French catheter scale
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The French scale or French gauge system is commonly used to measure the size of a catheter. It is most often abbreviated as Fr, but can often be seen abbreviated as Fg, Ga, FR or F. It may also be abbreviated as CH or Ch (for Charrière, its inventor).
The French size is three times the diameter in millimeters. A round catheter of 1 French has an external diameter of 1⁄3 mm, and therefore the diameter of a round catheter in millimetres can be determined by dividing the French size by 3:
D (mm) = Fr / 3
Fr = D (mm) * 3
For example, if the French size is 9, the diameter is 9/3 = 3.0; mm.
From the basic math equation C = πd, it follows that the catheter's circumference in mm is only slightly (about 4.7%) greater than the French size.
An increasing French size corresponds to a larger external diameter. This is contrary to needle-gauge size, where an increasing gauge corresponds to a smaller diameter needle.
The French size is a measure of the external diameter of a catheter (not internal drainage channel). So, for example, if a two-way catheter of 20 Fr is compared to a 20 Fr three-way catheter, they both have the same external diameter but the two-way catheter will have a larger drainage channel than the three-way. Three-way catheters accommodate an extra channel for irrigation within a similar external diameter.