Normal Ultrasound Measurements of the Hepatic and Common Bile Duct
The normal upper limit of the common bile duct (CBD) diameter on ultrasound is less than 6 mm, measured using the intraluminal diameter (inside wall to inside wall) in the transverse plane, with an age-adjusted increase of approximately 1 mm per decade of life. 1, 2
Standard Measurement Technique
- The CBD should be measured in the transverse plane using the intraluminal diameter (inside wall to inside wall), as this provides the most accurate measurement. 1
- The CBD is located by first identifying the portal vein in the porta hepatis, which it reliably accompanies. 1
- In a transverse view of the porta hepatis, the CBD and hepatic artery are typically seen anterior to the portal vein. 2
Normal Diameter Values
- The traditional upper limit of normal CBD diameter is 6 mm in adults, though some authorities accept up to 3 mm as the strict upper limit in younger patients. 1, 2
- Research data from healthy blood donors shows a mean CBD diameter of 2.5-2.8 mm, with 95% of healthy subjects having diameters less than 4 mm. 3
- The absolute upper limit in healthy subjects without biliary disease is 7 mm, beyond which pathology should be suspected. 3
Age-Related Adjustments
- Many authorities recommend adding 1 mm per decade of age to the normal upper limit, allowing for physiologic age-related ductal dilatation. 1, 2
- In elderly patients, diameters up to 8-10 mm may be considered acceptable, particularly in those over 70 years of age. 2, 4
- The mean CBD diameter increases from approximately 2.8 mm in patients 20 years or younger to 4.1 mm in patients over 71 years. 5
Post-Cholecystectomy Considerations
- After cholecystectomy, the CBD diameter may increase by 1-2 mm, with upper limits of 8-10 mm considered acceptable in post-surgical patients without obstruction. 2
- Mean CBD diameter in post-cholecystectomy patients is approximately 5.2-6.2 mm at various measurement points. 3, 5
Clinical Pitfalls to Avoid
- Do not rely on CBD diameter alone to diagnose choledocholithiasis—a diameter >10 mm is associated with only 39% incidence of stones, while <9.9 mm still carries 14% risk. 2, 6
- Avoid measuring the CBD in longitudinal views, as this may overestimate the diameter due to oblique angulation. 1
- Do not confuse the CBD with the portal vein or hepatic artery—the CBD lies anterior to the portal vein in the porta hepatis. 1, 7
- A CBD diameter of 4-6 mm in younger patients without cholecystectomy warrants close clinical follow-up and correlation with liver function tests, even if technically within "normal" range. 3