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The importance of diagnosis in chronic wound infection

Una Adderley
DN RGN MSc BSc BA
Community Tissue Viability Prescribing Nurse
Scarborough, Whitby and Ryedale PCT

The skin is the largest organ of the body and is naturally colonised by a wide variety of bacteria. When the skin's protection is breached, the resulting wound will inevitably be inhabited by bacteria. However, in some cases this may lead to the development of clinical infection. Accurate diagnosis and treatment of wound infection is essential, not just to encourage wound healing, but also to prevent wider complications. This article describes the signs and symptoms of wound infection and colonisation and discusses wound sampling and treatment options.

Wounds and bacteria - getting infected
The problem with wound infection is that it delays healing by prolonging the inflammatory phase of healing, disrupting the normal clotting mechanisms and preventing the development of new blood vessels and formation of granulation tissue. This leads to tissue breakdown and the extension of the wound. In terms of a patient's quality of life, wound infection may increase patient discomfort and cause malodour and the mess of excess exudate. At worst, it can lead to septicaemia and death.
Wounds become infected when bacteria are able to overwhelm the patient's defences. Whether a wound will become infected depends on the virulence of the bacteria balanced against the susceptibility of the patient. A bacteria's virulence depends on how well it is able to adhere to a mucosal surface or damaged tissue, invade cells, produce toxins, and avoid the body's defences. The susceptibility of the patient is already increased by the existence of a breach in their firstline defence, ie, the skin. Further individual issues, such as the patient's general level of nonspecific immunity, specific immunity (eg, antibodies), underlying conditions (eg, diabetes, peripheral vascular disease and trauma), and medically-induced susceptibility (eg, surgery, steroid medication or chemotherapy), will affect their susceptibility.

The importance of accurate diagnosis
Diagnosing wound infection is not an easy matter. For instance, the presence of bacteria within a wound does not necessarily indicate that a wound is infected. Equally, chronic wounds are likely to have a wide spectrum of bacteria in greater numbers than is usual on the surface of the skin. However, despite this colonisation, a wound may still progress to healing providing the patient is not particularly susceptible to infection. However, in more susceptible patients, the same degree of colonisation may cause significant problems. Similarly, a small quantity of a particularly virulent bacterium may cause a wound infection in an otherwise healthy person.
Accurate diagnosis of infection is important since patients should obviously not be submitted to unnecessary treatments. In recent times, the emergence of bacteria that are resistant to antibiotics, such as methicillin-resistant Staphylococcus aureus or MRSA, has highlighted the need for more accurate diagnosis of infection. MRSA was first identified in the 1960s and has since increased to become the most common cause of hospital-acquired infection. It is thought that although poor healthcare hygiene is largely to blame for the spread of MRSA, the overuse of antibiotics is also a contributing factor. The widespread use of antibiotics and the failure of many patients to finish a prescribed course of antibiotics mean that bacteria that survive a course of antibiotics are more likely to be drug resistant. This resistance is then passed onto the next generation of bacteria. Vancomycin has been the drug of last resort against MRSA but recently, American doctors have isolated a strain of vancomycin-resistant MRSA.
A further and more recent area of concern is the emergence of Clostridium difficile. The threat from C difficile differs to the MRSA bacteria since it affects the gut rather than causing particular problems within the wound bed. C difficile is carried harmlessly in the gut of a large proportion of the population. However, certain antibiotics can disturb the balance of the gut and allow C difficile to reach toxic levels that can cause life-threatening diarrhoea. Therefore to minimise the risk of a patient developing toxic levels of C difficile, patients should only receive antibiotic therapy if essential. A recent report by the Health Protection Agency noted that hospitals that had endeavoured to introduce and adhere to policies aimed at controlling antibiotic use appeared to be linked to a reduced incidence of C difficile.(1) Although this report was hospital focused, the need to consider carefully how antibiotics are used clearly has application within community healthcare.
Identifying wound infection in an acute wound is relatively simple providing that the normal inflammatory phase at the beginning of wound healing is not mistaken for inflammation due to infection. However, identifying wound infection in the chronic wounds that form the majority of community wound care is more challenging. Research by Gardner et al(2,3) has identified the following symptoms as indicative of wound infection in chronic wounds:

  • Increased intensity and/or change in character of pain.
  • Discoloured or friable granulation tissue.
  • Odour.
  • Wound breakdown.
  • Delayed healing.

However, it must be noted that the signs of infection may be reduced or masked by dermatological problems or an impaired immunological response.
To add further complexity, there is currently ongoing debate regarding the term "critical colonisation", which is used to describe the interim stage between microbial colonisation that is not increasing, persisting or causing delayed healing, and infection where the microbial growth is leading to cellular injury, illness and stopping wound healing.(4,5) Although there is currently no agreed definition of "critical colonisation", it does appear to be a clinically useful concept.
Distinguishing an infected chronic wound from a chronic "critically colonised" wound is a clinical judgment. A heavily colonised wound may be "indolent" in that it is failing to make the expected progress in healing. It may be covered in thick slough, which swiftly returns after rapid debridement techniques such as sharp debridement or the application of maggots, and there may be an intransigent odour. However, these are not necessarily signs of wound infection.
It has been said that the one piece of information that is not needed to diagnose infection is the findings of a wound swab processed by a microbiological laboratory! While it is certainly true that a swab result alone is of little use, laboratory information is extremely useful to identify which bacteria are present in a wound and which may be responsible for wound infection. However, since all chronic wounds contain bacteria, wound swabs will generally reveal a number of different types of bacteria present. In addition, false-positives and false-negatives are common.
A variety of samples may be collected from a wound (eg, pus, fine needle aspirants and tissue), but the most common is wound swabbing. The temptation is to collect a sample of the debris and exudate lying on top of the wound bed. However, these substances are likely to include a wide variety of colonising microorganisms rather than the causative agent. Therefore the wound swab should aim to sample the actual wound bed in order to achieve more accurate information.
The wound bed should be cleaned thoroughly and the swab moistened with saline or sterile water. The swab should then be pressed and rolled firmly against the tissue that looks most infected (ie, discoloured granulation rather than slough). The swab should then be replaced in its container along with the request form and both should be labelled accurately and thoroughly before being sent to the laboratory as quickly as possible. The information sent to the laboratory should include signs and symptoms of suspected infection, type of specimen, site of swab and type of wound.

Treatment
Wounds that have been diagnosed as clinically infected will require systemic antibiotic therapy. The selection of any antimicrobial treatment should consider the specificity, efficacy, cytotoxicity and allergenicity of the antimicrobial agent.6 Ideally this should be based upon the information given by the results of a high-quality wound swab. However, the condition of the patient may mean that it is necessary to initially treat blind until the laboratory results are available, at which point the most appropriate antibiotic should be prescribed. Wounds that are showing increasing signs of infection ("critical colonisation") may benefit from the application of a topical antimicrobial dressing such as one containing iodine or silver (see Figure 1). Infected and critically colonised wounds often present with a friable wound bed, malodour, heavy exudate levels and pain. Therefore dressing selection should aim for low-adherent but absorbent dressings.

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Conclusion
Developing the skills to accurately diagnose and appropriately treat wound infection is vital for both patients today and for those in the future who will need access to antibiotics that are still effective. Smelly, messy chronic wounds are fairly common in community nursing: true wound infections are fortunately far more rare. Community clinicians have a duty of care to develop the skills and confidence to differentiate between the two.

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References

  1. Health Protection Agency. Report on Clostridium difficile. London: HPA; 2006. Available from: http://www.hpa.org.uk/infections/topics_az/clostridium_difficile/documen...
  2. Gardner SE, Frantz RA, Doebbeling BN. The validity of the clinical signs and symptoms used to identify localized chronic wound infection. Wound Repair Regen 2001:May-June:178-86.
  3. Gardner SE, Frantz RA, Troia C, et al. A tool to assess clinical signs and symptoms of localized infection in chronic wounds: development and reliability. Ostomy Wound Manage 2001;47:40-7.
  4. Cooper RA. Understanding wound infection. EWMA Position Document: Identifying criteria for wound infection. Available from: http://www.ewma.org/english/english.htm
  5. Kingsley A. A proactive approach to wound infection. Nurs Standard 2001;15(30):50-8.
  6. Vowden P, Cooper RA. An integrated approach to managing wound infection. EWMA Position Document: Management of wound infection. Available from: http://www.ewma.org/english/english.htm


Resources

Health Protection Agency
W: www.hpa.org.uk
For information on MRSA and Clostridium difficile

European Wound Management Association
W: www.ewma.org/english/english.htm
For reliable and up-to-date position papers on wound infection

Department of Health Healthcare Associated Infection Information
W: www.dh.gov.uk/PolicyAndGuidance/Health
AndSocialCareTopics/HealthcareAcquiredInfection/fs/en
For general information on healthcare associated infection