Given the impact of infection on delayed wound healing, determining the presence of colonization and infection is imperative to achieving healed outcomes. Chronic wounds are always contaminated, and timely implementation of management and treatment interventions is a key component of the plan of care. Diagnosis of infection can be a very challenging task to say the least, and it is further complicated by the presence of biofilms for which no diagnostic tool is currently available. If not addressed in a timely manner, these local infections can become systemic, leading to sepsis, multiple organ failure, and death. The first steps are a complete and thorough history and a physical examination of the whole patient, not just the patient's wound, while taking into account both primary and secondary findings to understand the host response.¹

Having a thorough understanding of the principles of chronic wound care and of the current diagnostic modalities available is essential to the improvement of clinical outcomes and cost reduction related to the complication of wound infection. Our focus is on the challenges to diagnosing wound infection, including accurately determining risk factors, differentiating colonization from infection, and understanding the gold standard for diagnosing wound infection.

Risk Factors

Chronic wounds affect an estimated 20 million individuals worldwide, with the estimated cost in the United States alone exceeding $31 billion per year.² Also of significance are the indirect costs, which include reduced productivity and decreased quality of life. As the percentage of our older adult population rises, so will the incidence and significance of chronic wounds and wound infections. Women have a higher rate of chronic wounds than men, and many of these wounds, including those related to chronic venous insufficiency, peripheral arterial disease, and pressure injuries or ulcers, rarely heal quickly or without complications such as infection.

Conditions contributing to delayed wound healing and increasing the susceptibility to infection include diabetes, nutritional insufficiency, vascular insufficiency, neurologic defects, and age. Many times these factors are combined, increasing the potential for delayed wound healing and infection, a situation that is further complicated by the increasing numbers of antibiotic-resistant organisms and the development of biofilms protecting them. For this reason the pursuit of alternative adjunctive therapies should be considered.

Contamination, Colonization, and Infection

As stated earlier, all chronic wounds are contaminated with microorganisms. Wound contamination is the presence of non-replicating bacteria; the host remains in control of the environment, and healing is not impaired.³ These low levels of bacteria can actually stimulate wounds to repair themselves. However, when the microorganisms increase in the wound bed they can severely retard or prevent wound repair, so it is important to understand the difference between normal inflammation, which presents as an increase in periwound erythema, edema, warmth, and pain, and the clinical signs and symptoms of an infection, which can be very similar.⁴

With colonization the increased number and persistent presence of microoganisms can prolong the inflammatory phase of healing and lead to further tissue damage. When wounds are not managed well, such as with ineffective wound bed preparation, the bacteria can begin to replicate and proliferate. If there is an increase in the number of bacteria, depending on the virulence of those bacteria, this process can begin to overwhelm the host. The concept of critical colonization was invented to describe the idea that bacteria could play a role in non-healing wounds that do not have any obvious signs and symptoms of infection. In reality, this concept likely describes the presence of a biofilm, and as yet there is no diagnostic tool for its detection.⁴

When a wound is infected, replicating bacteria are present and are invading the tissue whether superficially or through deep penetration. Again, the host response will show a local reaction or a systemic reaction. The assessment of infection in a chronic wound is a clinical skill, and the decision to prescribe antibiotics or apply topical antimicrobial agents should be based primarily on clinical presentation.⁵ Clinical signs of a superficial infection may present as a delay in healing of the wound with only islands of healing or eroding wound edges. There may also be an increase in drainage, and granulation tissue may be abnormal in appearance, with discoloration, friability, or a flat and smooth appearance. There may be necrotic tissue or debris in the wound that pockets at the base, as well as an abnormally foul odor.⁴

Local signs and symptoms of a systemic infection may include changes in the periwound skin, such as an increase in temperature, additional breakdown, edema, induration, or erythema. The wound may have exposed bone or other structures, an increase in size, drainage, and odor. Periwound indicators can be helpful in determining the status of the wound. In a normal inflammatory response, during the first one to five days of wounding, erythema in the periwound area upward of 5cm is a normal physiologic response. After 5 days if there is still periwound erythema, and it is greater than 5cm in diameter, this may be a pathologic response and an indication of cellulitis. When periwound cellulitis starts red streaking and tracking in one direction away from the wound, this is a pathologic response and may be an indication of a systemic problem. Additional indications of a systemic infection may include an elevated body temperature, a change in laboratory values with an increase in white blood cells, elevated glucose levels, malaise, or a change in the level of consciousness.⁴

Gold Standard in Infection Diagnosis

Diagnosing an infection in a chronic wound can be difficult because different wound types may have various clinical presentations. Some signs and symptoms are more likely to be present than others, depending on wound location, onset, and type. In addition, a wound's surface is not sterile and can be colonized with various commensal, environmental, and possibly pathogenic microorganisms.⁶ There are multiple methods for identifying any pathogens in the wound bed. The most common is to perform a swab culture. A swab culture is not considered as accurate as other methods because if it is not performed correctly, it will mostly identify surface contaminants.⁷ Eschar, slough, or pus on the surface of the wound should not be cultured. The Levine technique is considered the best way to obtain a swab culture.

First, the wound bed is cleansed well with an antimicrobial wound cleanser. Once debris and exudate are well cleansed from the wound, the wound is rinsed thoroughly with sterile saline to remove any residual wound cleanser. By pressing against the cleanest portion of the wound base, fluid is expressed from a 1cm² area of the wound base from which a sample is obtained with a sterile culture swab.⁶ Another method for obtaining a wound culture is by aspiration of wound fluid from within the wound by needle aspiration. In this method a 5cm² area of skin around the infected wound is cleansed with antimicrobial wound cleanser and is allowed to dry for approximately 60 seconds. A fine-gauge needle is then inserted through the intact skin 1cm from the wound edge and passed tangentially approximately 2cm into the skin to reach the base of the wound.

The syringe is then used to aspirate while maintaining negative pressure for at least 10 seconds to obtain the specimen of wound fluid. If needed, the needle can be moved back and forth at different angles to obtain the fluid specimen. Once the specimen is obtained the needle is withdrawn, and the aspirate is transferred to a sterile specimen container and is immediately taken to the laboratory for analysis, as well as aerobic and anaerobic culture. Local anesthetics should not be used because they may have an antimicrobial effect.³ The gold standard method for acquiring a culture is by tissue biopsy. This is accomplished by using aseptic technique in obtaining a tissue sample usually by punch biopsy or with a scalpel. Despite its accuracy in determining infection, tissue biopsy for culture is not commonly done because it is an invasive procedure, requires a specific skill set, should be performed only in the appropriate health care setting, and is painful and costly.⁷

Other Considerations

Vast improvements in infection identification methods and their accuracy have been made over the last decade. Serologic tests such as erythrocyte sedimentation rate, bone biomarkers, and procalcitonin may be used. Radiography and magnetic resonance imaging are used, as well as newer hybrid imaging techniques such as single photon emission computed tomography/computed tomography and positron emission tomography/magnetic resonance imaging. Other newer technologies on the horizon include radiopharmaceuticals, ultrasonography, photographic and thermographic methods, and molecular microbial techniques that identify causative organisms, as well as their virulence factors and antibiotic resistance.

References

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2. Leaper D, Assadian O, Edmiston CE. Approach to chronic wound infection. Br J Dermatol. 2015;173(2):1-8. doi: 10.1111/bjd.13677.

3. Sudharsanan S, Sreenath GS, Sureshkumar S, Vijayakumar C, Sujatha S, Vikram K. Does fine needle aspiration microbiology offer any benefit over wound swab in detecting the causative organisms in surgical site infections? Wounds. 2017;29(9):255-61.

4. Stotts NA. Bioburden infection. In: Baranoski S, Ayello EA, eds. Wound Care Essentials: Practice Principles. 4th ed. Philadelphia, PA: Wolters Kluwer Health; 2015:93-119.

5. Shang J, Stone P, Larson E. Studies on nurse staffing and healthcare associated infection: methodologic challenges and potential solutions. Am J Infect Control. 2015;43(6):581-8. doi: 10.1016/j.ajic.2015.03.029.

6. Kallstrom G, Doern GV. Are quantitative bacterial wound cultures useful? J Clin Microbiol. 2014;52(8):2753-6.

7. Smith ME, Robinowitz N, Chaulk P, Johnson K. Comparison of chronic wound culture techniques: swab versus curetted tissue for microbial recovery. Br J Community Nurs. 2014;19(9):22-6. doi: 10.12968/bjcn.2014.19.