Skin and soft-tissue infections: Classifying and treating a spectrum
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.
KEY POINTS
- 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.
PHYSICAL EXAMINATION
The physical examination should include descriptions of the extent and location of erythema, edema, warmth, and tenderness so that progression or resolution with treatment can be followed in detail.
Crepitus can be felt in gas-forming infections and raises the concern for necrotizing fasciitis and infection with anaerobic organisms such as Clostridium perfringens.
Necrosis can occur in brown recluse spider bites, venous snake bites, or group A streptococcal infections.
Fluctuance indicates fluid and a likely abscess that may need incision and drainage.
Purpura may be present in patients on anticoagulation therapy, but if it is accompanying an SSTI, it also raises the concern for the possibility of sepsis and disseminated intravascular coagulation, especially from streptococcal infections.
Bullae can be seen in impetigo caused by staphylococci or in infection with Vibrio vulnificus or Streptococcus pneumoniae.19
Systemic signs, in addition to fever, can include hypotension and tachycardia, which would prompt closer monitoring and possible hospitalization.
Lymphangitic spread also indicates severe infection.
LABORATORY STUDIES
Simple, localized SSTIs usually do not require laboratory evaluation. Jenkins et al21 recently demonstrated that by using an algorithm for the management of hospitalized patients with cellulitis or cutaneous abscess, they could decrease resource utilization, including laboratory testing, without adversely affecting clinical outcome.
If patients have underlying disease or more extensive infection, then baseline chemistry values, a complete blood cell count, and the C-reactive protein level should be acquired.19 Laboratory findings that suggest more severe disease include low sodium, low bicarbonate (or an anion gap), and high creatinine levels; new anemia; a high or very low white blood cell count; and a high C-reactive protein level. A high C-reactive protein level has been associated with longer hospitalization.22
A score to estimate the risk of necrotizing fasciitis
This tool was developed retrospectively but has been validated prospectively. It has a high sensitivity and a positive predictive value of 92% in patients with a score of six points or more. Its specificity is also high, with a negative predictive value of 96%.20,24
Necrotizing fasciitis has a mortality rate of 23.5%, but this may be reduced to 10% with early detection and prompt surgical intervention.15 Since necrotizing fasciitis is very difficult to diagnose, clinicians must maintain a high level of suspicion and use the LRINEC score to trigger early surgical evaluation. Surgical exploration is the only way to definitively diagnose necrotizing fasciitis.
Blood cultures in some cases
Blood cultures have a low yield and are usually not cost-effective, but they should be obtained in patients who have lymphedema, immune deficiency, fever, pain out of proportion to the findings on examination, tachycardia, or hypotension, as blood cultures are more likely to be positive in more serious infections and can help guide antimicrobial therapy. Blood cultures are also recommended in patients with infections involving specific anatomic sites, such as the mouth and eyes.19
Aspiration, swabs, incision and drainage
Fluid aspirated from abscesses and swabs of debrided ulcerated wounds should be sent for Gram stain and culture. Gram stain and culture have widely varying yields, from less than 5% to 40%, depending on the source and technique.19 Cultures were not routinely obtained before MRSA emerged, but knowing antimicrobial susceptibility is now important to guide antibiotic therapy. Unfortunately, in cellulitis, swabs and aspirates of the leading edge have a low yield of around 10%.25 One prospective study of 25 hospitalized patients did report a higher yield of positive cultures in patients with fever or underlying disease,26 so aspirates may be used in selected cases. In small studies, the yield of punch biopsies was slightly better than that of needle aspirates and was as high as 20% to 30%.27
