Classification of Purulent Skin and Soft Tissue Infections

Definition and Criteria

The Infectious Diseases Society of America (IDSA) defines moderate purulent skin and soft tissue infections (SSTIs) by the presence of systemic signs of infection, including temperature >38°C, heart rate >90 beats per minute, respiratory rate >24 breaths per minute, or abnormal white blood cell count (<12,000 or >12,000 cells/μL) 1, 2, 3.
Severe purulent SSTIs are identified by treatment failure, systemic signs of infection, immunocompromised status, or clinical signs of deeper infection, such as bullae, skin sloughing, hypotension, or evidence of organ dysfunction 1, 2, 3, 4.

Systemic signs of infection, including fever, tachycardia, tachypnea, and abnormal white blood cell count, are used to distinguish between moderate and severe purulent SSTIs 1, 2, 3.
The presence of any systemic sign indicates bacteremia risk and necessitates antimicrobial coverage, with moderate infections requiring systemic antibiotics in addition to drainage 1, 2, 3.

Immunocompromised patients, including those with diabetes, HIV, or on immunosuppressive therapy, are automatically classified as severe due to unpredictable progression and higher mortality risk 1, 2, 3, 6.

Treatment failure is a severity marker, indicating either resistant organisms, inadequate source control, or deeper infection requiring IV therapy and possible hospitalization 1, 2, 3.
Rapid progression of infection, including signs like violaceous bullae, disproportionate pain, or rapid spread of erythema, suggests necrotizing infection and requires immediate surgical consultation 5, 6, 8.
Abnormal white blood cell counts, including both leukocytosis and leukopenia, indicate systemic involvement and qualify as moderate or severe depending on other factors 1, 3.