Skin and soft tissue infections in primary care
Year: 2025
Type: article
Abstract: Opening Vignette Johnny, a 55-year-old man, presents to your primary care clinic for a painful red rash on his right leg, which has worsened over 4 days. During a nature walk a week before, he had sustained an insect bite, which he had scratched. He denies fever or chills and is well otherwise. You note a poorly demarcated, warm, oedematous, erythematous and tender plaque on the right shin [Figure 1]. There are no signs of bullae, pustules, skin necrosis, fluctuance or crepitus. The distal foot pulses are well felt. There are no signs of chronic venous stasis. He does not have other significant past medical history and is not on any long-term medications.Figure 1: Clinical photograph of the patient shows a poorly demarcated, warm, oedematous, erythematous and tender plaque on the right shin.WHAT ARE SKIN AND SOFT TISSUE INFECTIONS? Skin and soft tissue infections (SSTIs) are infections of the skin, subcutaneous tissue, fascia or muscle resulting from compromised skin defences and microbial invasion, usually bacterial and less commonly, viral, fungal or parasitic in nature. They may progress and spread via the lymphatic system or haematogeneously. There are a few classification systems for SSTIs. One example is classification according to the depth of infection within the skin layer. Superficial infections, such as impetigo, erysipelas, folliculitis, furuncles and carbuncles, arise in the epidermis and dermis, while cellulitis involves the dermis and subcutaneous tissue. Deep infections extend below the dermis and present as complex abscesses, fasciitis or myonecrosis involving the subcutaneous tissue, deep fascia or muscle compartments. In the ambulatory setting, we suggest classifying SSTIs based on whether they are purulent or nonpurulent. In this article, we suggest the following approach to SSTIs in the ambulatory setting: (a) identify the red flags that suggest the need for inpatient management; (b) consider SSTIs in two broad presentations — purulent (pus-forming) or nonpurulent infection. HOW IS THIS RELEVANT TO MY PRACTICE? In the past decade, SSTIs have consistently ranked in the top ten causes of hospitalisation in Singapore, and they are also commonly encountered in primary care.[1] In the USA, about three-quarters of SSTIs are managed in the outpatient setting; these cases account for more than 14 million visits annually.[2] Primary care physicians play a crucial role in the management of SSTIs, and their antibiotic prescribing practices have major effects on bacterial antimicrobial resistance rates in the community. A local study reported inappropriate antibiotic use in 61% of Singapore residents, contributing to the growing public health threat of antimicrobial resistance locally.[3] A high incidence of methicillin resistance (35.3%) was discovered among Staphylococcus aureus hospital isolates in a national survey in 2006,[4] although there is little data on the prevalence of methicillin-resistant Staphylococcus aureus (MRSA) in the local community. Drug-resistant infections are difficult and costlier to treat, and patients with such infections have excess morbidity and mortality. Moreover, as there are not many new antimicrobials in the drug discovery pipeline, clinicians are left with few choices and must repurpose older, highly toxic drugs to treat patients with extensively drug-resistant infections. It is important and urgent to review our prescribing practices, as the impact of rampant, inappropriate antimicrobial use will adversely affect the health outcomes of present patients, as well as create a ripple effect on future generations. WHAT CAN I DO IN MY PRACTICE? History and physical examination A thorough history of predisposing factors for SSTIs (e.g., poor hygiene or skin care, preceding skin trauma such as animal bites or intravenous drug use, exposure to environmental pathogens in soil or seawater, comorbid conditions such as diabetes mellitus and immunocompromised states, etc.) allows physicians to identify the most likely causative pathogen and the risk factors for recurrent and nonresolving SSTIs. Next, a complete history of the presenting complaint (typically skin redness, warmth, swelling, pain or fever), including the tempo and progression of symptoms, and red flags for emergency department (ED) referral (e.g. rapid progression of infection, unstable comorbidities) should be taken. Finally, the physician should explore the treatments that patients had tried (e.g. topical medications, antibiotics, complementary medicine), their drug allergies and other social factors (e.g., home support, ability to return for review, functional loss), which will affect management decisions. Physical examination begins with a set of vital signs to identify systemic signs of severe infection (e.g., fever, hypotension and tachycardia). In the primary care setting, sepsis (defined as an infection leading to organ dysfunction) is probably best identified using the quick Sepsis-related Organ Failure Assessment (qSOFA), which includes altered mental status, hypotension and tachypnoea. Next, the location, extent and depth of infection (indicated by erythema, warmth, tenderness and swelling), presence of pus and any regional lymphadenitis should be documented. Physicians should also record the length and width of the area of infection and document the presence or absence of any red flags [Box 1], such as pain out of proportion to signs, oedema extending beyond the area of erythema, crepitus and skin bullae or necrosis suggesting necrotising infection.Box 1: Key considerations in the management of skin and soft tissue infections (SSTIs).If the SSTI can be managed in the outpatient setting, demarcate the initial lesion for monitoring of improvement or progression via any of the following methods: trace a dotted line along the edges of the infection with a skin marker, outline the edges on a transparent plastic sheet or take a clinical photograph with a ruler beside the lesion. A search for predisposing factors for SSTIs or sources of entry for pathogens should be conducted by carefully examining the skin (including the nails, nailfolds and web spaces) for skin breaches (excoriations, wounds), dermatological conditions affecting the skin barrier such as eczema, and signs of poor hygiene. The physician should also search for signs that support differential diagnoses, such as the presence of gouty tophi suggesting a gout flare or bilateral symmetrical lower limb signs suggesting venous stasis dermatitis. Box 1 is a comprehensive summary of red flags for ED referral and risk factors for more severe infection that should be carefully evaluated for during history-taking and examination. The primary care physician must be mindful that the immunocompromised patient with SSTI may not display typical clinical features and investigation findings due to dampened inflammatory response. Recurrent or nonresolving SSTIs affecting the immunocompromised patient may be due to multidrug-resistant bacteria, atypical bacteria or fungal infections, which warrant evaluation and/or empirical therapy at the hospital.[7] One may also wish to consider differential diagnoses where appropriate, such as eosinophilic cellulitis, dissecting cellulitis, hidradenitis suppurativa and erythema nodosum. Necrotising soft tissue infections Necrotising soft tissue infections (NSTIs) are rapidly progressive limb- and life-threatening infections that require early recognition, empirical polymicrobial antibiotic coverage and surgical debridement. They may be termed according to their anatomical locations (i.e., Fournier gangrene) or depth of invasion (e.g., necrotising cellulitis involves dermal and subcutaneous tissue, necrotising fasciitis involves deeper fascia and necrotising myositis involves muscle). Fournier’s gangrene is a severe type of NSTI involving the genital area or perineum, which presents with subcutaneous crepitations, patches of gangrene and foul-smelling purulent discharge.[8] Signs of and risk factors for necrotising infection are found in Box 1. The Laboratory Risk Indicator for Necrotising Fasciitis (LRINEC) score is used to distinguish necrotising fasciitis from other SSTIs but is not useful in the primary care setting where blood investigations are not promptly available. Hence, NSTI is primarily a clinical diagnosis. Investigations Besides a random blood glucose or glycated haemoglobin (HbA1c) test, which may be important in undiagnosed diabetes mellitus or identifying poorly controlled diabetes mellitus, laboratory investigations are not needed for most simple localised SSTIs. In ambiguous cases, a complete blood count may guide primary care physicians in the early identification of patients at high risk of developing sepsis and adverse outcomes. Imaging is not indicated for simple SSTIs, and a plain radiograph with normal findings cannot be used to exclude necrotising infection. Management and follow-up Figure 2 shows a simplified treatment algorithm for SSTIs. The first step in the management algorithm is to identify the red flags suggesting the need for inpatient management. Patients with severe or complicated infections, sepsis or unstable comorbid illnesses that increase the risk of severe infection require a referral to the ED for further investigations and management, including fluid resuscitation, parenteral antibiotics or surgical intervention. Patients with uncontrolled infection despite appropriate outpatient antimicrobial therapy or who cannot tolerate oral antibiotics should be referred to the ED for parenteral antibiotics and/or source control.Figure 2: Treatment algorithm for skin and soft tissue infections. MRSA: methicillin-resistant Staphylococcus aureusWe suggest grouping the SSTIs as purulent or nonpurulent [Box 2]. Purulent SSTIs may require a referral to the ED for source control via incision and drainage if facilities are not available in the primary care setting. If complete source control has been achieved, the subsequent use of antibiotics is controversial and clinical judgement should be exercised. Antibiotics with adequate Staphylococcus coverage (including cephalexin, cloxacillin, clindamycin, trimethoprim–sulfamethoxazole) may be needed depending on disease factors (e.g., surrounding cellulitis, poor response to initial source control, involvement of specific sites such as the hand where the risk of functional loss may be higher) and patient factors (e.g., children, elderly patients, significant comorbidities or immunosuppression).Box 2: Purulent and nonpurulent skin and soft tissue infections.Physicians should consider the severity of infection, exposure/pathogen, drug resistance, patient factors such as comorbidities and drug allergies, and culture results (if available) in deciding the choice of antibiotics. Nonpurulent infections are commonly caused by beta-haemolytic Streptococcus; antibiotics with adequate coverage include cephalexin, penicillin, amoxicillin and clindamycin. For nonresolving SSTIs in patients coming from long-term care facilities or who are known MRSA carriers, consider MRSA as a possibility. Oral antibiotics with MRSA coverage include clindamycin, trimethoprim–sulfamethoxazole and tetracycline. However, these antibiotics have variable and unreliable activity against MRSA strains, and clinical failure may be encountered in disseminated or undrained abscesses. Recurrent or nonresponding infection should prompt consideration of untreated predisposing conditions as well as noninfectious mimics such as gout, deep venous thrombosis and dermatological conditions (e.g., venous stasis dermatitis, contact dermatitis, eczema).[9] Predisposing factors picked up on clinical assessment must be addressed to increase the chances of successful outpatient treatment of the SSTI and to prevent future SSTIs. Patients with pilonidal cysts, hidradenitis suppurativa or foreign bodies are predisposed to recurrent purulent SSTIs and may require a surgical referral. Infrequently, children with recurrent SSTIs may have an underlying immune defect and will benefit from early specialist review.[10] Special mention: diabetic foot infections Diabetic foot infections (DFIs) are soft tissue or bone infections often associated with neuropathy or peripheral arterial disease in a diabetic patient, and are commonly seen in the ambulatory setting. In diabetic patients with a foot wound, DFIs should be ruled out. Physicians should assess for signs of inflammation (erythema, warmth, swelling, tenderness or pain), purulent discharge, friable or discoloured granulation tissue, undermined wound edges and foul odour. Most DFIs are polymicrobial, with staphylococci and streptococci being the most common causative pathogens. Gram-negative bacilli coinfection may be present in chronic infections or infections that have been pretreated with antibiotics, while anaerobes may be found in ischaemic or necrotic wounds. Diabetic foot infections are classified as mild (superficial, ≤2 cm erythema around ulcer), moderate (deeper than subcutaneous tissues, local erythema >2 cm) and severe (signs of severe sepsis).[11] The probe-to-bone test should be performed on any ulcer, as a positive test is predictive of osteomyelitis. Other causes of inflammatory response, such as gout or Charcot arthropathy, should also be excluded. Initial workup in the ambulatory setting may include an ankle–brachial index, plain radiograph of the affected foot and basic blood tests (blood glucose level, haemoglobin A1C, complete blood count). If soft tissue abscess or osteomyelitis is suspected, further imaging and tertiary care may be needed. Wounds without evidence of soft tissue or bone infection should not be treated with antibiotics but will require wound dressing, offloading and close monitoring to ensure wound healing. Mild, nonpurulent SSTI in a diabetic patient may be managed in the outpatient setting in a similar manner as nondiabetic patients. However, moderate and severe DFIs may require broad-spectrum, parenteral antibiotics and surgical intervention, and necessitate a referral to the ED. PATIENT EDUCATION As patients frequently have misconceptions about the use of antibiotics, their visit for an SSTI may be a good opportunity for public education on the role of antibiotics in treating bacterial infections and the need to complete the antibiotic course as prescribed to prevent the development of antibiotic resistance. Patients with SSTIs should also be educated on keeping the area clean and dry, elevating the limb above the level of their heart to improve drainage of oedema and inflammatory substances to facilitate recovery, and maintaining good skin care to prevent future SSTIs. Diabetic foot care advice should be given to diabetic patients. For nonpurulent SSTIs, primary care physicians may demarcate the area of erythema and advise patients to return if there is persistent fever, worsening of symptoms, or no improvement after 24–48 h of antibiotic initiation.[12] TAKE HOME MESSAGES For SSTIs, first exclude red flags that require inpatient management: rapidly progressive, severe or complicated infections, sepsis, unstable comorbid illnesses, surgical intervention is contemplated, diagnosis or prognosis is in doubt, or the patient cannot adhere to outpatient treatment. Next, distinguish between purulent and nonpurulent infections. Purulent infections may be effectively treated with incision and drainage alone or with adjunctive short course of antibiotics, while nonpurulent infections are treated with antibiotics. Antibiotic choice is guided by severity of infection, exposure/pathogen, drug resistance, patient factors (comorbidities, drug allergies) and culture results. Laboratory tests are not needed in most SSTIs treated in the outpatient setting, but a complete blood count can help triage severity of infection. Plain radiography cannot exclude necrotising infection. Most uncomplicated SSTIs can be managed in the outpatient setting, but adequate return advice should be given and a timely review should be arranged to ensure good response. Poor response to antibiotics should prompt considerations regarding diagnosis, causative pathogen, antibiotic treatment and patient compliance factors. Always exclude infection in any diabetic foot with a wound and consider antimicrobial coverage for gram-negative bacilli coinfection in chronic infections or infections pretreated with antibiotics, and anaerobic coverage for ischaemic or necrotic wounds. Primary care physicians play a pivotal role not only in the prompt diagnosis and management of SSTIs, but also in appropriate antibiotic prescribing to achieve good immediate and long-term outcomes for patients. Closing Vignette You confirm that Johnny does not have a history of recurrent cellulitis or risk factors for serious or prolonged infection, such as diabetes mellitus or peripheral vascular disease. You diagnose Johnny with mild cellulitis of the right leg and decide that it is appropriate to manage him on an outpatient basis, with a review in 1 week. You determine that the source is most likely a skin breach, and the most likely pathogen is Streptococcus pyogenes. After verifying that he has no drug allergies, you start him on empirical amoxicillin 500 mg three times daily for a week. You educate him on his condition and advise him on leg elevation to reduce swelling, as well as insect bite prevention. A week later, you are happy to see that his cellulitis has resolved, and he thanks you for your professional medical advice.Financial support and sponsorship Nil. Conflicts of interest How CH is the handling editor of the PILL series. SMC CATEGORY 3B CME PROGRAMME Online Quiz: https://www.sma.org.sg/cme-programme Deadline for submission: 6 pm, 17 March 2025
Source: Singapore Medical Journal
Authors: Jen Yi Ling, Choon How How, Jaime Mei Fong Chien, Vijo Poulose, Mark Chung Wai Ng
Institutions: Changi General Hospital, SingHealth Polyclinics
Language: English
Field: Medicine
Domain: Health Sciences
Open Access status: diamond
OpenAlex ID: W4407671367