Diabetes: prevalence and complications
Worldwide prevalence of diabetes mellitus was estimated at around 2.8% in 20001 and this is predicted to increase to 7.7% (affecting 439 million adults) by 2030,2 largely as a result of the obesity epidemic.3,4 In the USA, the prevalence of diabetes was 8.3% in 2012, which is a sharp increase (more than doubling) compared with the prevalence in 1990, which was 3.5%.5 Estimates from the USA predict that Americans born in 2000 will have a one in three lifetime risk of developing type 2 diabetes.6 In the UK, the prevalence of diabetes is 6.0% (3.2 million people),7 and Diabetes UK estimates that there are around 630,000 people with diabetes who have not yet been diagnosed.8 Treatment of diabetes in the UK cost approximately £23.7B in 2010/11, accounting for approximately 10% of the total health resource expenditure.
Both type 1 and type 2 diabetes can lead to serious health problems.9 Complications of diabetes, especially in patients with poorly controlled blood sugar levels, include damage to the eyes, kidneys, nerves and arteries. In the feet, diabetes-related peripheral neuropathy leads to changes to foot architecture (hence increasing pressure on plantar surfaces, including those unaccustomed to load-bearing),10,11 reduced sweating (hence dry, cracking skin), poor sensation (hence susceptibility to trauma) and accelerated atherosclerotic disease, which leads to reduced circulation, with consequent problems with healing wounds and fighting infection. These peripheral neuropathic and vascular changes, either alone or in combination, predispose the foot to ulceration and its sequalae.12,13
Diabetic ulcer infection: epidemiology and aetiology
It is estimated that the proportion of people with diabetes in the UK who have ever had a foot ulcer is around 6%,14 and that lifetime risk of ulceration is 15–25%.15,16 Diabetic foot ulcers (DFUs) can take many weeks or often months to heal and have a negative impact on patients’ functional ability and quality of life. Foot ulceration in diabetic individuals also has a wider societal impact, such as reduced work productivity, high health-care costs and financial loss.17–21 An open wound related to foot ulceration, combined with various immunological perturbations caused by diabetes, frequently results in infection. Prospective studies have found that about half of recent-onset DFUs are clinically infected at presentation.22 Diabetic foot infection is thought to be the most common cause of diabetes-related hospital admissions and precedes approximately 80% of non-traumatic lower limb amputations.15,23–27
Foot infections in people with diabetes can be hard to manage because of the associated impaired arterial supply to the legs, as well as impaired function of the immune system (especially those related to defects in polymorphonuclear leucocytes). This leads to an increased risk of progression of infection with contiguous spread to deeper tissues (including bone), and proximal extension up the foot and leg, as well as systemic spread into the blood stream. Therefore, many diabetic foot infections require some level of lower extremity amputation as a limb-sacrificing, but potentially life-saving, measure.14,28,29
The incidence of lower extremity amputations is 10–30% higher in people with diabetes than in the general population,28,29 and about 85% of these amputations are preceded by a foot ulcer infection.23,24,26,30–33 Limb amputation is associated with major consequences, as it dramatically reduces health-related quality of life, is expensive for both the patient and the health-care system and is associated with a 5-year mortality of over 50%.15 To reduce the risk of foot ulceration, accelerate the healing of open ulcers and identify and treat infection promptly, many health-care systems have deployed multidisciplinary foot teams to co-ordinate foot care. The prevention of foot ulceration and amputation involves optimising glycaemic control and foot care. This may include supplying pressure-relieving shoes or insoles, undertaking surgical interventions promptly and optimally treating any infection. Providing this care involves input from the specialties of general practice, diabetology, nursing, dietetics, podiatry, orthotics, vascular surgery and infectious diseases/clinical microbiology.
Wound infection: definition, identification and characterisation
All chronic wounds, including DFUs, have bacteria on their surface that may originate from the surrounding normal skin flora, as well as opportunistic bacteria, such as gut flora. Therefore, the presence of bacteria in a wound does not indicate the presence of infection. When the host tissues show no inflammatory response or incur no damage associated with the bacterial growth, then the wound is described as ‘colonised,’ rather than infected. At this stage, there is typically thought to be a ‘balance’ between the growth of the several species of bacteria and no single organism usually dominates. When a critical density or high virulence of colonising organisms causes damage to host tissues, the wound is deemed to be ‘infected’. Therefore, infection of chronic wounds is usually a clinical diagnosis based on signs and symptoms of host tissue inflammation, such as pyrexia, purulent secretions, pain or tenderness, erythema, warmth and induration.34–38 Although some investigators and clinicians also describe a quantitative diagnostic criterion for the presence of infection (e.g. a bacterial load of > 105 colony-forming units per gram of tissue), there is no agreement on this.39 In chronic wounds, a single cut-off point for bacteria has been found to be insufficient for defining infection; other factors, such as the number or virulence of the bacterial groups and the presence of biofilm, are also important.40
When a wound infection is diagnosed, the therapeutic approach depends on the whole clinical situation. Because in infected diabetic foot wounds the consequences of delayed antibiotic treatment can be profound, empiric antibiotic therapy should usually be initiated immediately. The antimicrobial regimen is usually selected in accordance with departmental protocols that are based on the probable causative organisms and their susceptibility patterns. Concurrently, samples for microbiological analysis are taken to identify the infecting organisms within the wound and their susceptibility to a range of antimicrobials. The resulting microbiological information guides subsequent modifications of the empiric antibiotic therapy required should the infection not improve and resistant organisms be isolated.34–38,41,42 The culture and sensitivity results also allow a change from broad- to narrow-spectrum antibiotic agents, thus following the principles of antibiotic stewardship.34–38,41
The microbiological analysis of specimens from the ulcer is useful only if the specimen is properly collected and processed and reported accurately and promptly. The aim is to acquire a wound sample that identifies all pathogens while avoiding colonising flora. First, the ulcer must be cleaned, which may involve debridement to remove necrotic material or callus and undermining tissues. Second, a specimen is taken from the site of infection, using one of a number of specimen-acquisition techniques, such as wound swabbing, fluid sampling using a fine-needle aspiration, or tissue sampling (by biopsy or curettage).36,37,42 Taking a tissue sample either uses a tool to extract a ‘punch biopsy’ or scrapes the base of a wound with a sharp-edged dermal curette or scalpel blade to obtain ulcer tissue from the debrided ulcer bed.43
It is important that the culture of the sample obtained reflects an accurate profile of the bacterial environment in the ulcer. Either failing to identify a true pathogen or identifying a coloniser as a pathogen can lead to inappropriate treatment of an infected wound. Therefore, it is important that health-care staff use a technique that will give a specimen that provides an accurate account of the bacteria present, including their number and sensitivity to antibiotics. Most published guidelines recommend obtaining a tissue specimen rather than a swab, in order to increase the likelihood of accurately reflecting the organisms associated with clinical infection at initial presentation.34,36,37,42
In clinical practice, however, samples from wounds are often taken with a cotton swab.44–47 The advantages of a wound swab include the almost universal availability of the equipment, the relative ease of the technique, the low cost of the swab and the fact that little training is needed to perform this correctly, which means that it can be done by non-clinician staff.45 Furthermore, there is little risk of harm using a swab to collect a tissue sample. The disadvantages of a swab include the concerns that it may not collect those bacteria responsible for the infection deep within the tissues (e.g. as happens if an appropriate technique is used), that it will collect the colonising bacteria on the wound surface, or that it will fail to provide an environment conducive to growth of obligate anaerobes and other fastidious organisms (i.e. those that may be present in the wound but die in a swab device that does not provide an adequate medium for their survival). To counter these problems, advocates of wound swabbing have specified how to prepare the ulcer bed (i.e. removing dead tissue that may contain non-pathogenic bacterial groups) and how to obtain a sample from deep in the ulcer (by pressing to collect fluid from deep in the subcutaneous tissues, as described by Levine et al.48 in 1976), as well as the optimal storage and transport procedures (use of charcoal swab, transport medium and swift delivery to the laboratory to maintain the viability of fastidious organisms).48
In contrast, the reported advantage of tissue sampling is that the specimen is likely to contain the pathogens responsible for tissue destruction and infection. However, tissue-sampling techniques require disruption or cutting of the ulcer bed to obtain a specimen and this may lead to bleeding or pain (although most DFUs are complicated by neuropathy, which reduces the ability to perceive pain). Some clinical staff may need additional training to be able to take these samples safely and they also require some basic equipment: sharp sterile blades (scalpel), dermal curettes or a biopsy cutter. Using appropriate storage and transport procedures (transport medium and swift delivery to laboratory to maintain the viability of fastidious organisms) is still required.
Processing method
Accurate characterisation of the bacterial flora depends on both the sample collection method and the processing method. Standard culture and plating techniques involve the multiplication of the bacteria in a medium, by growing them on various types of culture plates, identifying the organisms and assessing their sensitivity to antimicrobial agents. It is thought that some organisms do not survive collection and transport and, hence, a swab (or occasionally tissue sample) does not fully reflect the organisms causing the wound infection. These ‘fastidious’ organisms remain undetected in the laboratory but may be important pathogens.49 As these uncultured organisms cannot be identified by standard microbiological methods, appropriate antibiotic selection is problematic. This may partly account for the fact that approximately 10–20% of diabetic foot wounds fail to respond to initial antibiotic treatment. There is, therefore, some question over whether or not alternative techniques to identify bacteria within a sample, either instead of or in addition to sample plating and culture, may provide a more accurate picture of the wound flora. Modern molecular (or genotypic) techniques, such as polymerase chain reaction (PCR), have been proposed for this as the equipment for these tests become more readily available in hospitals.50 It is not yet clear, however, how the results of these molecular tests, which generally identify more pathogens, should be interpreted.51
The full report of culture results can take 4 or 5 days to be returned to the clinician. This delay in reporting, combined with the effects of antibiotic treatment given in the intervening period, means that the laboratory result may be out of date and that the wound flora may have changed. Therefore, a clinician reviewing an ulcer that has not improved with treatment cannot presume that the bacteria described in the microbiologists report are the same pathogens responsible for an infected ulcer 5 days later. Quicker techniques for microbiological analyses, such as genetic fingerprinting, that take 1 day or less, may help to address this delay.52
These newer microbiological analysis techniques multiply the genetic material of the bacteria rather than grow them in culture. Genetic fingerprint techniques are then used to identify the bacteria group from its deoxyribonucleic acid (DNA)/ribonucleic acid (RNA) signature.52 Culture-based methodology may not identify minor, although possibly important, components of a mixed bacterial population, whereas genetic fingerprinting techniques can.53 Therefore, we also conducted a small substudy to compare identification of ulcer pathogens using conventional culture versus PCR techniques. This enabled us to determine the agreement between analysis techniques, that is, how does the quicker molecular technique reflect the bacterial load captured by swabs and tissues samples in the foot ulcer compared with swab and curettage specimens (e.g. for those organism not identified via plating and culture).
Diabetic foot ulcer guideline recommendations for infection (diagnosis/identification and characterisation and treatment)
Several guidelines and consensus documents aimed at improving the care for people with DFUs have been published over the past decade.9,15,34–37,41,42,54,55 In this report, we have focused on three guidelines: (1) the UK National Institute for Health and Care Excellence (NICE) guidance on inpatient management of diabetic foot problems;37 (2) the Infectious Diseases Society of America (IDSA) guidelines for the diagnosis and treatment of diabetic foot infections;1,2,36 and (3) the International Working Group on the Diabetic Foot (IWGDF) guidelines on the management and prevention of the diabetic foot.42,56 IDSA guidelines were first published in 200434,35 and are widely used. The IWGDF guidelines were published in 200842 and the latest NICE guidance in 2011.37 The IDSA guidelines have recently been updated and provide details on the strength of the recommendations and the quality of the supporting evidence,36 making them the most current and comprehensive guidelines for the diagnosis and management of DFUs.
National Institute for Health and Care Excellence guidance37 recommends that clinicians should evaluate a diabetic patient presenting with a foot wound at three levels: the patient as a whole, the affected foot or limb and the infected wound. For infected wounds, an appropriately obtained specimen for culture is recommended prior to starting empiric antibiotic therapy, if possible. NICE guidance37 recommends sending a specimen for culture that is from deep tissue, obtained by biopsy or curettage and after the wound has been cleansed and debrided. The guidance advises against taking swab specimens, especially of inadequately debrided wounds, as they are likely to provide less accurate results. The IWGDF guidelines have the same message about obtaining the specimen but also mention the value of obtaining a Gram-stained smear of the wound in addition to culture.42 For infected wounds, the IDSA guidelines34–36 recommend sending a specimen for culture that is from deep tissue, obtained by biopsy or curettage after the wound has been cleansed and debrided. The guidelines also advise against obtaining for culture by swabbing the wound or wound drainage.36 In summary, all the clinical guidelines34–37,42 agree on their preference of tissue sample (obtained by biopsy, curettage or aspiration) to wound swab specimens.
The need for research
Although clinicians commonly use swab samples to provide information on the bacteria in a clinically infected wound, the current major guidelines all recommend tissue specimens over swab samples.34–38 This is mainly because swabs can be contaminated with colonising flora, can miss deep pathogens and may be less likely to grow anaerobic and some fastidious aerobic organisms. However, the strength of this recommendation was specified only in IDSA guidelines,36 where it was ‘moderate’ (i.e. further definitive research is likely to have an important impact on future recommendations).57
Three primary studies of culture techniques informing the guidelines were those conducted by Pellizzer et al.,58 Slater et al.59 and Bill et al.60 Pellizzer et al.58 assessed the reliability of results of ulcer swabbing versus deep tissue biopsy in 29 diabetic patients with a limb-threatening foot infection, who were neither recently treated with antibiotics nor hospitalised. This selected population does not reflect many of the patients with foot infections seen in outpatient clinics, who have often had recent antibiotic therapy. The study did not report on the agreement between swab and tissue samples, but, rather, simply on the number of bacterial colonies in each. Their conclusion that tissue samples are better than swab samples was based on a comparison of the numbers of isolates in only 21 participants remaining in the study at 30 days. Their finding may be due to chance as they performed 20 comparisons without adjustment for multiple testing. Furthermore, a method that identifies more colonies may be collecting more colonising bacteria and, therefore, is not necessarily ‘better’. The unpaired analysis presented means that we cannot readily compare the two techniques using appropriate statistical methods.
Slater et al.59 aimed to evaluate the accuracy of swab compared with deep tissue (obtained via needle aspiration) cultures in diabetic wounds of varying depth and severity. Their study, however, included only 30 people with ulcers (in a sample of 60, in which the other patients had deep abscesses, etc.) and it is not clear if the results were heterogeneous across types of wounds or apply to tissue samples collected using scalpel or curette. In 62% of the samples, there was a similar profile of organisms isolated from the swab and the deep tissue sample, whereas in 20% of samples the swab identified more organisms and in 18% the deeper tissue sample picked up more organisms. These data were not stratified by the presence or absence of an ulcer or by ulcer type (i.e. neuropathic or ischaemic). This study identified that there can be two forms of disagreement between swabbing and sampling, with swabs identifying more organisms or tissue samples identifying more organisms; hence, they did not consider either technique to be a gold standard.
In a 2006 systematic review of the diagnosis and management of infection in DFUs,61 only one study that evaluated sample acquisition and reported agreement in sufficient detail to allow appropriate analysis was identified. This study by Bill et al.60 included 18 patients with a pressure ulcer, 10 with a DFU, 5 with a venous leg ulcer, and 5 with an arterial ulcer. In this study, quantitative analysis of bacterial growth from a punch biopsy taken from the centre of the wound was compared with that of a wound swab. Using a definition of infection of a bacterial load of > 106 bacteria per gram of tissue in the punch biopsy, the authors reported a sensitivity for wound swabbing of 79%, meaning that the swab failed to detect approximately one in five wound infections as defined by punch biopsy. The derived likelihood ratios suggested that the wound swab was not a useful method of identifying infection in chronic wounds. Interpretation of this study’s findings is impeded by its small size and heterogeneity in the ulcer population. We cannot be sure that these data are directly transferable to the population of interest here, namely people with a DFU and a clinically diagnosed ulcer infection (there is no reason to sample uninfected ulcers and inclusion of people with uninfected foot ulcers may reduce the external validity of the study). In addition, there were potential sources of bias, such as no description of blind test verification and lack of clarity over whether or not the same clinical data were available when test results were interpreted as would be available when the test is used in practice.
Two studies62,63 have been published since the IDSA, NICE and IWGDF guidelines. Mutluoglu et al.62 assessed the reliability of cultures of superficial swabs by comparing them with cultures of concomitantly obtained deep tissue specimens in patients with DFUs. They retrospectively reviewed the notes from 54 patients from whom there were 89 pairs of samples, one a superficial swab and the other deep tissue. The results showed a 73% concordance between swab cultures and deep tissue biopsies, which dropped to 69.2% when sterile pairs of cultures were excluded. Compared with deep tissue specimens, in 11.2% of cases swabs detected additional species, in 9.0% of cases swabs detected fewer species and in 6.7% the two techniques identified totally different organisms. The study concluded that superficial swab cultures are not sufficiently accurate to identify the causative organisms in patients with an infected DFU. They described three forms of disagreement: swabs identified more organisms, tissue samples identified more organisms and the techniques found different organisms.
Demetriou et al.63 assessed the diagnostic performance of swabs versus tissue cultures in 50 consecutive diabetic patients with a foot ulcer, 28 of which were neuropathic and 22 of which were neuroischaemic. The authors stated that 36 (72%) wounds were infected, based on ‘the presence of at least 2 of the following criteria: local swelling or induration, erythema greater than 0.5 cm in any direction around the ulcer, local tenderness or pain, local increase of temperature, and purulent discharge’. Overall, the results showed that swabs reported significantly more isolates than tissue cultures, this difference being more evident in neuropathic than in neuroischaemic ulcers. They defined the tissue sample as the ‘gold standard’ for the diagnosis of infection, and reported swab culture sensitivity of 100% and negative predictive value of 100%, but the specificity was only 14.3% in neuropathic and 18.2% in neuroischaemic ulcers. They concluded that swabs are useful only to rule out infection. Given that guidelines do not recommend sampling/swabbing uninfected ulcers, the inclusion of 14 people in this study with uninfected ulcers reduces its external validity.
In summary, we concluded that the existing evidence regarding the results of cultures of specimens obtained by swabbing versus tissue sampling was derived from small, heterogeneous and, often, methodologically poor studies. Thus, there is a lack of robust evidence on the most appropriate method to use in routine clinical practice.
The question addressed in this study was not how to diagnose infection in a DFU, but rather what was the best way to collect a sample to characterise the bacterial flora. We therefore set out to describe the patterns of agreement and disagreement between swab and tissue samples. To help advise clinicians on the best technique to identify pathogens and to avoid colonising organisms in DFUs, we conducted a series of studies. The first was the ‘main study’, followed by three substudies:
main study: cross-sectional study to determine the patterns of agreement between culture results of contemporaneously collected swab versus tissue samples
substudy 1: independent ‘virtual’ clinical review of the appropriateness of empirical antimicrobial therapy based on the results of swabs compared with tissue samples to describe the potential clinical relevance of any differences in sampling results from swabs and tissue
substudy 2: a pilot comparative study of results of standard plating and culture techniques versus the molecular technique of PCR
substudy 3: a study of the prognosis of diabetic foot infection.
The main study was the first large, cross-sectional, multicentre study to examine agreement and disagreement of culture results between swab and tissue sampling techniques taken at the same time in a large group of patients with a clinically infected DFU.64 Each of the studies is described in detail in Chapters 2–5.
Study aims
The primary aim of the main study (patterns of agreement between swab sampling and tissue sampling) was to evaluate concordance between culture results from wound swabs and tissue samples from the same patient (see Chapter 2).
The aim of the clinical panel review study was to evaluate whether or not any differences in bacterial profiles from specimens obtained from swabs and tissue samples are clinically relevant. This was done by ascertaining from a panel of clinicians whether or not the reports from a swab or tissue sample would have resulted in a change in clinical management (see Chapter 3).
The aim of the pilot comparative study of standard plating and culture techniques versus PCR was to assess the concordance between results from specimens taken by conventional culture techniques and by molecular techniques (see Chapter 4).
The aim of the prognosis of foot infection study was to determine the outcome of patients with an infected DFU at 12 months post registration and to explore prognostic factors that may be related to time to wound healing (see Chapter 5).
Patient and public involvement
In this study, patient and public involvement was achieved by using our links with diabetes organisations at the national (Diabetes UK), regional (North West Diabetes Local Research Network) and local (School of Healthcare Service User Group) levels.
As this work was commissioned by the NHS Health Technology Assessment (HTA) programme, then there had been patient and public involvement engagement at the prioritisation stage, and this informed the commissioners as regards the importance and relevance of the clinical question.
During the study we were fortunate to recruit a patient representative, Mrs Christine Thomas, as a member of the Study Steering Committee (SSC). She played a key part in the SSC meetings and advised the study team at different stages, including at the writing of patient and public-facing information. She also had an important role in shaping all aspects of the communications with patients as regards consent, particularly when moving to verbal consent. Furthermore, Mrs Thomas advised the study team about the dissemination of the initial results to participants at the end of the study and reviewed draft communications.
