The “C-Swab” Test: A Technique for Identifying Bacteria in Sinus Tracts or Tunneled Wounds Utilizing a Cotton Swab and Bacterial Fluorescence Imaging

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Complex wounds encompass a diverse spectrum of injuries or tissue damage that present significant challenges in their management and healing. Delayed or impaired healing is a hallmark feature of complex chronic wounds. Bacteria can play a central role in the arrest of healing, even at clinically discrete concentrations not traditionally considered pathogenic due to other factors, such as chronicity, pathogen types, and their interactions; host responses; and other factors beyond merely bacterial concentration.^1-3 This phenomenon has been termed chronic inhibitory bacterial load.⁴ 

Detecting and managing bacterial presence and biofilm is extremely important to restart the healing process, but doing so is not straightforward. The chronic medical conditions that often underlie complex wounds, such as diabetes, vascular diseases, autoimmune disorders, and immunosuppression, not only compromise the body’s infection-fighting ability but delay or mask their clinical manifestations.⁵ This impedes timely detection and management and renders the evaluation of the effectiveness of the chosen treatments through standard methods (eg, Centers for Disease Control and Prevention criteria for vigilance and detection of superficial surgical infections⁶) challenging, leading to healing delays. As a result, wounds have time to evolve into larger, deeper injuries affecting multiple tissue layers, often accompanied by undermining, sinus tracts, or tunneling.⁷ The anatomical complexity of these conditions coupled with advanced bacterial survival mechanisms and the absence of obvious clinical signs demands a comprehensive, multidisciplinary approach to effective treatment with enhanced detection methods and clinician ingenuity. 

This report introduces a multifaceted solution regularly used in the practice of the authors, involving a point-of-care bacterial FL wound imaging device (MolecuLight i:X, MolecuLight) that detects pathogenic bacterial loads at or above a pathogenic threshold (10⁴ CFU/gr tissue) on the wound surface (up to a depth of approximately 1 mm) and periwound areas in real time through endogenous bacterial FL-producing components.^8,9 Although the device cannot quantify bacterial loads, the information it provides is invaluable in prompting and anatomically guiding localized management. 

To apply the benefit of this technology to tunneled, sinusoidal wounds (ie, bacteria located below the 1-mm penetration threshold) the authors of the current manuscript developed the C-Swab test. 

A sterile cotton applicator is used as a probe to obtain a sample of the deep cavities that is then scanned for bacteria with the FL imaging device before it is sent for traditional microbiology assessment. Thanks to the high diagnostic accuracy of this device,^5,8,9 the swab FL reliably detects the bacterial/infection status of the probed cavities, facilitating immediate and accurate treatment decisions, without needing to wait days for microbiology reports or resorting to empirical treatments.^9-11 Briefly, red signals represent most aerobes/anaerobes commonly found in wounds, and cyan signals represent Pseudomonas specifically. Examples of FL images are shown in Figure 1. 

Two clinical cases are presented in which the C-Swab test was used, resulting in positive clinical outcomes. 

Case 1
A 47-year-old male underwent a femoral-popliteal bypass for critical limb ischemia. Postoperatively, a small wound (secondary to graft tunneling trauma) developed adjacent to the main abdominal incision. It did not close for over 4 weeks despite management with silver alginate dressings. Infected wounds associated with arterial bypass procedures increase the risk of graft infection, which can lead to the need for additional surgery, graft removal, amputation, or death.⁷

At baseline, clinical assessments and FL imaging were done for both wounds. Positive red FL signals were limited to the larger surgical incision. The smaller wound appeared FL-negative. The dressings and wick were removed and imaged for FL; both were highly positive (red FL), signaling deep bacterial presence and the need for probing (C-Swab test). This was done before irrigation or cleansing through the open, smaller wound to maintain intact closure of the surgical wound. A tunnel extended from the smaller wound toward the suture line but not onto the graft. After probing, the sterile applicator (ie, cotton swab [Q-tips]) was FL-positive for deep bacteria (Figure 2). 

By objectively confirming pathogenic bacterial loads deep within the wound and tunnel at the bedside, it was possible to adjust treatment immediately. The surface of the incision line was cleaned, and the smaller wound was irrigated with an antibacterial solution, followed by focused debridement of the second wound and tunnel. Repeat FL imaging and “C-Swabbing” appeared negative. Antibacterial dressings (BlastX [Next Science] and Hydrofera Blue Transfer [Hydrofera]) and 2 weeks of antibiotic therapy (doxycycline 100 mg twice a day) were prescribed to prevent graft infection.

Due to the positive bacterial FL findings, a closer follow-up was planned, focusing on repeat FL scanning. Bacterial FL imaging of the wound, dressing, and C-Swab remained negative on weekly follow-up visits. Wound healing progressed, and the wound closed at 4 weeks. The wound remains closed after 12 months.

Case 2
A 78-year-old male presented with debilitating localized edema of the thighs, refractory to traditional treatments (Figure 3A). This condition rendered him immobile, severely impairing his ability to walk and perform activities of daily living. Four hospitalizations in 4 months were needed due to recurring thigh wounds and cellulitis episodes.

Focal massive lymphedema was resected as a last resort to control the multifaceted condition (Figure 3B, immediately postoperatively). Surgical site infection and dehiscence occurred 7 days postoperatively, prompting referral to the authors’ specialized wound care clinic, where erythema, warmth, and pain to touch toward the lower (distal) third of the wound were noted. There was also evidence of a wound created from a 2- to 3-cm–long dehiscence in the distal third of the surgical incision (Figure 3C). Red FL signals were present over the closed two-thirds of the wound (Figure 3E) and were easily eliminated through wound cleansing and superficial debridement. Next, a sterile cotton applicator was inserted through the dehisced portion of the wound and into the deeper cavity that had formed under the closed incision. This C-Swab test indicated red FL (Figure 3D), prompting immediate commencement of systemic antibiotics to manage/contain the surgical site infection. Thorough wound cleaning and irrigation were performed at each visit. The patient was seen in the clinic on a weekly basis, and the wound healed in 2 months.

Diagnosing wound infection and bacteria or biofilm in chronic wounds based on clinical assessment alone misses a significant number of at-risk wounds.⁵ This is even more challenging in the typical complex wound patient who cannot exhibit clear signs of inflammation and/or infection.^12-14 It can be difficult to discern the effectiveness of treatments and whether a change in approach is warranted. Consequently, wound care specialists may overmedicate or struggle to anticipate and promptly address complications. The current case report discusses 2 nonhealing postoperative infected wounds that were successfully managed through the symbiotic merger of clinical ingenuity and medical technology. Surgical site infections are the costliest and most common type of complicated wound in the United States, accounting for $12 billion to $13 billion of Medicare spending every year.¹⁵

Even with FL imaging, challenges in infection diagnosis are further exacerbated by the presence of sinus tracts, undermining, and tunneling. This imaging modality cannot penetrate beyond a tissue depth of 1 mm, per published evidence.¹⁰ In sinus tracts or tunneled wounds, bacteria lie deeper within and concealed by mostly intact skin; thus, the bacteria remain undetected. The C-Swab test allows clinicians to leverage the benefits of the technology over this shortcoming. 

The advent of objective assessment methods and technologies has considerably advanced the practice of wound care and the management of complex and chronic wounds, with the potential to reduce costs and human burden. FL imaging unveils pathogenic bacterial loads that, once identified, change the course of treatment toward a more proactive approach to bacterial management,^5,17 eventually leading to shortened healing times and fewer complications.^18,19 In patients with tunneling wounds, timely infection detection is crucial due to these patient’s weakened immunity and the frequent presence of deeply dwelling bacteria, both of which enable spread to surrounding tissues and heighten the risk of severe infection and complications. In the cases discussed in the current report, real-time bedside FL information facilitated timely treatment adjustments relating to antibiotics, cleansing, and debridement, contributing significantly to successful outcomes.

This report has limitations. This study is limited by the sample size and single institution setting. Further studies are required to quantify the usefulness of this technique over standard microbiological assessment. 

The C-Swab test pairs an old sampling technique with new imaging technology for improved bacterial management and monitoring of tunneled, sinusoidal, and cavernous wounds. It helps avoid tissue disruption, such as enlarging the wound for inspection, and is a great example of cost efficiency and a beneficial partnership with technology in which a humble sterile applicator combined with FL bacterial imaging can avoid tremendously costly and significant consequences stemming from undetected bacterial loads in anatomically complex wounds. 

Affiliation: Madigan Army Medical Center, Vascular/Endovascular/Limb Preservation, and Wound Care Services, Tacoma, WA

Disclosure: Technical and educational support was provided by MolecuLight Inc. No economic support was provided.

Disclaimer: The information included in this manuscript, including brand names, is provided for clarity and general information only and does not constitute professional advice. Opinions expressed in this manuscript are of the authors and not of the US Army or Dept of Defense. 

Ethical Approval: The patients provided consent for the publication of their case and related images.

Correspondence: Charles A. Andersen, MD; Madigan Army Medical Center, Vascular/Endovascular/Limb Preservation and Wound Care Services, 9040 Jackson Ave, Tacoma, WA 98431; Cande98752@aol.com

Manuscript Accepted: May 24, 2024

Andersen CA, McLeod K. The “C-Swab” test: a technique for identifying bacteria in sinus tracts or tunneled wounds utilizing a cotton swab and bacterial fluorescence imaging. Wounds. 2025;37(2):46-50. doi:10.25270/wnds/23153