A 52-year-old woman presents with acute right upper quadrant pain, fever (38.5°C), and positive Murphy's sign. Serum bilirubin is normal. Ultrasound shows a thickened gallbladder wall, pericholecystic fluid, and a single 1.8 cm stone impacted in the Hartmann's pouch. What is the investigation of choice to confirm acute cholecystitis and assess for complications?

Explanation

## Investigation of Choice for Acute Cholecystitis **Key Point:** HIDA scan (hepatobiliary scintigraphy) is the gold standard investigation for confirming acute cholecystitis when ultrasound findings are equivocal or clinical suspicion is high despite normal ultrasound. ### Why HIDA Scan is Superior in This Case **High-Yield:** HIDA scan has the highest sensitivity (95–98%) and specificity (90–95%) for acute cholecystitis because it directly assesses cystic duct patency. Non-filling of the gallbladder within 4 hours after IV injection of the tracer indicates cystic duct obstruction — the pathophysiologic hallmark of acute cholecystitis. **Clinical Pearl:** In this patient, ultrasound has already identified the stone and wall thickening. HIDA scan adds functional confirmation: if the gallbladder does not fill, cystic duct obstruction is confirmed, validating the diagnosis of acute cholecystitis and justifying urgent cholecystectomy. ### Diagnostic Accuracy Comparison | Investigation | Sensitivity | Specificity | Best Use | |---|---|---|---| | **Ultrasound** | 80–95% | 80–95% | First-line; detects stones, wall thickening, pericholecystic fluid | | **HIDA scan** | 95–98% | 90–95% | **Confirms cystic duct obstruction; gold standard for acute cholecystitis** | | **CT abdomen** | 70–85% | 85–95% | Complications (perforation, gangrene, abscess); not for uncomplicated cholecystitis | | **MRCP** | 90–95% | 95–98% | Choledocholithiasis, biliary obstruction; not for acute cholecystitis diagnosis | | **EUS** | 95–98% | 95–98% | Stones in common bile duct; not for acute cholecystitis | ### Mechanism of HIDA Scan in Acute Cholecystitis 1. Tracer (iminodiacetic acid derivative) is taken up by hepatocytes and excreted into bile. 2. Bile flows through the cystic duct into the gallbladder. 3. **Non-filling of gallbladder** = cystic duct obstruction (stone, edema, mucus plug). 4. Positive result confirms acute cholecystitis; negative result excludes it with high confidence. **Warning:** Do not confuse HIDA scan with CT or MRCP — HIDA is functional (assesses duct patency), while CT and MRCP are anatomic (show stones, ducts, inflammation).