What is the appropriate management for a 20-year-old male with a swollen and warm knee, diagnosed with cellulitis and possible septic bursitis, who was started on cephalexin (cephalexin) after hitting his knee 3 days ago and has X-ray findings of prepatellar soft tissue swelling with possible fluid in the prepatellar bursa?

Management of Prepatellar Bursitis with Surrounding Inflammation

This patient requires immediate bursal aspiration with Gram stain and culture, followed by empiric IV vancomycin 15-20 mg/kg every 8-12 hours plus continuation of oral cephalexin, as this represents septic prepatellar bursitis with surrounding inflammation—not simple cellulitis—and requires drainage as the primary treatment. 1

Critical Distinction: Septic Bursitis vs. Cellulitis

The terminology matters clinically: this is "septic prepatellar bursitis with surrounding inflammation," not "cellulitis with bursitis." 1 The IDSA explicitly states that the term "cellulitis" is inappropriate for cutaneous inflammation associated with purulent collections such as septic bursitis. 1 The primary treatment of septic bursitis is drainage of pus, with antimicrobial therapy having a subsidiary role, whereas cellulitis requires antibiotics as primary treatment. 1

Immediate Management Algorithm

Step 1: Bursal Aspiration (Within Hours)

• Perform immediate needle aspiration or surgical drainage of the prepatellar bursa to obtain fluid for Gram stain, culture, and cell count. 2
• Bursal fluid WBC counts in septic bursitis vary widely (350-392,500 WBC/mm³), so a "negative" tap doesn't exclude infection. 2
• Staphylococcus aureus causes 78-80% of septic bursitis cases, making empiric anti-staphylococcal coverage mandatory. 2, 3

Step 2: Antibiotic Selection

• Initiate IV vancomycin 15-20 mg/kg every 8-12 hours immediately after aspiration while awaiting culture results. 4
• Continue oral cephalexin 500 mg every 6 hours to maintain streptococcal coverage for the surrounding inflammation. 4
• This combination addresses both MRSA (the most likely pathogen in septic bursitis) and streptococci (which cause the surrounding inflammatory response). 1, 2

Step 3: Assess for Hospitalization

• Admit if any of the following are present: systemic inflammatory response syndrome (fever >38°C, tachycardia >90 bpm), extensive surrounding cellulitis (which occurred in 89% of septic bursitis cases), profound limb edema (11% of cases), or failure to respond to initial oral antibiotics. 2, 4
• The 3-day delay since trauma and current presentation on cephalexin suggests possible treatment failure, warranting more aggressive management. 2

Why Cephalexin Alone Is Insufficient

Cephalexin provides excellent coverage for streptococci and methicillin-sensitive S. aureus but has no activity against MRSA, which is the predominant pathogen in septic bursitis. 4, 2 The patient's lack of improvement after 3 days on cephalexin strongly suggests either:

1. MRSA infection requiring vancomycin
2. Inadequate drainage of the purulent bursal collection
3. Both

Treatment Duration and Monitoring

• Total antibiotic duration: 7-14 days based on clinical response, not the standard 5 days used for uncomplicated cellulitis. 4
• Clinical resolution in septic bursitis is notoriously slow, occurring at a mean of >5 weeks (range up to 20 weeks to return to baseline). 2
• Reassess at 48-72 hours: if no improvement with vancomycin plus drainage, consider repeat aspiration or surgical bursectomy. 2

Essential Adjunctive Measures

• Elevate the affected leg above heart level for at least 30 minutes three times daily to promote gravity drainage of edema. 4
• Immobilize the knee temporarily to reduce mechanical irritation of the inflamed bursa. 2
• Examine for predisposing factors: occupational kneeling (51% of cases), recent trauma (23%), or pre-existing bursal disease (11%). 3

Common Pitfalls to Avoid

• Do not treat this as simple cellulitis with antibiotics alone—the purulent bursal collection requires drainage as primary therapy. 1
• Do not assume cephalexin failure means the patient needs broader gram-negative coverage—the issue is inadequate MRSA coverage and lack of drainage. 4
• Do not delay aspiration waiting for imaging confirmation—the X-ray showing "possible fluid" is sufficient clinical indication for diagnostic/therapeutic aspiration. 2
• Do not use clindamycin monotherapy without confirming local MRSA resistance rates are <10%, as resistance is common. 4

Transition to Oral Therapy

Once clinically improved (typically after 4-5 days of IV vancomycin):

• Transition to oral clindamycin 300-450 mg every 6 hours if local MRSA clindamycin resistance is <10%. 4
• Alternative: linezolid 600 mg twice daily, though significantly more expensive. 4
• Continue oral therapy to complete 7-14 days total treatment duration. 4