In a 67‑year‑old female on postoperative day 4 after total abdominal hysterectomy and bilateral salpingo‑oophorectomy with emergency exploratory laparotomy, enterolysis, lavage, ileal repair, appendectomy and drain placement, who now has fever, abdominal pain, leukocytosis (WBC 24 000, neutrophils 90 %) and a normal chest X‑ray while receiving Piperacillin‑Tazobactam, what is the most likely diagnosis, the key differential diagnoses, and recommended management?

Post-Operative Intra-Abdominal Infection: Diagnosis and Management

This patient most likely has post-operative peritonitis, and you must obtain urgent CT abdomen/pelvis with IV contrast immediately and prepare for potential surgical re-exploration, as delayed intervention beyond 24 hours significantly increases mortality. 1

Most Likely Diagnosis

Post-operative peritonitis secondary to anastomotic leak or ongoing intra-abdominal infection is the primary diagnosis given:

• Persistent fever and abdominal pain on POD 4 1
• Marked leukocytosis (WBC 24,000) with 90% neutrophils 2
• Recent ileal repair (enterorrhaphy) with high-risk anastomosis 1
• Failure to improve on appropriate antibiotics (Piperacillin-Tazobactam) 1

The combination of fever, tachycardia (likely present given the clinical picture), and persistent abdominal pain are significant predictors of serious postoperative complications requiring immediate intervention. 2

Critical Differential Diagnoses

1. Anastomotic Leak at Ileal Repair Site

• Most common cause of post-operative peritonitis 1
• High-risk given emergency surgery context and likely compromised bowel 1
• Presents exactly as described: fever, leukocytosis, abdominal pain 2

2. Intra-Abdominal Abscess

• May be localized or multiple collections 1
• Can occur despite adequate initial source control 1
• Requires imaging for definitive diagnosis 2

3. Missed Bowel Injury or Incomplete Source Control

• Given the emergency exploratory laparotomy for perforated ileum 1
• Multiple adhesions (enterolysis performed) increase risk of unrecognized injury 1

4. Ongoing Peritoneal Contamination

• Inadequate initial source control 1
• Persistent bacterial peritonitis despite antibiotics 1

Immediate Diagnostic Workup

Imaging (URGENT - Do Not Delay)

CT abdomen and pelvis with IV contrast is mandatory and should be obtained immediately 2:

• Sensitivity 88-94% and specificity 93% for intra-abdominal abscess 2
• Gold standard for postoperative complications 2
• Look specifically for: free air, fluid collections, bowel wall thickening >5mm, anastomotic dehiscence, abscess formation 3

Critical pitfall to avoid: Do NOT delay imaging for "observation" in a patient with fever and peritoneal signs—this increases mortality 2

Laboratory Studies (Obtain Simultaneously with Imaging)

• C-reactive protein (CRP): Superior diagnostic accuracy compared to WBC alone 3, 4
  • CRP ≥159 mg/L on POD 3-4 indicates high suspicion for complications 4
  • Should be declining by POD 4 in uncomplicated recovery 4
• Procalcitonin: Higher specificity than CRP for bacterial infection 4
• Serum lactate: Assess for bowel ischemia or sepsis 3
• Blood cultures: Before any antibiotic changes 1

Drain Fluid Analysis

• Send existing drain output for culture and cell count 1
• Elevated amylase suggests anastomotic leak 1

Management Algorithm

Step 1: Resuscitation and Stabilization

• Ensure adequate IV access and fluid resuscitation 1
• Monitor hemodynamics closely 1
• NPO status, nasogastric decompression if not already in place 3

Step 2: Continue Current Antibiotics While Awaiting Results

Piperacillin-Tazobactam 4.5g IV Q6H is appropriate empiric coverage 5, 6, 7:

• Excellent spectrum for post-operative intra-abdominal infections 5, 7
• Covers gram-positive, gram-negative, and anaerobic organisms 5, 7
• Clinical cure rates 88-91% in intra-abdominal infections 7
• However, failure to improve on appropriate antibiotics suggests inadequate source control 1

Step 3: Surgical Decision Based on CT Findings

If Localized Abscess Without Peritonitis:

• Percutaneous CT-guided drainage may be attempted 1
• Only if: hemodynamically stable, no signs of generalized peritonitis, accessible collection 1
• Continue antibiotics for approximately 4 days after adequate source control 1

If Generalized Peritonitis, Free Air, or Failed Drainage:

Prompt surgical re-exploration is mandatory 1:

• Complete surgical source control must be performed as soon as patient is maximally resuscitated 1
• Delayed re-laparotomy >24 hours results in higher mortality 1
• Early re-laparotomy is most effective for post-operative peritonitis 1

Step 4: Intraoperative Considerations

Damage control surgery principles may apply if 1:

• Hemodynamic instability despite resuscitation 1
• Severe physiological derangement (acidosis, hypothermia, coagulopathy) 1
• Extensive visceral edema 1
• Persistent source of infection requiring staged approach 1

Consider: Resection of compromised anastomosis, diversion with ostomy, extensive lavage, drain placement, temporary abdominal closure if indicated 1

Critical Pitfalls to Avoid

1. Do NOT dismiss fever as "normal postoperative" when accompanied by abdominal pain, tenderness, and leukocytosis 2
2. Do NOT delay imaging or surgical consultation - mortality increases significantly with delays >24 hours 1
3. Do NOT rely on plain radiographs - only 49% sensitivity for complications 2
4. Do NOT continue conservative management if no improvement within 24-48 hours 3
5. Do NOT assume antibiotics alone will resolve the problem - inadequate source control is associated with intolerably high mortality 1

Antibiotic Duration After Source Control

Once adequate source control is achieved 1:

• Fixed duration of 4 days is as effective as longer courses 1
• No need to wait for normalization of inflammatory markers if source controlled 1
• Reassess daily for appropriateness and need for continuation 1

Prognosis and Monitoring

• Post-operative peritonitis carries high mortality rates (18% in some series) 1
• Inability to control septic source is associated with significantly elevated mortality 1
• Serial clinical examinations every 4-6 hours until improvement 3
• Monitor for signs of septic shock: hypotension, decreased urine output, respiratory distress 1

Bottom line: This patient requires urgent CT imaging NOW, surgical team at bedside for results, and very low threshold for re-exploration given the high-risk scenario and failure to improve on appropriate antibiotics.