What is the likely cause and recommended management for a postoperative knee surgery patient with leukocytosis (WBC 18.3 ×10⁹/L) and neutrophilia (84% neutrophils, absolute neutrophil count ≈15.3 ×10⁹/L)?

Postoperative Leukocytosis After Knee Surgery

This leukocytosis (WBC 18.3 with 84% neutrophils) is most likely a normal physiologic response to surgery rather than infection, and in the absence of clinical signs of infection (fever, wound drainage, erythema), no additional workup is warranted.

Expected Postoperative Response

• Leukocytosis is extremely common after knee arthroplasty, occurring in 38-50% of patients 1, 2
• The average postoperative WBC count increases to approximately 3 × 10⁶ cells/μL over the first 2 postoperative days, then declines to slightly above preoperative levels by day 4 1
• Your patient's WBC of 18.3 falls well within the expected range for normal postoperative inflammation 1
• The neutrophil predominance (84%) with absolute neutrophil count of 15.3 represents typical surgical stress response and leukocyte activation that peaks between postoperative days 1-3 3

When to Suspect Infection vs. Normal Response

Clinical context is paramount—laboratory values alone should NOT drive infection workup 2:

• If the patient has fever, wound drainage, erythema, or other clinical signs of infection, then further evaluation is warranted 2
• If the patient is clinically well without concerning physical findings, this leukocytosis requires no additional testing 2
• White blood cell count has poor specificity (only 46%) for diagnosing early periprosthetic infection, meaning most patients with leukocytosis do NOT have infection 1

Diagnostic Thresholds for Early Periprosthetic Joint Infection

If infection is clinically suspected, the following thresholds apply for synovial fluid analysis (not peripheral blood):

Within 6 weeks postoperatively 4:

• Synovial fluid WBC ≥8,676 cells/μL (sensitivity 83%, specificity 90%)
• Synovial neutrophils ≥88% (sensitivity 67%, specificity 78%)
• CRP ≥82 mg/L (sensitivity 70%, specificity 77%)

Between 6-12 weeks postoperatively 4:

• Synovial fluid WBC ≥1,983 cells/μL (sensitivity 80%, specificity 85%)
• Synovial neutrophils ≥76% (sensitivity 80%, specificity 81%)
• CRP ≥34 mg/L (sensitivity 90%, specificity 93%)

Important Caveats

• ESR and CRP remain persistently elevated for weeks after arthroplasty and are NOT sensitive markers for early infection 5
• The neutrophil-to-lymphocyte ratio (NLR) in your patient is approximately 7.4 (15.3/2.1), which is elevated but represents normal surgical stress 6, 5
• Diagnostic testing based solely on laboratory values without clinical findings leads to unnecessary tests, patient discomfort, and increased costs 2
• Only 10% of additional tests performed for isolated fever or leukocytosis yield clinically relevant results 2

Recommended Management

Monitor clinically without additional workup unless concerning signs develop 2:

• Daily assessment for fever, wound erythema, drainage, or increasing pain
• If clinical infection is suspected, obtain synovial fluid aspiration with cell count, differential, and culture 7
• Withhold antibiotics until cultures are obtained if infection is suspected, as antimicrobial therapy significantly reduces culture yield 7