What is the initial assessment and diagnostic workup for a patient with suspected small bowel obstruction?

Initial Assessment and Diagnostic Workup for Small Bowel Obstruction

For suspected small bowel obstruction, immediately begin IV fluid resuscitation and nasogastric decompression while obtaining CT abdomen/pelvis with IV contrast as the definitive diagnostic test—this imaging has >90% accuracy and should replace plain radiographs as the initial study in most cases. 1, 2

Immediate Clinical Assessment

History Taking

• Document prior abdominal surgeries (85% sensitivity, 78% specificity for adhesive SBO) 2
• Ask about last bowel movement and passage of flatus 2
• Inquire about previous diverticulitis, chronic constipation, rectal bleeding, or unexplained weight loss 2
• Review medications affecting peristalsis (can cause pseudo-obstruction) 2

Physical Examination

• Assess for abdominal distension (positive likelihood ratio 16.8, negative likelihood ratio 0.27) 2
• Examine all hernia orifices and previous surgical incision sites 2
• Perform digital rectal examination to detect blood or masses 2
• Look for peritonitis signs (fever, diffuse tenderness, guarding, rebound)—though physical exam has only 48% sensitivity for strangulation 2, 3
• Check vital signs for shock indicators: tachycardia, tachypnea, cool extremities, mottled skin, slow capillary refill, oliguria 1

Laboratory Workup

Order the following tests immediately: 1, 2

• Complete blood count (marked leukocytosis >10,000/mm³ suggests peritonitis)
• Electrolytes (hypokalemia is common and requires correction)
• Renal function (BUN/creatinine to assess dehydration)
• Lactate level (elevated in intestinal ischemia)
• CRP (>75 may indicate peritonitis)
• Liver function tests
• Coagulation profile (essential given potential need for emergency surgery)

Immediate Supportive Management

Begin these interventions before imaging is complete: 1, 2

• IV crystalloid resuscitation with isotonic dextrose-saline containing supplemental potassium
• Nasogastric tube placement for decompression and aspiration prevention (feculent aspirate suggests distal SBO)
• Foley catheter to monitor urine output
• Anti-emetics and bowel rest

Diagnostic Imaging

First-Line Imaging: CT Abdomen/Pelvis with IV Contrast

This is the preferred initial imaging study—NOT plain radiographs. 1, 2

CT advantages: 1, 2

• 90% diagnostic accuracy for SBO

• ~90% accuracy predicting strangulation and need for urgent surgery
• Identifies obstruction site, cause, and complications
• Multiplanar reconstructions increase accuracy for locating transition zones

CT protocol specifics: 1

• Use IV contrast (evaluates bowel perfusion and ischemia)
• No oral contrast needed for suspected high-grade obstruction—non-opacified fluid provides adequate intrinsic contrast
• Oral contrast delays diagnosis, increases aspiration risk, and can mask abnormal bowel wall enhancement

Critical CT findings indicating need for immediate surgery: 1, 2

• Abnormally decreased or increased bowel wall enhancement
• Intramural hyperdensity on non-contrast images
• Bowel wall thickening
• Mesenteric edema
• Ascites
• Pneumatosis intestinalis or mesenteric venous gas
• Closed-loop obstruction
• Free intraperitoneal air

Plain Radiographs: Limited Role

Plain abdominal X-rays should NOT be the primary diagnostic tool. 1, 2

• Only 50-60% diagnostic, 20-30% inconclusive, 10-20% misleading 1
• Sensitivity 74% (only marginally better than clinical assessment at 57%) 1
• Cannot exclude SBO or assess for complications 1
• May prolong evaluation and delay definitive imaging 1

Alternative Imaging Modalities

Ultrasound: 1, 2

• 90% sensitivity, 96% specificity for SBO
• Useful in pregnancy, children, or when CT unavailable
• Operator-dependent
• Can detect free fluid suggesting need for urgent surgery

MRI: 1

• Consider in pregnant patients when ultrasound is inconclusive
• Provides anatomical information without radiation

Water-Soluble Contrast Studies

For adhesive SBO managed non-operatively (after initial CT diagnosis): 1

• Administer 50-150 mL orally or via nasogastric tube
• Timing: Can give at admission or after 48 hours of conservative management
• 48-hour delay reduces aspiration and dehydration risk (allows adequate rehydration first)
• Obtain abdominal X-ray at 24 hours
• If contrast has NOT reached colon at 24 hours: highly predictive of non-operative management failure—proceed to surgery 1
• Has both diagnostic (96% sensitivity, 98% specificity) and therapeutic value 1

Contraindications and precautions: 1

• Ensure adequate gastric decompression before administration (prevents aspiration pneumonia)
• Can cause dehydration in children and elderly (high osmolarity shifts fluid into bowel lumen)
• Rare anaphylactoid reactions possible
• Can dilute with water to reduce dehydration risk

Decision Algorithm: Operative vs. Non-Operative Management

Immediate Surgery Required: 1, 2, 3

• Signs of peritonitis on exam
• CT evidence of bowel ischemia (see findings above)
• Strangulation indicators
• Closed-loop obstruction
• Free intraperitoneal air
• Hemodynamic instability despite resuscitation

Trial of Non-Operative Management: 1, 2

• Partial obstruction without ischemia signs
• No peritonitis
• Hemodynamically stable
• Continue for 24-48 hours maximum
• Delays beyond this increase morbidity and mortality 1

Critical Pitfalls to Avoid

• Do NOT rely on plain radiographs as definitive imaging—proceed directly to CT in most cases 1, 2
• Do NOT give oral contrast for suspected high-grade obstruction—delays diagnosis and increases aspiration risk 1
• Do NOT delay surgery when ischemia signs present—mortality reaches 25% with ischemia 1, 2
• Do NOT mistake incomplete obstruction with watery diarrhea for gastroenteritis 2
• Do NOT overlook SBO in elderly patients where pain may be less prominent 2
• Do NOT proceed to surgery without correcting electrolyte abnormalities first 2
• Do NOT attempt prolonged non-operative management beyond 48 hours—increases complications 1