that the effectiveness of these and other recommended methods have not been validated in rigorous laboratory experiments or in the clinical scenario9. The above recommendations apply to all endocavitary examinations and are therefore applicable to general gynecology, but they are only appropriate if the transducer cover has not been breached. If the cover splits, or has been applied incorrectly, and the transducer itself has been directly exposed, even potentially, to tissue, disinfection should be of the same standard as that recommended for ‘critical’ probes. This involves thorough cleaning of the transducer until all macroscopic debris has been removed, followed by immersion of the probe in a high-level disinfectant for an appropriate period of time; options include 5 min in 250 ppm chlorine dioxide (Tristel Fuse™ for Laboratories, Tristel Solutions Ltd, Snailwell, Cambs, UK) or 12 min in 0.55% ortho-phthalaldehyde (Cidex ® OPA Solution, Advanced Sterilization Products c/o Johnson & Johnson Medical Ltd, Ascot, Berks, UK)9, 14. The transducer should then be rinsed under running tap water and dried with a paper towel or equivalent. Work by Hignett and Claman13 suggests that all medium risk (semi-critical) procedures conducted with an endocavitary transducer should be considered as high risk (critical) when there is significant contamination by body fluids. They compared two different probe covers commercially available at the time of the study (Cook Inc, Stouffville, Ontario, Canada and SweMed International Lab, Billdal, Sweden) used during endovaginal oocyte retrieval and found that perforation affected at least 75–80% of the covers used. Even more worryingly, at least 25% of those covers not used also contained small perforations. Very few studies have assessed the potential of the coupling acoustic gel to act as a vehicle of cross contamination. Outbreaks of both Klebsiella4 and Burkholderia6 infection have been reported within ultrasound departments and the source of infection traced back to the ultrasound gel. A study by Muradali et al.3 showed that the gel could act as a culture medium and permitted bacterial growth. They recommended that, after the final ultrasound examination of the day, all transducers should be cleaned with a liquid cleaning solution such as chlorhexidine (0.05% weight/volume) to remove all traces of coupling gel which could otherwise support the overnight growth of bacteria. General hygiene during scanning is undeniably of paramount importance and its visible compromise is a common cause of complaint. Measures of general hygiene such as the use of disposable gloves and hand washing, both before and after the examination, are important and have been shown to reduce the risk of nosocomial infection and microbial transmission. Care must also be taken to ensure that contaminated gloves do not come into contact with the ultrasound machine's control panel or the transducer cable. After endocavitary scanning, first the transducer cover and then the gloves should be removed and both items disposed of carefully to prevent contamination of the surroundings with body fluids. These items must be treated as clinical waste and there should be a local policy and training provided to ensure that all ultrasound practitioners are aware of how to dispose of such material correctly and of the medical and ethical reasons, including cost implications, of incorrect disposal. Sanitary wear and clinical waste disposal systems must be in place. If a patient with bleeding has been examined it is important to check the examination couch for evidence of this and to clean the bed accordingly. The examination couch should be cleaned in accordance with the above recommendations for ultrasound transducers and must be made of material that allows for this. In conclusion, all ultrasound examinations present a genuine risk for cross infection. The exact level of risk is dependent on the type of examination conducted and is understandably higher when endocavitary transducers are used. In the absence of clear guidelines and advice, both from microbiologists and the manufacturers of ultrasound machines and transducers, it would seem sensible to adopt a standard protocol for all examinations; this should include, at the very least, removal of any residual ultrasound gel from the transducer before cleaning it with a disinfectant wipe or spray. The probe should be immersed in a suitable disinfectant if it has been used in a procedure or on a patient considered to be of ‘critical’ risk or if a probe cover has been breached during an endocavitary examination. Immersion should, in addition, be considered as a routine procedure at the end of each working day or after each completed patient list. It is important not to forget simple measures such as hand washing and the correct disposal of clinical waste, which will reduce the overall risk within any department. Further work is required to quantify the precise risk of cross infection within the field of obstetrics and gynecology and the efficacy and safety of the various decontamination procedures, both in terms of the patient and the ultrasound equipment itself and this is best achieved through collaboration between ultrasound practitioners, microbiologists and our industrial partners.

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