Is Ultrasound Gel a Reservoir for Bacterial Pathogens? Results from a Tertiary Care Vascular Laboratory

Introduction

Acoustic gel is ubiquitous in ultrasound laboratories. Cost containment and considerations for patient comfort have driven the use of bulk gel containers and bottle warmers in most facilities. Many laboratories, like ours, refill the hand-held bottles from the bulk container and keep them in a warmer throughout the workday. In 2002, Ruggiero ¹ described how refilling ultrasound gel bottles and using bottle warmers could lead to gel contamination, because gel is a moist medium that, when heated, provides an optimal environment for bacterial growth. In addition, many hospital-based laboratories routinely scan patients on contact precautions, because of infection with resistant bacteria, thus providing a potential mechanism of nosocomial spread of infections through ultrasound gel contamination. The purpose of this study is to determine whether our current practice of bulk refilling and warming of gel bottles leads to bacterial contamination.

Materials and Methods

The setting of the study is a university teaching hospital–based vascular laboratory that performs approximately 300 vascular examinations per month. From October 2002 to January 2003, 96 bottles of ultrasound gel were evaluated for bacterial contamination for 12-weeks. Six 0.25-liter bottles of gel, marked A through F, along with two random gel bottles, were left uncovered on laboratory shelves at room temperature and then rotated through a bottle warmer.

All duplex ultrasound that used the gel were performed on GE Logic 700 (General Electric, WI) or ATL HDI Ultramark 9 (Philips, WA) with a 3–6 MHz or 5–10 MHz linear array transducer or 2–4 MHz sector array transducer. Gel was applied directly to the skin and transducer surfaces, with the tip of the bottle never coming into contact with either surface. Aquasonic 100 Ultrasound Transmission gel (Parker Laboratories, Inc. NJ) was used and heated in a Thermasonic (Parker Laboratories, Inc. NJ) multibottle gel warmer. The Aquasonic 100 is a viscous gel formation containing the preservatives propyl paraben and methyl paraben in bacteriostatic concentrations. ² Sterile ultrasound gel was also cultured as a control. Aquasonic's sterile gel (Parker Laboratories, Inc., NJ) is prepared in wrapped, sterilized foil pouches and is used on patients with nonintact skin. Open wounds were first covered with a Tegaderm transparent dressing (3M Health Care, MN) before scanning proceeded around the area.

Specimens were collected from gel residue with a BBL CultureSwab Collection and Transport System (Becton, Dickinson and Company, NJ). Swabbing was performed on the upper interior portion of the bottles, and the specimens were then sent to the microbiology laboratory for bacteriologic examinations.

Results

A total of 104 cultures was collected and analyzed for bacterial growth (Table 1). No bacterial growth was noted in the initial control samples of the new gel bottles, sterile gel, and the 5-liter bulk container. Propionibacterium speices, a common skin contaminant of clinical specimens, was cultured intermittently from bottle A, once from bottle F, and on two occasions from the random samples. Propionibacterium species represented 50% of all positive isolates (6% of all samples) observed in our study. Although found in bottle A on three different samples, the bacteria were not present in the same bottle in subsequent weeks. Diphtheroids, another common skin contaminant, were present in two different bottles in the same collection week. Staphylococcus aureus, an important nosocomial pathogen, was isolated only once in one bottle (bottle C, week 7) during the sampling period. Straphylococcus epidermidis (coagulase-negative Staphylococcus) was also noted in one isolate (bottle D, week 7). An unidentified Micrococcus species and anaerobic gram-positive cocci, both known to colonize skin and the upper respiratory tract, were also found in only one isolate in separte weeks.^3,4 Overall, 12% of samples were positive for bacterial growth, but persistent colonization was not observed in any of the refilled bottles.

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Table 1

Occurrence of Positive Cultures Serially Obtained from Bulk Refilled and Heated Ultrasound Gel Bottles

Discussion

Acoustic transmission gel is required for vascular ultrasound. The aqueous gel eliminates the air layer between the transducer, and the skin and facilitates the passage of sound waves into and out of tissue. However, skin contact with gel and ultrasound probes provides an opportunity for the spread of pathogens, especially in a hospital-based practice, where a significant number of patients are on isolation precautions because of infection with drug-resistant bacteria. The use of bulk refilling and warmers for ultrasound gel could facilitate colonization and spread of these organisms, as suggested by Ruggiero, ¹ although no formal studies have addressed this issue. This study has shown that over 12 weeks, there was no persistent colonization of gel bottles that were bulk refilled and warmed. Intermittent colonization did occur, mostly with skin flora, which represented 92% of positive cultures. In all but one case, the positive cultures did not persist on the next sampling, suggesting the possibility of contamination while obtaining or handling the culture swabs. Micrococcus species and anaerobic gram-positive cocci are generally skin flora.^3,4 The most concerning isolate was Staphylococcus aureus, which was identified in bottle C at week 8 or 12. This was likely acquired from the environment but did not lead to persistent colonization of the container.

Analysis of the gel composition reveals that the bacteriostatic agents propyl paraben (propyl 4-hydroxybenzoate) and methyl paraben (methyl 4-hydroxybenzoate) are used. Parabens are antiseptics widely used in food, cosmetics, and pharmaceutical preparations. It is reasonable to conclude from our data that these agents are effective in preventing persistent colonization of gel, even under conditions that would favor bacterial inoculation and growth.

Proper scanning technique and handling of the gel containers is important for avoiding contamination. The dispensing tip should not come into contact with the skin or probe surface. Open wounds in the scanning area should be covered with an occlusive dressing, and the probes and console should be wiped down with a bactericidal cleanser after scanning. This practice is particularly important when scanning patients on contact precautions, which represented 5% of our patient volume.

Conclusion

Our study suggests that bulk refilling and warming of ultrasound gel containing bacteriostatic agents does not lead to persistent bacterial colonization in a setting at high risk for the acquisition and spread of pathogens. Because our study used a 12-week sampling period and there are no data regarding long-term colonization of gel bottles, it is advisable to discard the gel bottles after 3 months to prevent long-term contamination.