Yes, This is Entirely Possible and Expected
A gallstone identified on ultrasound can absolutely be missed on CT scan performed the next day—this is a well-documented limitation of CT imaging for gallstone detection. The sensitivity of CT for detecting gallstones is only approximately 75%, meaning CT misses about 1 in 4 gallstones that are actually present 1.
Why CT Misses Gallstones That Ultrasound Detects
The fundamental issue is that CT detection depends entirely on the density difference between the stone and surrounding bile 1. Many gallstones are:
- Cholesterol stones: These are often isodense (same density) as bile, making them invisible on CT
- Minimally calcified or rim-calcified stones: These subtle stones are easily overlooked, even on contrast-enhanced CT 2, 3
- Non-calcified stones: These appear as the same gray shade as bile and simply disappear on CT images
Only densely calcified gallstones are readily apparent on CT 1. The stone composition in your South Asian patient likely falls into one of the radiolucent categories.
Clinical Implications for Your Patient
Your ultrasound finding is the more reliable result for detecting the gallstone itself 4, 5. The ACR Appropriateness Criteria explicitly state that ultrasound remains the initial test of choice for right upper quadrant pain and suspected biliary disease 1. Recent comparative studies confirm ultrasound has higher sensitivity for detecting cholelithiasis (95.9% vs CT) and gallbladder wall thickening (98.5% vs 66.4%) 6, 5.
What the CT Actually Tells You
The CT scan is not useless—it serves a different purpose:
- Rules out complications: CT is superior for identifying gangrenous cholecystitis, perforation, abscess formation, or alternative diagnoses 1, 4
- Evaluates surrounding structures: Better visualization of liver, pancreas, and other organs
- Negative predictive value: A negative CT has ~90% negative predictive value for acute cholecystitis complications 1
Common Pitfall to Avoid
Do not dismiss your ultrasound finding based on the negative CT. Multiple case reports document exactly this scenario—gallstones confirmed at surgery that were visible on ultrasound but completely missed on CT, even with contrast enhancement 2, 3, 7. In one reported case, a 3.5 cm calcified gallstone causing bowel obstruction was not clearly identified on CT despite being present 7.
Management Approach for Your Patient
Given the patient presented to the ER with abdominal pain:
Trust the ultrasound for gallstone presence—it has 95-98% sensitivity for cholelithiasis 6, 5
Use the CT findings to assess for:
- Acute cholecystitis complications (wall enhancement, pericholecystic inflammation, abscess)
- Alternative causes of pain
- Bile duct involvement
If clinical suspicion for acute cholecystitis remains high despite equivocal imaging, consider HIDA scan (gold standard for acute cholecystitis) or MRI/MRCP (85-100% sensitivity for cholelithiasis/choledocholithiasis) 1, 4
Base surgical decision-making on the ultrasound finding plus clinical picture, not the negative CT for stone detection
The discordance between your office ultrasound and the ER CT is a recognized limitation of CT technology, not an indication that the stone disappeared or that your ultrasound was incorrect.