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US Probes: Risk of Cross Infection and Ways to Reduce It-Comparison of Cleaning Methods¹

¹ From the Departments of Radiology (C.F.) and Microbiology (D.M.), University Hospital of Wales, Heath Park, Cardiff, CF4 4XW United Kingdom. From the 1998 RSNA scientific assembly. Received June 29, 1998; revision requested July 31; revision received October 14; accepted February 12, 1999. Address reprint requests to D.M.

	Abstract

TOP Abstract Introduction Materials and Methods Results Discussion References

 
After their use at ultrasonography (US) in the intensive therapy unit, probes were used to directly inoculate blood agar plates before and after various cleaning procedures. The uncleaned probes transmitted large numbers of clinically important microbes. Simple cleaning methods were effective in reducing transmission among certain patients: fit patients, double paper wipe; patients at risk of contracting infection, single paper wipe followed by alcohol wipe; patients with a potential source of infection, single paper wipe followed by alcohol wipe.

Index terms: Ultrasound (US), biological effects

	Introduction

TOP Abstract Introduction Materials and Methods Results Discussion References

 
The problem of multiresistant organisms in hospitals is increasing. Of particular concern is methicillin-resistant Staphylococcus aureus (MRSA).

It is essential that vulnerable patients (ie, neonates or patients in the intensive therapy unit, with hematologic or renal disease, or with unhealed wounds) are not exposed to multiresistant organisms. This can be ensured if such patients are isolated from those who are a potential source of infection by means of an effective infection control policy.

Ultrasonographic (US) machines are ideal vectors for cross infection. A busy machine may be used to scan 30 patients a day including both patients who may act as a source of infection and those who are vulnerable. The probe of the US machine could act as a vector between these groups unless there is effective cleaning.

To our knowledge, the best practice has yet to be established. The limited literature is divided regarding the propensity of US probes to act as a source for cross infection and the way this may be prevented (1–3). The manufacturer's recommendation, to soak the probe for 20 minutes in weak sodium hypochlorite solution, is generally impractical due to time constraints. The lack of an effective and easy cleaning method for US probes may place patients at risk.

Our aim was to assess the potential for US probes to act as vectors for cross infection and to compare cleaning methods that can be used to prevent this.

	Materials and Methods

TOP Abstract Introduction Materials and Methods Results Discussion References

 
A prospective study was carried out to assess the ability of US probes to transfer colony-forming units (CFU) from patients to blood agar plates after various cleaning procedures.

Forty specimens were obtained from patients in the intensive therapy unit after abdominal US performed to exclude an abdominal source of sepsis. Thirty-four of these specimens were taken from patients who were known to have been infected with MRSA.

The specimens were divided randomly into two equally sized study groups: the double paper wipe group (n = 20) and the alcohol wipe group (n = 20).

The samples were collected by one operator (C.F.), who performed standardized abdominal US (Toshiba, Tokyo, Japan) with 10 sweeps across the lower abdomen of each patient with use of a sector probe and sterile gel. For each sample, the uncleaned probe was used to directly inoculate the first of a set of three blood agar plates by sweeping the dirty face of the probe over a defined area of the plate (n = 40). The probe was then wiped to dryness with soft nonsterile paper, and the inoculation process was repeated on the second plate (n = 40). The probes allocated to the double paper wipe group (n = 20) were then wiped a second time with soft paper, whereas those in the alcohol wipe group (n = 20) were wiped with a 70% alcohol wipe and allowed to dry fully before being used to inoculate the third plate.

The plates were spread and incubated, and the number, identity, and antibiotic sensitivities of CFU were established after 24 hours.

	Results

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The number of CFU isolated from the two groups of inoculated plates versus cleaning methods are shown in the Table. On average, 128 CFU were transferred by the uncleaned probes, 21 CFU by the single paper wipe probes, 2 CFU by the double paper wipe probes, and 0.05 CFU by the single paper and alcohol wipe probes.

The most important organisms isolated included MRSA and vancomycin-resistant enterococci. When the patient was known to have been MRSA positive, the transmission rate of MRSA was 41% (14 of 34 patients). Other organisms transmitted were coagulase-negative staphylococci, methicillin-sensitive S aureus, enterococci, Klebsiella pneumoniae, Enterobacter cloacae, and Proteus species.

	Discussion

TOP Abstract Introduction Materials and Methods Results Discussion References

 
To our knowledge, the published literature contains three articles regarding cross-infection risk with US probes.

Muradali et al (1) concluded that a single paper wipe was as effective as immersion in chlorhexidine. However, neither cleaning method removed Pseudomonas aeruginosa from the probe, and the single paper wipe did not remove MRSA. They described the growth of these as not clinically important and considered a single paper wipe to be an adequate cleaning method.

Spencer and Spencer (2) found that 66% of swabs taken at random from probes of US machines in constant use grew bacteria. Swabs taken from the probes after they were wiped with alcohol were sterile. They recommended routine use of an alcohol wipe before a probe was used to scan any wound. Tesch and Fröschle (3) found that many organisms were transmitted after a single paper wipe and that immersion in glyoxal for 10 minutes was required to prevent growth.

In this study, we found that the transmission of clinically important infection by US probes can be prevented by applying appropriate simple cleaning methods.

An average of 128 CFU was transferred from probe to plate when there was no probe cleaning after abdominal US in patients in the intensive therapy unit.

A single paper wipe reduced transmission by a factor of approximately 10 and yet allowed a large number of organisms to be transmitted. We agree with the conclusions of Spencer and Spencer (2) and Tesch and Fröschle (3) that a single paper wipe is inadequate to prevent cross infection.

Double paper wipe reduced transmission by a factor of about 100. Single paper wipe followed by alcohol wipe reduced transmission by a factor of about 1,000. Although alcohol wipe was found to be the most effective of the cleaning methods tested, routine use cannot be recommended as the alcohol may degrade the rubber seal and shorten the working life of the probe.

We believe the cleaning method, therefore, needs to be tailored to the clinical situation to achieve an appropriate cost-to-benefit ratio, and we have adopted the following approach. Before the examination of outpatients and fit short-stay inpatients, a double paper wipe appears to be an adequate cleaning method. Before the examination of patients at risk for contracting infection (ie, neonates or patients in the intensive therapy unit, with hematologic or renal disease, or with unhealed wounds), a single paper wipe followed by an alcohol wipe is adequate. After the examination of patients who may be a potential source of infection (those with MRSA-positive results, who are in the intensive therapy unit, or who have undergone multiple antibiotic courses), single paper wipe followed by an alcohol wipe provides adequate cleaning to protect the next patient from cross infection.

	Footnotes

 
Abbreviations: CFU = colony-forming units MRSA = methicillin-resistant Staphylococcus aureus

Author contributions: Guarantors of integrity of entire study, C.F., D.M.; study concepts and design, C.F., D.M.; definition of intellectual content, C.F., D.M.; literature research, C.F., D.M.; clinical studies, C.F., D.M.; experimental studies, C.F., D.M.; data acquisition and analysis, C.F., D.M.; statistical analysis, D.M.; manuscript preparation, C.F.; manuscript editing, C.F., D.M.

	References

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1. Muradali D, Gold WL, Phillips A, Wilson S. Can ultrasound probes and coupling gel be a source of nosocomial infection in patients undergoing sonography? an in vivo and in vitro study. AJR 1995; 164:1521-1524.[Abstract/Free Full Text]
2. Spencer P, Spencer RC. Ultrasound scanning of post-operative wounds: the risks of cross infection. Clin Radiol 1988; 39:245-246.[Medline]
3. Tesch C, Fröschle G. Sonography machines as a source of infection. AJR 1997; 168:567-568.[Medline]

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