What is the next best step for a 24-year-old male with worsening cellulitis of the right foot, currently on doxycycline, with new pustules and discoloration turning purple, despite initial improvement with ibuprofen for pain management?

Management of Worsening Cellulitis Despite Doxycycline Treatment

The patient requires immediate change of antibiotic therapy to a broader-spectrum regimen with coverage for MRSA, such as trimethoprim-sulfamethoxazole, clindamycin, or linezolid, along with hospital admission for consideration of intravenous antibiotics due to treatment failure and progression to possible necrotizing infection.

Clinical Assessment and Concerns

The 24-year-old male presents with concerning features that indicate treatment failure and potential progression to a more serious infection:

• Worsening symptoms after 5 days of doxycycline therapy
• Development of new pustules on the great toe
• Progression of erythema to purple discoloration
• Persistent pain (though improved with ibuprofen)

These findings suggest that:

1. The current antibiotic regimen is inadequate
2. The infection may be caused by resistant organisms
3. The infection may be progressing to a more serious form

Immediate Management Steps

1. Change antibiotic therapy:

   • Doxycycline has failed despite 5 days of treatment
   • According to the Infectious Diseases Society of America guidelines, for skin and soft tissue infections that are not responding to initial therapy within 72 hours, patients should be reevaluated and given parenteral therapy 1
   • Consider MRSA coverage with one of the following:
     • Trimethoprim-sulfamethoxazole (not as single agent if Group A Streptococcus is suspected)
     • Clindamycin (if local resistance patterns permit)
     • Linezolid

2. Consider hospital admission for:

   • Intravenous antibiotics
   • Close monitoring for progression
   • Surgical evaluation if needed

3. Wound culture and drainage:

   • The pustules should be cultured to identify the causative organism and antimicrobial susceptibility
   • Incision and drainage of any abscesses is recommended 1

Differential Diagnosis

The purple discoloration and worsening despite appropriate therapy raise concerns for:

1. MRSA infection - Common cause of treatment failure with doxycycline
2. Necrotizing fasciitis - The purple discoloration is particularly concerning for deeper infection
3. Deep tissue abscess - May not respond to oral antibiotics alone
4. Non-infectious etiology - Consider vascular or inflammatory conditions

Monitoring and Follow-up

• Daily assessment of the extent of erythema (mark borders with pen)
• Monitor for systemic symptoms (fever, hypotension)
• Watch for signs of compartment syndrome or deep tissue involvement
• Follow-up wound cultures to guide targeted therapy

Special Considerations

1. Elevation of the affected limb is an important and often neglected aspect of treatment that promotes gravity drainage of edema and inflammatory substances 1

2. Pain management should continue with ibuprofen as it has been effective, but monitor for masking of symptoms

3. If necrotizing infection is suspected, urgent surgical consultation is required as this is a surgical emergency

4. Common pitfalls to avoid:

   • Continuing ineffective antibiotics beyond 72 hours
   • Failing to obtain cultures before changing antibiotics
   • Overlooking the possibility of deeper infection when superficial signs improve
   • Discharging patients too early when they show initial improvement

Prevention of Recurrence

After resolution of the current episode, consider:

• Treating any underlying conditions (e.g., tinea pedis)
• Keeping skin well hydrated with emollients
• Good hygiene practices including daily change of socks and careful drying between toes 2
• If frequent infections occur despite preventive measures, prophylactic antibiotics may be reasonable 1

The purple discoloration and development of pustules despite appropriate antibiotic therapy are red flags that warrant immediate intervention to prevent serious complications.