What labs should be ordered for a patient with an abscess?

Laboratory Tests for Patients with Abscess

For patients with abscess, essential laboratory tests should include complete blood count, inflammatory markers (C-reactive protein, procalcitonin), and culture of the abscess material, with additional tests based on location and severity of infection. 1

Basic Laboratory Evaluation

All Abscess Patients:

• Complete blood count (CBC) - To assess for leukocytosis and systemic inflammatory response
• Culture of abscess material - To identify causative organisms after drainage
• C-reactive protein (CRP) - To assess inflammatory response and monitor treatment effectiveness

Patients with Systemic Symptoms:

• Serum creatinine and electrolytes - To assess renal function and electrolyte imbalances
• Blood cultures - Particularly in patients with signs of sepsis or systemic spread
• Procalcitonin - For patients with suspected sepsis 1

Location-Specific Testing

Anorectal Abscess:

• Serum glucose, HbA1c, and urine ketones - To identify undetected diabetes mellitus (strongly recommended) 1
• Pus sampling - Recommended in high-risk patients or those with risk factors for multidrug-resistant organisms 1

Brain Abscess:

• HIV testing - Should be considered in all patients with non-traumatic brain abscess 1, 2
• Blood cultures - Positive in approximately 28% of brain abscess cases 1, 2
• Additional microbiological samples - Based on suspected source of infection 1

Special Considerations

Immunocompromised Patients:

• More extensive workup is warranted, including:
  • Additional cultures (blood, tissue)
  • Broader antimicrobial susceptibility testing
  • Fungal and mycobacterial cultures when indicated 1, 3

Large or Complex Abscesses:

• Consider additional testing for antibiotic penetration into abscess fluid
• Vancomycin and ciprofloxacin often have inadequate penetration into abscess cavities 4

When to Avoid Routine Cultures

• Simple, uncomplicated abscesses in immunocompetent patients may not require routine cultures 5, 3
• Cultures should be reserved for:
  1. Immunocompromised patients
  2. Abscesses in the central facial region
  3. Abscesses containing gas
  4. Involvement of muscle or fascia
  5. Treatment failures
  6. Recurrent abscesses 3, 6

Monitoring Response

• Serial laboratory testing (CBC, CRP) should be performed to monitor response to treatment
• For brain abscesses, serial imaging (every 2 weeks) is recommended to assess treatment response 2

Practical Algorithm

1. Initial assessment: Determine if patient has systemic symptoms or is immunocompromised
2. Basic labs: CBC, CRP for all patients with suspected abscess
3. Drainage procedure: Obtain cultures during drainage
4. Additional testing: Based on location, severity, and host factors
5. Monitor response: Follow inflammatory markers to assess treatment efficacy

Remember that while laboratory tests are important, surgical drainage remains the cornerstone of abscess management, with antibiotics playing a supportive role in specific circumstances 1, 5.