Skin and soft-tissue infections: Classifying and treating a spectrum

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ABSTRACTSkin and soft-tissue infections (SSTIs) are a common presenting problem in both inpatients and outpatients. SSTIs have been broadly classified as complicated or uncomplicated, but specific disease processes and patient characteristics are important in guiding clinical management. Early recognition of the extent of infection, close follow-up, and familiarity with local antibiotic susceptibility data are critical to successful treatment.


  • Categories and definitions of specific subtypes of infections are evolving and have implications for treatment.
  • Methicillin-resistant Staphylococcus aureus (MRSA) and streptococci continue to be the predominant organisms in SSTIs.
  • A careful history and examination along with clinical attention are needed to elucidate atypical and severe infections.
  • Laboratory data can help characterize the severity of disease and determine the probability of necrotizing fasciitis.
  • Although cultures are unfortunately not reliably positive, their yield is higher in severe disease and they should be obtained, given the importance of antimicrobial susceptibility.
  • The Infectious Diseases Society of America has recently released guidelines on MRSA, and additional guidelines addressing the spectrum of SSTIs are expected within a year.



Skin and soft-tissue infections (SSTIs) are a common reason for presentation to outpatient practices, emergency rooms, and hospitals.1–5 They account for more than 14 million outpatient visits in the United States each year,1 and visits to the emergency room and admissions to the hospital for them are increasing.2,3 Hospital admissions for SSTIs increased by 29% from 2000 to 2004.3


The increase in hospital admissions for SSTIs has been attributed to a rising number of infections with methicillin-resistant Staphylococcus aureus (MRSA).3–5

In addition, strains once seen mostly in the community and other strains that were associated with health care are now being seen more often in both settings. Clinical characteristics do not differ between community-acquired and health-care-associated MRSA, and therefore the distinction between the two is becoming less useful in guiding empiric therapy.6,7

After steadily increasing for several years, the incidence of MRSA has recently stabilized. The US Centers for Disease Control and Prevention maintains a surveillance program and a Web site on MRSA.8

At the same time, infections with group A, B, C, or G streptococci continue to be common. The SENTRY Antimicrobial Surveillance Program for the United States and Canada collected data from medical centers in five Canadian provinces and 32 US states between 1998 and 2004. The data set represents mostly complicated infections (see below). Staphylococcus was the most commonly retrieved organism (Table 1).9 However, streptococci were likely underrepresented, since mild or superficial streptococcal cellulitis may not require hospital admission, and positive cultures can be difficult to obtain in streptococcal infection.


Complicated skin and skin structure infections is a relatively new term coined in a 1998 US Food and Drug Administration (FDA) guideline for industry on developing antimicrobial drugs.10 Subsequent trials of antibiotics and reviews of skin infections used the guideline and its definitions. However, the category of complicated skin infections contained widely disparate clinical entities ranging from deep decubitus ulcers to diabetic foot infections (Table 2).10

The intent of the 1998 guideline was to provide not a clinical framework but rather a guide for industry in designing trials that would include similar groups of infections and therefore be relevant when compared with each other. In 2008, the Anti-Infective Drugs Advisory Committee was convened,11 and subsequently, in August 2010, the FDA released a revision of the guide.12

The revised guidelines specifically exclude many diagnoses, such as bite wounds, bone and joint infections, necrotizing fasciitis, diabetic foot infections, decubitus ulcers, catheter site infections, myonecrosis, and ecthyma gangrenosum. Notably, the word “bacterial” in the title excludes mycobacterial and fungal infections from consideration. The diagnoses that are included include cellulitis, erysipelas, major cutaneous abscess, and burn infections. These are further specified to include 75 cm2 of redness, edema, or induration to standardize the extent of the infection—ie, the infection has to be at least this large or else it is not “complicated.”

The terms “complicated” and “uncomplicated” skin and skin structure infections persist and can be useful adjuncts in describing SSTIs.13–16 However, more specific descriptions of SSTIs based on pathogenesis are more useful to the clinician and are usually the basis for guidelines, such as for preventing surgical site infections or for reducing amputations in diabetic foot infections.

This review will focus on the general categories of SSTI and will not address surgical site infections, pressure ulcers, diabetic foot infections, perirectal wounds, or adjuvant therapies in severe SSTIs, such as negative pressure wound care (vacuum-assisted closure devices) and hyperbaric chambers.


SSTIs vary broadly in their location and severity.

Although the classic presentation of erythema, warmth, edema, and tenderness often signals infection, other diseases can mimic SSTIs. Common ones that should be included in the differential diagnosis include gout, thrombophlebitis, deep vein thrombosis, contact dermatitis, carcinoma erysipeloides, drug eruption, and a foreign body reaction.17,18


Specific exposures. A detailed history can point to possible organisms and appropriate therapy. Table 3 lists several risk factors or exposures that may be elicited in the history and the pathogens they suggest.14

Wounds. Skin infections are usually precipitated by a break in the skin from a cut, laceration, excoriation, fungal infection, insect or animal bite, or puncture wound.

Impaired response. Patients with diabetes, renal failure, cirrhosis, chronic glucocorticoid use, history of organ transplantation, chronic immunosuppressive therapy, HIV infection, or malnourishment have impaired host responses to infection and are at risk for both more severe infections and recurrent infections. Immunocompromised hosts may also have atypical infections with opportunistic organisms such as Pseudomonas, Proteus, Serratia, Enterobacter, Citrobacter, and anaerobes. Close follow-up of these patients is warranted to ascertain appropriate response to therapy.19

Surgery that includes lymph node dissection or saphenous vein resection for coronary artery bypass can lead to impaired lymphatic drainage and edema, and therefore predisposes patients to SSTIs.


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