What's the next step to confirm cholecystitis (inflammation of the gallbladder) and rule out pulmonary embolism (blockage of an artery in the lungs)?

Diagnostic Approach for Suspected Cholecystitis and Pulmonary Embolism

For suspected cholecystitis, ultrasound (US) should be the initial diagnostic imaging test, followed by CT if US is equivocal/non-diagnostic and clinical suspicion persists. 1

Diagnostic Algorithm for Cholecystitis

Step 1: Initial Imaging

• Abdominal ultrasound is the first-line imaging modality for suspected cholecystitis due to its shorter study time, ability to evaluate gallbladder morphology, confirm presence/absence of gallstones, assess bile ducts, and identify alternative diagnoses 1
• Key ultrasound findings to look for include:
  • Presence of gallstones (number, size, mobility, acoustic shadowing)
  • Gallbladder wall thickening >3mm
  • Pericholecystic fluid
  • Sonographic Murphy sign (though this has relatively low specificity)
  • Gallbladder distension 2, 3

Step 2: If Ultrasound is Equivocal/Non-diagnostic

• Obtain abdominal CT scan with IV contrast 1
• CT can confirm or refute cholecystitis diagnosis with a negative predictive value approaching 90% 1
• CT is particularly useful for detecting complications such as gangrene, gas formation, intraluminal hemorrhage, and perforation 1, 4
• Common CT findings include wall thickening, pericholecystic stranding, distension, high-attenuation bile, pericholecystic fluid, and subserosal edema 5

Step 3: If Both US and CT are Equivocal/Non-diagnostic

• Obtain either hepatobiliary iminodiacetic acid (HIDA) scan or MRI/MRCP 1
• HIDA scan has higher sensitivity (97%) and specificity (90%) for acute cholecystitis compared to US (sensitivity 88%, specificity 80%) 1
• Gallbladder non-visualization on HIDA scan is highly accurate for diagnosing acute cholecystitis 1, 6

Diagnostic Approach for Pulmonary Embolism

For suspected pulmonary embolism (PE), the diagnostic approach should be separate from cholecystitis evaluation:

• CT pulmonary angiography (CTPA) is the gold standard for diagnosing PE
• D-dimer testing may be appropriate before imaging in patients with low clinical probability
• Consider ventilation-perfusion (V/Q) scan if CTPA is contraindicated (e.g., renal insufficiency, contrast allergy)

Important Considerations

• Ultrasound has higher sensitivity than CT for detecting cholecystitis when using a two-sign criterion (80.9% vs 70.0%) 3
• Ultrasound is more sensitive for detecting cholelithiasis and thickened gallbladder wall, while CT is more sensitive for detecting pericholecystic fluid and gallbladder distension 3
• In pregnant patients, both US and MRI are appropriate initial imaging modalities for suspected cholecystitis 1
• The usefulness of ultrasound is limited in critically ill patients where gallbladder abnormalities are common in the absence of acute cholecystitis 1
• Patients with cholecystitis may develop pulmonary embolism postoperatively, especially those with risk factors for thromboembolism 7

Pitfalls to Avoid

• Relying solely on sonographic Murphy sign for cholecystitis diagnosis, as it has relatively low specificity 1, 2
• Misinterpreting increased common bile duct diameter as definitive evidence of choledocholithiasis, especially in elderly patients 2
• Failing to consider complications of cholecystitis (hemorrhagic, gangrenous, emphysematous) which may require more aggressive management 4
• Overlooking the possibility of concurrent cholecystitis and pulmonary embolism, as both can present with right-sided pain 7