Correlation of Semi-Quantitative Swab Cultures to Quantitative Swab Cultures from Chronic Wounds

Introduction The relationship between tissue bioburden and wound healing has been established by several studies and confirms that high bioburden delays wound healing.1,2 The reports also indicate that tissue bioburden has a greater effect on wound healing than the presence of systemic diseases, such as diabetes and cardiovascular disease. When high levels of bacteria in the wound are suspected to be the cause of nonhealing, a culture needs to be obtained. The gold standard for determining wound bacterial level is quantitative tissue biopsy, but this modality is not generally used because of potential damage to healing tissue, potential to cause pain to the patient, and the lack of expertise to process the biopsies. The use of quantitative tissue swab culture has been suggested for determining infection in the chronic wound. Most agree that greater than 105 organisms per gram of tissue is diagnostic of infection and delayed wound healing. Bill, et al.,3 studied 38 patients with clean chronic wounds using quantitative tissue biopsies and quantitative swabs. With biopsy, 74 percent of wound samples (28) contained greater than 105 bacteria. Simultaneous quantitative swab culture of these 28 biopsies indicated infection in 22 of the 28 cases for a correlation of 79 percent. They concluded that quantitative swab culture provides a valuable adjunct for monitoring bioburden in the management of chronic wounds. Processing quantitative swabs requires several steps, and many routine microbiology services may not want to deal with a process that complex. In contrast, the procedures for processing a semi-quantitative swab are routine in most laboratories. The only materials required are a blood agar plate, sterile loops, and an incubator. The clinical validity of using semi-quantitative swabs instead of tissue biopsies to monitor tissue levels of bacteria has been documented for burn wounds.4,5 The purpose of this study was to compare quantitative swab to semi-quantitative swab to determine the clinical acceptability of semi-quantitative swabs in identifying clean chronic wounds with greater than 105 bacteria. If proven acceptable, the use of semi-quantitative swabs to monitor bioburden in chronic wounds could be established in most wound care facilities to direct care plans. Methodology A nonrandomized prospective design was used to swab patients seen in a university-based wound care clinic from November, 2001, to April, 2002. All wounds were present for more than six months and included any type of cutaneous wound at any body site. No patients who had gross surface contamination, necrotic tissue, purulent drainage, or eschar were cultured. A total of 124 wounds were cultured. Before obtaining the samples, a sterile 4cm by 4cm gauze was moistened with sterile saline, and the wound was cleaned to remove surface contamination. Next, using sterile technique, an alginate-tipped applicator (Fisherbrand, Houston, Texas) was rotated over a 1cm by 1cm area for five seconds with sufficient pressure to cause tissue fluid to be expressed. The tip of the swab was then broken off into a sterile transport tube. This swab was processed using the semi-quantitative technique. Following the acquisition of this swab sample, another swab was obtained from the same site using the same technique. The tip of this swab was broken off and placed into a sterile transport tube containing 5mL of normal saline. This swab was processed using a quantitative technique. Both samples were immediately transported to the laboratory for processing. Serial dilutions of the quantitative swabs were performed and plated on sterile agar medium. All plated specimens were incubated under aerobic conditions at 37 degrees C. After 24 hours, the plates were visually inspected and colonies of bacteria counted. Colony-forming units (CFU) were then utilized to determine the total bacterial count on each plate. Soft-tissue infection was defined as greater than 105 CFU per cm2 for swab culture. For the semi-quantitative method, a blood agar plate was streaked three times on one quadrant and then three times on each remaining quadrant using a sterile loop for each quadrant. This procedure is thought to create dilutions of the original swab in each quadrant. All plated specimens were incubated under aerobic conditions at 37 degrees C. After 24 hours, the plates were visually inspected and colonies of bacteria counted in the four quadrants. Results A total of 124 wounds were swabbed and processed using both the semi-quantitative and quantitative techniques. The patients were 74 men and 50 women. Forty-four of the wounds were pressure ulcers, 27 were chronic venous insufficiency ulcers, 29 were neuropathic or diabetic ulcers, eight were lower-extremity arterial disease ulcers, and 16 were wounds of other various etiologies. Using the results obtained from the cultures, a table was constructed comparing the two methods, semi-quantitative and quantitative (Table 1). Sensitivity reflects the ability of a test to identify a positive result. Fifty-three quantitative swabs contained 105 or more bacteria/cm2. These wounds were defined as infected. For the semi-quantitative swabs, this correlated with 42 swabs that had growth in quadrant III or quadrants III and IV for a sensitivity of 79 percent (42/53). If infection is predicted by the semi-quantitative technique by bacterial growth in quadrant III or quadrants III and IV, then noninfected wounds should be predicted by growth in quadrants less than III. In this case, 64 semi-quantitative swabs correlated with 71 quantitative swabs that were less than 105 bacteria/cm2 for a specificity of 90 percent (64/71). If growth in quadrant III or quadrants III and IV in the semi-quantitative technique is defined as infected, then in this study there was a 10-percent (7/71) incidence of false positives where the quantitative swab was actually 104 bacteria/cm2 and not the required 105 bacteria/cm2. These wounds would have been considered infected and treated with an appropriate topical antimicrobial for an additional two weeks. From the opposite perspective, 11 quantitative swabs were 105 or greater, and there was no growth in quadrant III of the semi-quantitative technique. This means an incidence of 21 percent (11/53) false negatives. In this situation these wounds would not have received topical antimicrobial therapy for two weeks. Linear regression analysis demonstrated a statistically significant correlation between the two techniques with a coefficient of r = 0.84 with p 3 Based on that study and the results of this study, it can be inferred that semi-quantitative swabs also would be correlated with tissue biopsy results. Alginate swabs were chosen over cotton-tipped swabs because the cotton-tipped swabs may be bacteriostatic secondary to the oxidative sterilization procedure.1 The technique for obtaining swabs for both techniques was identical. The difference between the techniques was how the swabs were processed in the lab. Patients universally felt the swabs caused little or no discomfort since the swab technique was noninvasive. The cost of the alginate swab is relatively inexpensive although slightly more expensive than a cotton-tipped swab. When high levels of bacteria in the wound are suspected as the cause of nonhealing, then a quantitative swab culture needs to be obtained.1 Even though tissue biopsy is the gold standard, there is excellent correlation between tissue biopsy and quantitative swabs. However, many facilities do not have the capability to perform quantitative swab counts. A more practical method of determining wound infection needs to be established. Semi-quantitative swabs do not require complex procedures, so they can be performed in most microbiology labs. The efficacy of semi-quantitative swabs has been demonstrated in the burn literature. Basak, et al.,6 evaluated 171 burn wounds to correlate surface swab culture and quantitative tissue biopsy. The authors noted a correlation of 72 percentbetween surface swab culture and wound biopsy culture. However, the authors only looked at the ability of the two techniques to isolate bacteria from the wound and did not determine total bioburden. Additionally, the authors did not describe the method for obtaining the swabs or the type of swab used. Rudensky, et al.,7 compared surface wound swabs and tissue aspiration with wound biopsy in 72 pressure ulcers. They found that 97 percent (29 out of 30) of swab cultures were positive for bacteria. However, the method for the collection of the swab cultures was not discussed, and the authors stated that the positive results were a reflection of surface contamination. Bacterial bioburden of the wounds was not discussed. Thomson, et al.,4 compared tissue biopsy and semi-quantitative swabs in 24 burn patients. The swabs were streaked onto a blood agar plate and then streaked over the plate with a sterile loop. Plates were incubated and colonies were counted. Linear regression analysis demonstrated a statistically significant correlation between the two techniques at p 30 colonies). They concluded that surface swabs may be used as an indicator of numbers of tissue organisms with reliability. Herruzo-Cabera, et al.,5 also studied whether semi-quantitative cultures of burn eschar are as reliable as tissue biopsy in 159 burn wounds. They used a classic semi-quantitative method for the surface swab cultures, which was the technique used in this study. The technique involves sequential streaking of the initial swab over four quadrants and determining the highest quadrant with bacterial growth. They found a sensitivity of 87.7 percent and a specificity of 52.8 percent for the semi-quantitative surface culture as a predictor of invasive burn wound infection. They concluded that semi-quantitative burn wound swab cultures are useful and are equivalent to quantitative biopsy cultures in the prediction of invasive wound infection. Bacteria are known to impede wound healing.2,3 Most chronic wounds are chronic because of the continued presence of high levels of bacteria. Wound management procedures, such as debridement, cleaning, and the use of absorptive dressings, are effective in reducing the bioburden. When the wound is clean and healing is not occurring, it may be due to high levels of bacteria. At this point, the use of the semi-quantitative swab would be helpful in making wound care decisions. If the wound contains high levels of bacteria, more aggressive cleansing, more absorptive dressings, and adjunctive therapies, such as hyperbaric oxygen, ultraviolet light, or a topical antimicrobial cream, may be used for a two-week period. The organisms isolated on the blood agar plate of the semi-quantitative assay can be used to determine the bacteria’s sensitivity to various topical antimicrobial creams, such as silver sulfadiazine, sulfamylon, and gentamicin.8 At the end of the two-week regimen, the semi-quantitative swab should be repeated to determine if the new wound care program is providing benefit. The results of the semi-quantitative swab can become an integral part of the clinician’s wound care program. Currently, no other scientific studies exist in the literature that evaluate the semi-quantitative swab as a method for determining bacterial bioburden in chronic wounds. This method is inexpensive and readily available to anyone in healthcare regardless of the practice setting. More studies evaluating this technique in chronic wounds are needed. For example, additional research might compare cotton swabs and calcium alginate-tipped swabs to see if any differences exist between the two swabs. Conclusion Semi-quantitative swabs can serve as an important adjunct in managing chronic wounds. The authors found a significant correlation between the techniques of semi-quantitative and quantitative swabs. Research has shown that chronic wounds with high bacterial bioburdens do not heal at an optimum rate. However, a systematic method for determining bacterial bioburden is not routinely practiced by clinicians. The use of semi-quantitative swabs may be very helpful for clinicians who want to adjust wound care based on bacterial levels.