In a Foley catheter, does a lower French size indicate a smaller or larger diameter catheter?

French Size Scale in Foley Catheters

A lower French (Fr) number indicates a smaller diameter catheter. The French scale is a direct measurement where each French unit equals approximately 0.33 millimeters in outer diameter, meaning a 14 Fr catheter is physically smaller than an 18 Fr catheter 1.

Understanding the French Scale

• The French sizing system is standardized across all catheters, where 1 French = 0.33 mm of outer diameter 2
• Common adult Foley catheter sizes range from 12 Fr to 24 Fr, with each increment representing a larger physical diameter 1, 3
• A 14 Fr catheter has a smaller outer diameter than a 16 Fr, which is smaller than an 18 Fr, and so on 1

Standard Adult Sizing

• The typical adult Foley catheter size is 14-16 Fr, with 16 Fr being the most commonly used standard size for routine catheterization 1
• For routine adult catheterization, starting with a 16 Fr catheter is recommended 1
• Smaller sizes (12-14 Fr) are used when minimizing urethral trauma is paramount 1, 4
• Larger sizes (18-22 Fr) may be used for specific indications such as preventing blockages from blood clots or debris 3, 5

Clinical Principle: Smallest Appropriate Size

• The smallest appropriate catheter size should be selected to minimize trauma while maintaining adequate drainage 1
• In general, the smallest diameter catheter and minimum number of lumens should be used to minimize the risk of catheter-related complications 2
• Oversized catheters (>18 Fr) increase the risk of patient discomfort 1

Important Caveat About Larger Sizes

• Silicone Foley catheters become progressively stiffer as the French size increases 3
• A size 12 Fr silicone Foley catheter has a soft and flexible tip, while a size 24 Fr silicone Foley catheter is distinctly stiff 3
• This increased stiffness with larger sizes can increase the risk of bladder perforation, particularly in patients with chronically inflamed or neuropathic bladders 3
• Beyond 18 Fr, larger catheter sizes provide diminishing marginal returns in flow improvement; upsizing from 18 Fr to 20 Fr only reduces resistance by 19% 6

Pediatric Sizing Reference

• For pediatric patients, median catheter sizes are: 5 Fr for infants younger than 1 year, 7 Fr for children ages 1-5 years, and 8 Fr for children older than 5 years 4
• Neonatal long lines are available down to 28G (approximately 0.9 Fr) for specialized use 1