What is the likely cause of colicky right upper quadrant (RUQ) and right flank pain in an adult patient, particularly a female of childbearing age, with normal complete blood count (CBC), normal lipase level, and normal urinalysis (UA)?

Differential Diagnosis for Colicky RUQ and Right Flank Pain with Normal Labs

The most likely diagnosis is biliary colic from cholelithiasis or functional gallbladder disease, and you should order right upper quadrant ultrasound immediately as the first-line imaging study. 1

Primary Diagnostic Considerations

The clinical presentation of colicky RUQ and right flank pain with completely normal laboratory values (normal CBC, normal lipase at 145, normal UA) significantly narrows the differential diagnosis:

• Biliary colic from cholelithiasis is the most common cause of colicky RUQ pain and can present with entirely normal laboratory values between episodes of obstruction 1, 2
• Functional gallbladder disorders (biliary dyskinesia or hyperkinesia) can cause identical symptoms with normal imaging and labs, representing an often-overlooked diagnosis 3
• Intermittent choledocholithiasis can cause colicky pain with transiently normal labs if the stone passes or obstruction is incomplete 4

Why Other Diagnoses Are Less Likely

• Acute cholecystitis is unlikely because the patient has a normal CBC (no leukocytosis) and presumably no fever, which are typically present in acute inflammatory disease 1
• Pancreatitis is effectively ruled out by the normal lipase level of 145 (though note that lipase can be mildly elevated in non-pancreatic conditions like colitis) 5
• Nephrolithiasis is unlikely given the normal urinalysis, though small non-obstructing stones could theoretically be present 1
• Hepatobiliary infection or cholangitis is unlikely without elevated white blood cell count or liver function test abnormalities 4

Recommended Diagnostic Algorithm

Step 1: Order RUQ ultrasound immediately

• The American College of Radiology rates ultrasound 9/9 (usually appropriate) as the initial imaging test for RUQ pain 1
• Ultrasound has 81% sensitivity and 83% specificity for acute cholecystitis, but more importantly detects gallstones with 96% accuracy 1, 4
• Ultrasound can identify gallbladder wall thickening, pericholecystic fluid, bile duct dilatation, and alternative diagnoses 1, 4

Step 2: If ultrasound shows gallstones but is otherwise normal, consider HIDA scan

• Cholescintigraphy with calculation of gallbladder ejection fraction can diagnose functional gallbladder disorders (biliary dyskinesia or hyperkinesia) that present with normal labs and imaging 1, 3
• HIDA scan has 96% sensitivity and 90% specificity for acute cholecystitis, superior to ultrasound, but is most useful here for evaluating gallbladder function 1
• Biliary hyperkinesia (GBEF >85%) is an underrecognized cause of biliary colic that responds to cholecystectomy 3

Step 3: If ultrasound is negative or equivocal, proceed to MRCP

• MRCP has 85-100% sensitivity and 90% specificity for detecting choledocholithiasis and provides comprehensive visualization of the entire biliary tree 4
• MRCP is superior to CT for evaluating suspected biliary sources of RUQ pain and can detect stones, strictures, and anatomic abnormalities that ultrasound misses 4
• MRCP visualizes the common bile duct and cystic duct better than ultrasound, which is critical when intermittent obstruction is suspected 4

Step 4: If ultrasound and MRCP are negative, consider CT with IV contrast

• CT should be reserved for when biliary imaging is unrevealing and you need to evaluate for alternative diagnoses including hepatic, pancreatic, renal, or colonic pathology 1, 6
• CT has limited value for detecting gallstones (only 39-75% sensitivity) and exposes the patient to unnecessary radiation if ordered first-line 4

Critical Clinical Pitfalls to Avoid

• Do not assume normal labs exclude biliary disease - biliary colic from gallstones frequently presents with completely normal laboratory values between episodes of obstruction 1, 2
• Do not skip ultrasound and proceed directly to CT - ultrasound is more appropriate for initial evaluation, avoids radiation exposure, and has superior sensitivity for gallstones 1, 4
• Do not dismiss functional gallbladder disorders - if ultrasound shows gallstones but no acute inflammation, and symptoms are severe enough to impair quality of life, HIDA scan can identify biliary dyskinesia or hyperkinesia that responds to cholecystectomy 1, 3
• Do not overlook the right flank component - while this could represent referred pain from the gallbladder, ensure the ultrasound includes renal evaluation to exclude nephrolithiasis despite the normal UA 1

Additional Diagnostic Considerations

• Over one-third of patients with acute RUQ pain do not have acute cholecystitis, so maintaining a broad differential is essential even when biliary disease seems most likely 2, 6
• Colicky pain specifically suggests intermittent obstruction rather than constant inflammatory pain, making cholelithiasis or functional gallbladder disease more likely than acute cholecystitis 1, 3
• The combination of RUQ and right flank pain could represent hepatic flexure pathology or referred pain from biliary colic, both of which would be evaluated by the recommended imaging algorithm 7