What is the upper limit of normal for the common bile duct on ultrasound: how much do you want it to be?

Conventionally, the upper limit of normal for the common bile duct as measured by ultrasound is considered to be 6 mm. This review is a somewhat personalized account of how that number became the convention and cautions the reader to avoid being slavish in the use of this number. Two specific cautions are not to apply this limit to older patients and to consider where in the common bile duct the measurement was taken, as measurements taken closer to the pancreas may be larger than ones closer to the liver.