What is the normal size of the common bile duct (CBD)?

Normal Size of the Common Bile Duct

The normal common bile duct (CBD) diameter is less than 6 mm in adults, measured as the intraluminal diameter in the transverse plane on ultrasound. 1

Standard Normal Values

• The upper limit of normal CBD diameter is traditionally 6 mm in adults without biliary disease 1
• The common hepatic duct should measure less than 4 mm in diameter 1
• Mean CBD diameter in normal adults ranges from 3.4 to 4.1 mm across multiple studies 2, 3, 4

Age-Related Variations

CBD diameter increases physiologically with age, approximately 1 mm per decade, which is a critical consideration to avoid overdiagnosis of pathology 1

• In patients aged 20 years or younger: mean diameter 2.8 mm 3
• In patients aged 71 years or older: mean diameter 4.1 mm 3
• In elderly patients (>75 years), CBD diameters up to 8-10 mm can be considered normal in the absence of other pathological findings 1, 5
• The very elderly (>75 years) have a mean CBD diameter of 6.5 mm, with an upper limit of 10 mm being reasonable 5

Post-Cholecystectomy Considerations

Post-cholecystectomy patients may have CBD diameters up to 10 mm without necessarily indicating pathology 1

• Mean CBD diameter after cholecystectomy is 4.5-4.7 mm in younger populations 2, 3
• In elderly post-cholecystectomy patients (>75 years), mean diameter increases to 8.7 mm, with measurements up to 14 mm potentially occurring 5

Measurement Technique

The CBD must be measured using the intraluminal diameter (inside wall to inside wall) in the transverse plane for accurate assessment 1, 6

• The CBD is located by identifying the portal vein, with the CBD and hepatic artery typically seen anterior to the portal vein in transverse view 6

Critical Clinical Pitfall

An increased CBD diameter alone is NOT sufficient to diagnose common bile duct stones (CBDS) and requires further diagnostic testing 7, 1

• A CBD diameter larger than 10 mm is associated with only a 39% incidence of CBDS, while diameter smaller than 9.9 mm is associated with CBDS in only 14% of cases 7, 1
• Direct visualization of a stone in the CBD on ultrasound is a much stronger predictor of choledocholithiasis than diameter measurements alone 1, 8
• In patients with acute calculous cholecystitis, mean CBD diameter is 5.8 mm without CBDS versus 7.1 mm with CBDS 7

Summary of Normal Ranges by Clinical Context

• Standard adult (<50 years): <6 mm 1
• Elderly (>75 years): <10 mm 1, 5
• Post-cholecystectomy (any age): <10 mm 1
• Post-cholecystectomy (>75 years): up to 14 mm may be acceptable 5