Share this“As a person that deals with numerous catheter manufactures, I find it very challenging when discussing the size of tubing for various catheters. It normally requires that the supplier educate me on their terminology before we can even discuss the specifics of what I am looking to purchase.”
Do you remember the first time you tried to find a catheter with the right dimensions for a specific vessel? Chances are the various units of measure, abbreviations, and acronyms encountered during your search were enough to make you pull your hair out! Problems can arise when the researcher and catheter supplier are not using the same vocabulary when discussing catheter sizes. A researcher may indicate that they are looking for a “23 gauge” catheter for a particular use or a “3 French” catheter for a different use. Sounds simple enough, but what sizes are we really describing? Is a 23 gauge (G) catheter the size of the outer diameter of the catheter, or a catheter that can accept a 23G connector or plug? If the catheter accepts a 23G plug, then what is the outer diameter?
French and gauge are scales of measure used to refer to the diameter of catheters and tubing. While each have certain advantages, misinterpreting these standards can lead to confusion between the researcher and catheter manufacturer. “OD” and “ID” are abbreviations for outer diameter and inner diameter, respectively, and are generally measured in inches or millimeters.
The catheter sizing classification most commonly used in the medical industry is the French (Fr) scale, also known as the “Charrière’s system.” The French scale is an easy method to use to describe the basic size of a catheter, because it specifies the most obvious component, the outer diameter. French sizing has uniform increments starting with 1Fr with no upper end point.1 Each increment of French sizing equals 0.33mm, for example, a 3Fr catheter equals 1mm outer diameter. The disadvantage of the French scale is that it does not specify the inner diameter of the catheter or tubing. A 3Fr polyurethane catheter, perfect for a rat jugular vein, has a typical OD of 1mm (0.040”) and an ID of approximately 2Fr (0.024”). A 3Fr silicone catheter, on the other hand, has an ID that is slightly smaller than 2Fr (0.020”). Knowing this difference, while seemingly very small, is essential to ensure a secure connection between the catheter and connector, plug, or swivel. Thus, the French sizing used alone, is not specific enough to adequately describe small catheters, where internal diameter is critical. This scale is much more useful to specify larger catheter sizes such as 5Fr–9Fr, (used for large animal species) where the ID is not as critical.
A second method of sizing catheters is the gauge measurement originally developed for wire sizing in the early 19th century by Peter Stubs2 and later incorporated into needle sizing. Gauge is a descending scale, opposite the French scale of measurement which ascends with corresponding size. The higher the gauge size the smaller the tubing, for example, a 23G tubing has an OD/ID of 0.025”x 0.012” and 18G tubing has an OD/ID of 0.050” x 0.033”. Measuring a catheter based on a gauge measurement is common because laboratory personnel are familiar with standard needle sizes. However, while there are standards for medical needle sizing, wall thickness from one manufacture to another may vary, so again, it can be challenging to determine the inner diameter of a catheter or tubing simply by using gauge alone.
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