What is the Infectious Diseases Society of America (IDSA) classification and management of diabetic (DM) foot infections?

IDSA Classification of Diabetic Foot Infections

The IDSA classifies diabetic foot infections into four severity grades based on clinical findings: Grade 1 (uninfected), Grade 2 (mild), Grade 3 (moderate), and Grade 4 (severe), with osteomyelitis designated by adding "(O)" to any grade. 1

Diagnostic Criteria for Infection

Infection is diagnosed clinically when at least 2 classic signs of inflammation are present: erythema, warmth, tenderness, pain, induration, or purulent secretions. 1 Secondary signs may include nonpurulent secretions, friable or discolored granulation tissue, undermining of wound edges, or foul odor. 1

Classification System

Grade 1: Uninfected

• No local or systemic symptoms or signs of infection 1
• Wound present but no inflammatory response 1

Grade 2: Mild Infection

• At least 2 of the following present: 1
  • Local swelling or induration
  • Erythema >0.5 cm but <2 cm around the wound
  • Local tenderness or pain
  • Local increased warmth
  • Purulent discharge
• Infection limited to skin and subcutaneous tissue only 1
• No systemic manifestations 1
• Exclude other causes of inflammation (trauma, gout, Charcot arthropathy, fracture, thrombosis, venous stasis) 1

Grade 3: Moderate Infection

• Erythema extending ≥2 cm from the wound margin, and/or 1
• Infection involving tissues deeper than skin and subcutaneous tissues (tendon, muscle, joint, bone) 1
• No systemic inflammatory response syndrome (SIRS) criteria present 1

Grade 4: Severe Infection

• Any foot infection with ≥2 SIRS criteria: 1
  • Temperature >38°C or <36°C
  • Heart rate >90 beats/min
  • Respiratory rate >20 breaths/min or PaCO₂ <32 mmHg (4.3 kPa)
  • White blood cell count >12,000/mm³ or <4,000/mm³, or >10% immature (band) forms
• Indicates systemic toxicity or metabolic instability 1

Osteomyelitis Designation

• Add (O) to any grade when bone infection is present 1
• If osteomyelitis exists without ≥2 signs of local inflammation, classify as Grade 3(O) if <2 SIRS criteria or Grade 4(O) if ≥2 SIRS criteria 1

Three-Level Assessment Approach

The IDSA mandates evaluating patients at three distinct levels: 1

Level 1: Patient as a Whole

• Document vital signs to identify SIRS criteria 2
• Assess metabolic status including glucose control and renal function 2
• Evaluate for systemic toxicity or metabolic perturbations 1

Level 2: Affected Limb

• Assess arterial ischemia (strong recommendation, moderate quality evidence) 1
• Evaluate venous insufficiency 1
• Test for loss of protective sensation 1
• Identify biomechanical problems 1

Level 3: Infected Wound

• Debride necrotic tissue and surrounding callus 1
• Classify infection severity using the grading system above 1
• Obtain post-debridement specimens for culture (tissue preferred over swabs) 1

Management Implications by Grade

Mild Infections (Grade 2)

• Outpatient oral antibiotics targeting aerobic gram-positive cocci (especially Staphylococcus aureus) 1
• Duration: 1-2 weeks, possibly extending to 3-4 weeks 2
• Wound care and pressure off-loading 1

Moderate Infections (Grade 3)

• May require hospitalization depending on patient factors 1
• Consider parenteral antibiotics initially 1
• Duration: 2-4 weeks depending on debridement adequacy and vascularity 2
• Broader spectrum coverage if chronic, previously treated, or at risk for resistant organisms 1

Severe Infections (Grade 4)

• Hospitalization mandatory 1
• Intravenous broad-spectrum antibiotics pending culture results 1
• Urgent surgical consultation for abscess drainage, extensive bone involvement, crepitus, substantial necrosis, or necrotizing fasciitis 2
• Duration: 2-4 weeks for soft tissue; minimum 4-6 weeks if osteomyelitis with retained infected bone 2

Osteomyelitis (Any Grade with "O")

• Minimum 4-6 weeks of antibiotics if infected bone remains 2
• Shorter duration acceptable if all infected bone surgically removed 2
• MRI is the preferred imaging modality for diagnosis 1
• Bone biopsy provides definitive diagnosis and guides antibiotic selection 1

Critical Pitfalls to Avoid

Do not treat clinically uninfected ulcers with antibiotics (moderate evidence against use). 1 All wounds are colonized with microorganisms, but colonization does not equal infection. 1

Peripheral arterial disease is present in up to 40% of diabetic foot infections, making vascular assessment critical for treatment success. 3 Ischemic wounds may require revascularization before healing can occur. 1

Systemic inflammatory markers may be absent in up to half of patients even with severe infection, so their absence does not exclude serious disease. 2 However, their presence indicates more extensive tissue involvement or virulent pathogens. 2

Multidisciplinary Team Approach

A coordinated multidisciplinary diabetic foot care team significantly improves outcomes (strong recommendation, moderate quality evidence). 1 The team should include or have access to infectious disease specialists, surgeons with diabetic foot expertise, vascular specialists, podiatrists, and wound care specialists. 1, 4