What labs should be ordered for an infected cat bite?

Laboratory Tests for Infected Cat Bites

For an infected cat bite, aerobic and anaerobic cultures from the wound are essential for identifying the causative pathogens, with blood cultures recommended if there are signs of systemic infection. 1, 2

Primary Laboratory Tests

• Wound Culture:

  • Aerobic and anaerobic cultures from the wound site 1, 3
  • Gram stain to assess for inflammation markers (neutrophils), contamination (epithelial cells), and microorganisms 1
  • Tissue biopsy preferred over surface swabs for accurate pathogen identification 1

• Blood Tests:

  • Complete blood count (CBC) to assess for leukocytosis indicating infection
  • Blood cultures if systemic infection is suspected (fever, lymphangitis, or other signs of bacteremia) 1

Microbiology Considerations

• Target Pathogens: Focus on common cat bite pathogens 1, 2:

  • Pasteurella multocida (found in 70-90% of infected cat bites) 4, 3
  • Staphylococcus species
  • Streptococcus species
  • Anaerobic bacteria (present in 16% of cases alongside P. multocida) 3
  • Capnocytophaga canimorsus (especially in immunocompromised patients) 5

• Specimen Collection Best Practices:

  • Clean wound thoroughly before specimen collection 1
  • Remove topical antimicrobials that could affect culture results 1
  • For deep infections, obtain tissue samples rather than surface swabs 1
  • Collect specimens before starting antibiotic therapy when possible

Additional Tests Based on Clinical Presentation

• For Hand Infections (higher risk of complications) 2, 6:

  • X-ray to rule out foreign body or bone involvement
  • Consider MRI if deep space infection, osteomyelitis, or tenosynovitis is suspected

• For Systemic Symptoms:

  • Basic metabolic panel
  • C-reactive protein and erythrocyte sedimentation rate to assess inflammation severity
  • Liver function tests if sepsis is suspected

Special Considerations

• Immunocompromised Patients:

  • More extensive laboratory workup including broader culture panels 2
  • Consider fungal cultures in addition to bacterial cultures

• Timing Considerations:

  • Obtain cultures before starting antibiotics whenever possible
  • Don't request "culture everything that grows" - focus on clinically relevant pathogens 1

Common Pitfalls to Avoid

1. Inadequate Sampling: Surface swabs may miss deep pathogens; tissue samples are preferred 1

2. Delayed Culture: Initiating antibiotics before obtaining cultures may lead to false-negative results

3. Overlooking Anaerobes: Ensure proper anaerobic culture techniques are used, as these organisms are common in cat bite infections 3

4. Missing Systemic Infection: Failure to obtain blood cultures in patients with fever or other signs of systemic spread 1

Remember that cat bites have a significantly higher infection rate (20-80%) compared to dog bites (3-18%), making proper laboratory assessment crucial for appropriate management 4, 5.