Diagnostics & Classification in SSTIs and Acute Osteomyelitis

1. Clinical Evaluation

A structured history and focused physical exam are critical to distinguish simple cellulitis from deeper infections and to identify red flags for necrotizing infection or bone involvement.

History

• Symptom duration: Less than 2 weeks suggests an acute infection, while symptoms lasting over 6 weeks raise concern for chronic osteomyelitis or indolent pathogens.
• Systemic signs: Fever, chills, and malaise can indicate bacteremia or a deep-seated infection.
• Local factors: Inquire about recent trauma, ulcers, prosthetic hardware, prior antibiotic use, and diabetic foot wounds.
• Comorbidities: Conditions like diabetes, peripheral vascular disease, and immunosuppression can alter the clinical presentation and increase risk.

Physical Exam

• Cellulitis: Presents as ill-defined erythema, warmth, tenderness, and nonfluctuant edema.
• Abscess: Characterized by a fluctuant mass, localized tenderness, and often associated with overlying pustules.
• Necrotizing infection: Suspect this with severe pain out of proportion to exam findings, rapid spread of erythema, bullae, crepitus (subcutaneous gas), skin necrosis, or skin anesthesia.
• Osteomyelitis: Look for point tenderness over bone, sinus tracts, draining fistulae, or a decreased range of motion in an adjacent joint.

2. Laboratory Assessment

Routine laboratory tests are used to support clinical suspicion, stratify the severity of infection, and monitor the patient’s response to therapy.

• Complete blood count (CBC): Leukocytosis (WBC >12,000/µL) is common in moderate-to-severe SSTIs but may be absent in early or chronic osteomyelitis.
• C-reactive protein (CRP): This inflammatory marker rises within 6–12 hours of infection onset. Serial measurements are valuable for guiding response and determining the duration of therapy.
• Procalcitonin: While more specific for systemic bacterial infection, it has limited sensitivity for localized bone and deep-tissue infections.
• Blood cultures: Indicated for purulent SSTIs with systemic signs, suspected hematogenous osteomyelitis, or in immunocompromised hosts. The yield is low in simple cellulitis (<5%) but can be higher in osteomyelitis (up to 50% in pediatric hematogenous cases).

3. Microbiologic Diagnostics

Culture samples should be obtained from the deep infection site to ensure accuracy. Avoid relying on superficial swabs, which often grow colonizing flora, except as adjunctive information.

• Purulent SSTIs: An ultrasound-guided needle aspirate of an abscess provides the highest correlation with the causative pathogens.
• Nonpurulent cellulitis: In severe, recurrent, or atypical cases, an edge-of-lesion tissue biopsy or aspirate may identify streptococci or staphylococci.
• Osteomyelitis: A bone biopsy, obtained either percutaneously or via an open procedure, remains the gold standard for diagnosis, yielding the pathogen in over 80% of cases.
• Molecular diagnostics: Broad-range PCR and 16S rRNA sequencing can detect fastidious or culture-negative organisms, which is especially useful in biofilm-associated infections on prosthetic material.

4. Imaging Modalities

The choice of imaging should be based on the specific clinical question, such as identifying an abscess, detecting gas, or evaluating for bone marrow involvement.

• Ultrasound: An excellent bedside tool for detecting superficial abscesses and guiding needle aspiration. Its main limitation is poor assessment of deep fascial planes or bone.
• Computed Tomography (CT): Useful for identifying gas in soft tissues (a sign of necrotizing infection), delineating deep fluid collections, and assessing cortical bone destruction. It is a valuable alternative when MRI is unavailable or contraindicated.
• Magnetic Resonance Imaging (MRI): Considered the gold standard for diagnosing both osteomyelitis and necrotizing SSTIs. T2-weighted/STIR sequences highlight edema in marrow and fascia, while gadolinium enhancement helps differentiate an abscess from phlegmon. MRI has a sensitivity and specificity of over 90% for osteomyelitis and is superior for defining the soft-tissue and intramedullary extent of infection.

5. Classification & Severity Scoring

Applying standardized classification systems helps stratify risk, guide antibiotic selection, and determine the need for inpatient care or surgical intervention.

Cellulitis Classification

• Uncomplicated: Localized erythema without systemic signs of infection. Typically managed with outpatient oral therapy targeting streptococci.
• Complicated: Associated with purulence, systemic toxicity, rapid progression, or deeper tissue involvement. Requires hospitalization and empiric coverage for MRSA when indicated.

LRINEC Score for Necrotizing Fasciitis

The Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score uses common lab values to estimate risk.

Cierny-Mader Staging of Osteomyelitis

This system classifies chronic osteomyelitis based on the anatomic location of the infection and the physiologic status of the host, which guides surgical and medical management.

6. Indications for Surgical Exploration

Persistent infection despite medical management or signs of deep necrosis warrant prompt surgical consultation for source control.

• Failure of adequate antibiotic therapy after 48–72 hours (e.g., rising CRP, persistent fever).
• Clinical or imaging evidence of a drainable abscess, necrotic fascia, or gas in the tissues.
• High clinical suspicion for necrotizing infection (extreme pain, systemic toxicity, crepitus), even if the LRINEC score is low.
• Osteomyelitis with sequestra (dead bone), hardware involvement, or draining sinus tracts that do not heal.