What is the most appropriate initial investigation for a patient with a history of perforated duodenal ulcer repair, now presenting with fever, RUQ pain, and tenderness?

Ultrasound (US) is the Most Appropriate Initial Investigation

In a stable patient presenting 3 weeks post-duodenal ulcer repair with fever, RUQ pain, and signs suggesting a subphrenic or hepatic abscess, ultrasound should be performed as the initial imaging modality. 1

Clinical Context and Differential Diagnosis

This patient's presentation is highly suggestive of a post-operative intra-abdominal complication, most likely:

• Subphrenic abscess (most probable given RUQ localization, fever, and basal atelectasis/pleural effusion) 2
• Hepatic abscess
• Localized peritoneal collection 2

The timing (3 weeks post-surgery), fever, localized RUQ tenderness, and respiratory findings (decreased breath sounds, dullness, basal atelectasis with mild pleural effusion) are classic for subphrenic abscess formation following upper GI perforation repair. 2

Why Ultrasound First in This Stable Patient

Guideline-Based Rationale

• The 2024 IDSA guidelines explicitly recommend abdominal ultrasound as the initial diagnostic imaging modality for adults with RUQ pain and fever (conditional recommendation, very low certainty of evidence). 1
• While these guidelines focus on cholecystitis/cholangitis, the principle applies to any RUQ pathology in stable patients where US can effectively evaluate for fluid collections, abscesses, and hepatobiliary pathology. 1

Clinical Advantages in This Scenario

• US has 88% sensitivity for detecting intra-abdominal fluid collections and abscesses 3
• Readily available, no radiation, no contrast needed, and can be performed at bedside 1
• The patient is vitally stable, making immediate CT or laparotomy unnecessary 4
• US can effectively identify subphrenic and perihepatic collections, which are the most likely diagnoses given the clinical presentation 1

When to Proceed to CT

If ultrasound is equivocal, non-diagnostic, or fails to identify a clear source despite high clinical suspicion, CT abdomen with IV contrast should be obtained as the next step. 1, 3

CT provides superior characterization of:

• Extent and precise location of abscesses 3, 5
• Relationship to surrounding structures 1
• Guidance for potential percutaneous drainage 4

The World Society of Emergency Surgery confirms that CT has 93-96% sensitivity and 93-100% specificity for detecting intra-abdominal complications of perforation, including abscess formation. 3, 5

Why NOT Laparotomy Initially

Immediate laparotomy is NOT indicated because:

• The patient is vitally stable (no signs of septic shock or peritonitis) 4, 6
• Imaging must first define the nature, location, and extent of the complication to guide appropriate intervention (percutaneous drainage vs. surgery) 4, 2
• Many post-operative abscesses can be managed with percutaneous drainage and antibiotics, avoiding unnecessary surgery 4, 2

Critical Pitfall to Avoid

Do not skip imaging in stable patients with suspected post-operative complications. Even with classic clinical findings, imaging is essential to:

• Confirm the diagnosis 5, 4
• Determine if percutaneous intervention is feasible 4
• Rule out other complications (recurrent perforation, anastomotic leak, etc.) 2, 6

The incidence of intra-abdominal abscess after peptic ulcer perforation is substantial (12% in one series), and these complications are significantly more frequent when perforation occurred >24 hours before initial operation. 2