Classification of Infections
Infections are classified by multiple complementary systems: anatomical site, causative organism, severity/clinical presentation, and whether they are necrotizing or non-necrotizing—with the most clinically actionable approach being severity-based classification that directly guides treatment intensity and hospitalization decisions.
Primary Classification Systems
By Anatomical Site
The IDSA organizes infections by anatomical location, which provides a practical framework for diagnosis and treatment 1:
- Bloodstream and cardiovascular system infections
- Central nervous system infections
- Respiratory tract infections (upper and lower)
- Gastrointestinal tract infections
- Genitourinary infections
- Skin and soft tissue infections
- Bone and joint infections
- Ocular infections
By Causative Organism
Infections are classified by pathogen type, which determines antimicrobial selection 1:
- Bacterial infections (further subdivided by specific organism: gram-positive cocci, gram-negative bacilli, anaerobes)
- Viral infections
- Fungal infections
- Parasitic infections (blood and tissue parasites)
- Arthropod-borne infections 1
The CDC provides specific case definitions requiring laboratory confirmation for reportable infectious conditions, categorizing them as "confirmed" or "probable" based on clinical and laboratory criteria 1.
By Severity and Clinical Presentation
For skin and soft tissue infections specifically, the most clinically useful classification is severity-based 1:
- Uncomplicated SSTIs: Superficial infections (impetigo, simple abscesses, furuncles) requiring only antibiotics or simple drainage; low risk for life- or limb-threatening complications 1
- Complicated SSTIs: Deep infections (necrotizing infections, infected ulcers, major abscesses) requiring significant surgical intervention; high risk for life-threatening infection 1
The Eron classification further stratifies by systemic toxicity and comorbidities 1:
- Class 1: No systemic toxicity or comorbidities
- Class 2: Systemically unwell with stable comorbidities OR systemically well with complicating comorbidity (diabetes, obesity)
- Class 3: Toxic appearance (fever, tachycardia, tachypnea, hypotension)
- Class 4: Sepsis syndrome or life-threatening infection (e.g., necrotizing fasciitis)
By Tissue Depth and Necrosis Status
The necrotizing versus non-necrotizing distinction must always be specified, as this fundamentally changes management 1:
- Non-necrotizing infections: Managed primarily with antibiotics ± minor drainage
- Necrotizing soft tissue infections (NSTIs): Spreading infection with tissue necrosis requiring urgent surgical debridement 1
NSTIs can be further described by:
- Anatomical depth: Necrotizing cellulitis (dermal/subcutaneous), necrotizing fasciitis (fascial), necrotizing myositis (muscular) 1
- Microbiology: Type 1 (polymicrobial), Type 2 (monomicrobial β-hemolytic streptococci or CA-MRSA), Type 3 (monomicrobial gram-negative bacilli) 1
However, these microbiologic and anatomic distinctions are less clinically useful than severity assessment, as all NSTIs require similar urgent surgical and antimicrobial management 1.
For Diabetic Foot Infections
Diabetic foot infections use a specific severity classification 1:
- Mild: Superficial, limited in size and depth
- Moderate: Deeper or more extensive
- Severe: Accompanied by systemic signs or metabolic perturbations
This classification, combined with vascular assessment, determines hospitalization need, imaging requirements, and surgical intervention 1.
Critical Clinical Pitfalls
The most dangerous error is delayed recognition of necrotizing infections—always differentiate necrotizing from non-necrotizing character, as delay in diagnosis or treatment correlates with poor outcomes and multiple organ failure 1. The LRINEC score (≥8 indicates 75% risk of NSTI) can aid early identification in emergency settings 1.
For severity assessment in NSTIs, classify patients as high risk versus mild/moderate risk of poor outcome to identify those requiring immediate surgical treatment and intensive care 1.
When classifying any infection, independently assess: necrotizing versus non-necrotizing character, anatomical extension, purulent versus non-purulent characteristics, and patient clinical condition 1.