What is the most likely diagnosis and first‑line treatment for a patient with acute erythema, warmth, and swelling of the shin, no pitting edema, and negative ultrasound for deep‑vein thrombosis and normal X‑ray for fracture?

Most Likely Diagnosis and First-Line Treatment

The most likely diagnosis is cellulitis of the lower leg, and first-line treatment is oral antibiotics targeting streptococci and staphylococci, specifically cephalexin 500mg four times daily or dicloxacillin 500mg four times daily for 5-10 days, combined with leg elevation and warm compresses. 1

Diagnostic Reasoning

The clinical presentation—erythema, warmth, swelling, and tenderness localized to the shin without pitting edema—meets the diagnostic criteria for cellulitis, which requires at least 2 classic inflammatory signs (erythema, warmth, tenderness, pain, or induration). 1 The negative ultrasound for DVT and normal X-ray effectively exclude the two most critical differential diagnoses that require different management. 1

Why DVT is Excluded

• The negative duplex ultrasound from inguinal ligament to ankle (complete compression ultrasound) is sufficient to exclude DVT with high confidence. 2
• Even if the Wells score was ≥2 (DVT "likely"), a negative complete duplex ultrasound rules out DVT without need for repeat testing. 1, 3
• The absence of pitting edema further argues against DVT, as pitting edema confined to the symptomatic leg scores 1 point on the Wells criteria. 2

Why Fracture is Excluded

• Normal X-ray excludes acute fracture, bone destruction, and early osteomyelitis (though plain films may miss very early osteomyelitis within the first 10-14 days). 4

Classification of Cellulitis Severity

This patient appears to have moderate cellulitis based on the following criteria: 1

• Mild cellulitis: Erythema 0.5-2 cm, involving only skin/subcutaneous tissue, no systemic signs
• Moderate cellulitis: Erythema >2 cm OR deeper tissue involvement, no systemic signs
• Severe cellulitis: Any local infection PLUS systemic inflammatory response syndrome (fever >38°C or <36°C, heart rate >90, respiratory rate >20, WBC >12,000 or <4,000)

The shin location and described swelling suggest erythema likely exceeds 2 cm, placing this in the moderate category. 1

First-Line Antibiotic Treatment

Empirical oral antibiotics targeting Gram-positive organisms (streptococci and staphylococci) are indicated: 1

• Cephalexin 500mg orally four times daily OR
• Dicloxacillin 500mg orally four times daily 5
• Treatment duration: 5-10 days, extending based on clinical response 1

These agents provide excellent coverage for group A beta-hemolytic streptococcus, which causes 85% of cellulitis cases. 6 If MRSA risk factors are present (recent hospitalization, injection drug use, known MRSA colonization), consider adding coverage with trimethoprim-sulfamethoxazole or doxycycline. 1

Essential Adjunctive Measures

Beyond antibiotics, the following are critical: 7, 6

• Leg elevation and bed rest to reduce swelling and promote healing 7
• Warm compresses to the affected area 1
• Identify and treat the portal of entry: Examine for tinea pedis (athlete's foot), which is the most common entry point for lower extremity cellulitis 7, 8
• Consider prophylactic anticoagulation if the patient has risk factors for venous thromboembolism during the period of reduced mobility 7

Red Flags Requiring Urgent Surgical Consultation

Seek immediate surgical evaluation if any of the following develop: 1

• Rapid progression of infection despite appropriate antibiotics
• Crepitus or tissue gas on examination
• Bullae, especially hemorrhagic
• Pain out of proportion to physical findings (suggests necrotizing fasciitis)
• New neurologic deficits
• Failure to improve after 48-72 hours of appropriate therapy

Common Pitfalls to Avoid

Do not obtain blood cultures or extensive laboratory workup for uncomplicated cellulitis, as they rarely change management and are not cost-effective. 7 The diagnosis is clinical. 1, 8

Do not prescribe antibiotics targeting anaerobes or gram-negative organisms unless there are specific risk factors (diabetic foot infection, water exposure, immunocompromised state). 1

Do not miss cellulitis mimics that present similarly but require different treatment: 9

• Venous stasis dermatitis (bilateral, chronic, associated with varicosities)
• Contact dermatitis (history of exposure, pruritus predominates)
• Superficial thrombophlebitis (palpable cord along superficial vein)
• Necrotizing fasciitis (pain out of proportion, rapid progression, systemic toxicity)

Follow-Up and Recurrence Prevention

Reassess in 48-72 hours to confirm clinical improvement (decreased erythema, warmth, and tenderness). 1 If symptoms worsen or fail to improve, consider hospitalization for IV antibiotics or alternative diagnosis. 1

Address predisposing factors to prevent recurrence: 8

• Treat tinea pedis with topical antifungals
• Manage chronic edema with compression stockings
• Optimize diabetes control if applicable
• Maintain skin integrity and barrier function

For patients with frequent recurrences despite addressing underlying conditions, long-term antimicrobial prophylaxis with penicillin or erythromycin may be considered. 7, 8