Background: Computed tomography (CT) is widely used in diagnosis of biliary disorders in adults yet there is no data on the normal common bile duct (CBD) size of the average Thai adult. This study attempts to establish a reference normal measurement for CBD diameter for the Thai population.
Material and Method: In this retrospective study, CBD measurements were performed on axial oblique images, perpendicular to the long axis of the distal CBD. The mean diameter of the normal CBD was measured in 277 patients. The patients were divided into 7 groups according to their age. Analysis of variance (ANOVA) was used to compare data obtained from the seven age groups.
Results: Mean diameter of CBD ranged from 2.3 to 9.5 mm. The mean of the mean diameter of 277 subjects was 4.65+1.39 mm. The diameter of the CBD significantly increased with age (p<0.05), and are not significantly related to gender.
Conclusion: Diameter of CBD shows a considerable increase with age. The largest diameter of the CBD is up to 6.4 mm in patients who less than 60 years. An upper limit of 8 mm appears reasonable after the age of 60.

Keywords: Common bile ducts, Computed tomography