What lab tests should I order postoperatively to evaluate for signs of infection?

Postoperative Laboratory Markers for Infection Detection

Complete blood count (CBC), C-reactive protein (CRP), and procalcitonin (PCT) are the most important laboratory tests to evaluate for postoperative infection. 1

Primary Infection Markers

First-Line Tests

• Complete Blood Count (CBC)

  • Monitors leukocytosis (WBC >12,000/μL) which may indicate infection
  • Note: Leukocytosis is common after surgery (38% of patients) and represents a normal physiologic response in the first 2-4 days 2
  • Sensitivity for infection: 79%, Specificity: only 46% 2

• C-Reactive Protein (CRP)

  • Higher sensitivity and specificity than WBC count for detecting infections 1
  • Peaks 48-72 hours postoperatively
  • CRP ≥5 mg/dL has high specificity for postoperative complications 1
  • Excellent negative predictive value for ruling out anastomotic leaks and abscess formation 1

• Procalcitonin (PCT)

  • More specific than CRP for bacterial infections 1
  • Rises early in the course of infection
  • Particularly useful when blood culture results are not yet available
  • Helps guide antibiotic therapy decisions 1

Second-Line Tests

• Serum lactate levels

  • Elevated in tissue hypoperfusion and sepsis
  • Not reliable as a single marker for internal herniation or localized ischemia 1

• Blood cultures

  • Indicated if fever is present, symptoms have acute onset, or if patient has conditions that increase risk of bloodstream infection 1
  • Low yield (6.3% true positives) in postoperative patients 3
  • Most useful when drawn >4 days after surgery in patients not receiving preoperative antibiotics 3

Timing Considerations

Early Postoperative Period (0-72 hours)

• Routine blood cultures not recommended unless:

  • Fever with signs of sepsis
  • Acute onset of symptoms
  • Conditions increasing risk of bloodstream infection (e.g., S. aureus infection) 1

• Urinalysis and urine culture not mandatory unless:

  • Indwelling catheter present >72 hours
  • Symptoms suggesting urinary tract infection 1

Late Postoperative Period (>72 hours)

• Lower threshold for obtaining cultures if fever persists
• Consider blood cultures if no obvious source of infection identified

Special Considerations

Joint Replacement Surgery

• For suspected prosthetic joint infection (PJI):
  • Erythrocyte sedimentation rate (ESR) and CRP should be performed 1
  • Diagnostic arthrocentesis with synovial fluid analysis including:
    • Total cell count and differential leukocyte count
    • Culture for aerobic and anaerobic organisms 1

Bariatric Surgery

• Combination of CBC, serum electrolytes, CRP, PCT, and serum lactate levels recommended 1
• High CRP and leukocytosis are predictors of abdominal emergencies following bariatric surgery 1
• Consider vitamin levels (B1, B6, B12, D, folic acid) due to risk of deficiencies 1

Common Pitfalls

1. Overreliance on WBC count alone

   • Normal WBC doesn't rule out infection
   • Postoperative leukocytosis is common (38% of patients) 2

2. Ignoring CRP trends

   • Failure to trend CRP levels may miss evolving infections
   • Single normal CRP doesn't exclude infection possibility 1

3. Premature blood cultures

   • Blood cultures within first 4 days have low yield 3
   • Most useful when drawn >4 days after surgery in patients not receiving preoperative antibiotics

4. Neglecting glycemic control assessment

   • Poor glycemic control (HbA1c >7%) associated with increased postoperative infections 4
   • Consider HbA1c in diabetic patients with suspected infection

In summary, while leukocytosis is common after surgery, the combination of CBC with CRP and procalcitonin provides the most reliable laboratory assessment for postoperative infections. These tests should be interpreted in the context of clinical signs and symptoms, with blood cultures reserved for patients with fever, acute symptom onset, or specific risk factors for bloodstream infection.