Workup for Cellulitis from Cat Bite
For cat bite cellulitis, obtain wound cultures (aerobic and anaerobic) and blood cultures if systemic signs are present, check tetanus status, and initiate empiric therapy with amoxicillin-clavulanate 875/125 mg twice daily immediately while awaiting culture results 1.
Initial Clinical Assessment
Focus your evaluation on these specific high-risk features that predict serious infection:
- Timing: Cat bites developing cellulitis within 12-24 hours strongly suggest Pasteurella multocida infection 2
- Location: Hand and forearm wounds carry the highest infection risk and may harbor deep tissue involvement including purulent flexor tenosynovitis or tendon lesions 3
- Wound characteristics: Puncture wounds (which cat bites typically are) penetrate deeply through skin in >90% of cases 2
- Systemic signs: Fever, lymphangitis, or shock indicate bacteremia risk 4
Diagnostic Workup Algorithm
Microbiological Studies
Wound cultures are essential - obtain both aerobic and anaerobic cultures from any serosanguineous or purulent drainage 2. P. multocida is isolated in over 50% of cat bite wounds 5 and grows readily on standard media, but you must specifically request anaerobic cultures as well since polymicrobial infection is common 1.
Blood cultures should be obtained if the patient has:
- Fever or systemic symptoms
- Rapidly progressive cellulitis
- Immunocompromised status
- Signs of sepsis or shock 4
Imaging Considerations
For hand or forearm bites presenting >24 hours after injury with limited painful mobilization, swelling, and erythema, obtain plain radiographs initially to evaluate for:
- Foreign bodies
- Gas in soft tissues
- Underlying osteomyelitis (though this is typically a late finding)
Consider ultrasound or MRI if deep tissue infection (abscess, tenosynovitis) is suspected based on examination findings of severe pain with passive range of motion 3.
Laboratory Studies
- Complete blood count: Assess for leukocytosis indicating systemic infection
- C-reactive protein: Elevated levels (>50 mg/dL) indicate significant inflammation and can be used to monitor treatment response 6
- Basic metabolic panel: Especially if systemic antibiotics will be used, to assess renal function for dosing
Tetanus Prophylaxis
Administer tetanus toxoid if the patient has not received vaccination within 10 years - preferably Tdap over Td if Tdap has not been previously given 1. This is a strong recommendation that should never be overlooked.
Critical Pitfalls to Avoid
Do not delay cultures for empiric therapy - the rapid onset of cellulitis (often 12-24 hours) means you must act quickly, but obtain cultures first 2.
Do not underestimate hand wounds - all nine patients in one series who presented late (>24 hours) had hand/forearm involvement, and 67% required hospital admission for deep tissue infection 3. These warrant aggressive workup including consideration of surgical exploration.
Do not assume simple cellulitis - cat bites can cause bacteremia and shock even with minimal initial local findings 4. The clinical picture may evolve rapidly over 48 hours to extensive erysipelas-like cellulitis.
Risk Stratification for Admission
Admit patients with:
- Hand/forearm involvement presenting >24 hours after bite
- Signs of deep tissue infection (tenosynovitis, abscess)
- Systemic symptoms or bacteremia
- Immunocompromised status
- Failed outpatient management 3
The mean admission duration for complicated cat bites is approximately 6 days, requiring IV antibiotics and often surgical drainage 3.