What is the most appropriate initial test for a patient with generalized abdominal pain, nausea, vomiting, and a rigid abdomen, presenting with tachycardia, fever, and normal blood pressure?

Most Appropriate Initial Test for Suspected Perforated Viscus

An upright chest X-ray is the most appropriate initial test for this patient with a rigid, scaphoid abdomen, as it is the fastest and most accessible method to detect pneumoperitoneum (free air under the diaphragm), which would confirm a perforated viscus requiring emergent surgical intervention.

Clinical Presentation Analysis

This 21-year-old presents with classic signs of peritonitis:

• Rigid, scaphoid abdomen - pathognomonic for perforated viscus with peritoneal irritation 1
• Tachycardia (HR 105) and fever (38.5°C) - indicating systemic inflammatory response 1
• Generalized abdominal pain with nausea/vomiting - consistent with acute surgical abdomen 1

The rigid abdomen is the critical finding that mandates immediate evaluation for perforation, as this represents a surgical emergency with high morbidity and mortality if delayed 1.

Why Upright Chest X-Ray First

Diagnostic Utility

• Upright chest X-ray detects pneumoperitoneum in up to 75-80% of perforated viscus cases and can be obtained within minutes at the bedside or in the radiology suite 1
• Free air under the diaphragm on upright CXR is diagnostic and immediately directs management toward surgical consultation 1
• The test is rapid, widely available, and does not delay definitive treatment 1

Guideline Support

• The ACR Appropriateness Criteria specifically recommends imaging evaluation for patients with acute nonlocalized abdominal pain and fever, with the understanding that plain radiography can detect pneumoperitoneum 1
• For suspected bowel perforation, lateral and anteroposterior plain X-ray films of the chest, abdomen, and pelvis are recommended to identify pneumoperitoneum 1

Why Not the Other Options Initially

Erect Abdominal X-Ray (Option B)

• While erect abdominal films can show free air, upright chest X-ray is superior for detecting small amounts of pneumoperitoneum because the diaphragm provides better contrast 1
• Abdominal radiography has low sensitivity (limited role) for sources of abdominal pain and fever compared to chest imaging for perforation 1

Abdominal Ultrasound (Option C)

• Ultrasound is not the initial test for suspected perforation - it has limited sensitivity for detecting free air 2
• Guidelines recommend against routine abdominal ultrasound in patients with fever and abdominal pain without specific indications like recent surgery or hepatobiliary symptoms 1
• Ultrasound is operator-dependent and time-consuming in this emergent setting 1

Abdominal CT Scan (Option D)

• While CT is more sensitive than plain radiography for detecting perforation and identifying the source, it should not be the initial test in a hemodynamically stable patient with clear peritonitis 1, 2
• CT delays surgical intervention and is unnecessary if upright CXR shows obvious pneumoperitoneum 1
• CT is indicated when initial workup (including CXR) is equivocal or when the exact source of perforation needs identification for surgical planning 1

Recommended Diagnostic Algorithm

1. Immediate upright chest X-ray - to detect pneumoperitoneum 1
2. If CXR shows free air: Proceed directly to surgical consultation for emergent laparotomy 1
3. If CXR is negative but clinical suspicion remains high (rigid abdomen persists): Proceed to CT abdomen/pelvis with IV contrast for definitive evaluation 1, 2
4. Concurrent laboratory workup: CBC, comprehensive metabolic panel, lactate, and blood cultures given fever and signs of sepsis 3, 4

Critical Pitfalls to Avoid

• Do not delay imaging for extensive laboratory workup - the rigid abdomen demands immediate imaging to rule out perforation 1
• Do not skip straight to CT without attempting plain radiography first in a stable patient, as this wastes time if free air is obvious on CXR 1
• Do not rely on abdominal radiography alone - upright chest X-ray is superior for pneumoperitoneum detection 1
• Do not assume negative CXR excludes perforation - if clinical suspicion remains high with a rigid abdomen, CT is mandatory as CXR misses 20-25% of perforations 1

Answer: A. Upright chest X-ray