Foley Catheter Size Comparison: 16 French vs 18 French
An 18 French (Fr) Foley catheter is larger than a 16 French (Fr) Foley catheter, with each French unit representing 0.33 mm in diameter.
Understanding French Catheter Sizing
The French (Fr) scale is the standard measurement system used for catheter sizing:
- Each French unit equals 0.33 mm in external diameter
- The larger the French number, the larger the catheter diameter
- An 18 Fr catheter is approximately 6 mm in diameter
- A 16 Fr catheter is approximately 5.3 mm in diameter
Clinical Implications of Catheter Size Selection
Flow Characteristics
- Larger catheters provide better drainage capacity
- An 18 Fr catheter offers approximately 19% less resistance to flow compared to a 16 Fr catheter 1
- Beyond 18 Fr, there are diminishing returns in flow improvement when upsizing catheters 1
Patient Comfort and Trauma Risk
- Smaller catheters generally cause less urethral trauma and discomfort
- Guidelines recommend using the smallest possible catheter size that will adequately drain the bladder 2
- For most routine catheterizations, 14-16 Fr catheters are sufficient 3
Special Clinical Scenarios
Post-surgical Drainage
- For post-surgical bladder drainage after repair of bladder injuries, a standard urethral catheter is recommended over suprapubic cystostomy 2
- In cases with significant hematuria or complex bladder repairs, a larger catheter (18 Fr) may be preferred to prevent clot obstruction 2
Patients with Artificial Urinary Sphincters
- For patients with artificial urinary sphincters, catheter size selection is critical:
- 16 Fr catheters can pass through 4.0 cm cuffs with mild risk of trauma
- 18 Fr catheters can safely pass through 4.5 cm or larger cuffs 4
Long-term Catheterization
- For long-term catheterization, smaller catheters (16 Fr) are generally preferred to reduce:
- Risk of urethral erosion
- Bladder spasms
- Catheter-associated urinary tract infections 3
- Larger catheters (18 Fr) may be necessary for patients with:
- Significant hematuria
- Sediment in urine
- History of frequent catheter blockage 1
Potential Complications of Oversized Catheters
- Increased risk of urethral trauma and stricture formation
- Greater patient discomfort
- Increased bladder irritation and spasms
- Higher risk of catheter-associated urinary tract infections
- Risk of bladder wall damage with stiffer, larger catheters 5
Recommendations for Catheter Selection
- Default to smaller sizes (14-16 Fr) for routine catheterization
- Consider 18 Fr catheters when:
- Managing hematuria with clots
- Dealing with thick urinary sediment
- Addressing recurrent catheter blockage
- Avoid unnecessarily large catheters as they provide minimal additional drainage benefit beyond 18 Fr but increase complication risks 1
Remember that proper catheter fixation (preferably on the upper thigh rather than abdomen) can also significantly reduce catheter-associated lower urinary tract symptoms regardless of size 6.