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Kit and numbers
🚿 Catheters
Sizes, lengths, and when a bladder scan changes the plan
Sized in French (Fr, or Ch — the same thing). One French is a third of a millimetre of outer diameter, so a 16Fr catheter is about 5.3 mm across the outside.
The instinct to go bigger is usually wrong. A larger catheter is not better for ordinary retention: it is more painful, more traumatic to the urethra, and more likely to cause a stricture. Bigger is for clots.
Size
| 12–14Fr | Standard adultRetention, monitoring output, most inpatients. Start here. |
|---|---|
| 16Fr | Larger standardIf a 14Fr will not pass or drains poorly. Common in men with prostatic obstruction. |
| 18–22Fr, three-way | Haematuria and clotsThe third channel is for irrigation. Frank haematuria with clot retention needs this, not a bigger two-way. |
| 6–10Fr | PaediatricBy age and size — ask paediatrics rather than estimating. |
Length
| Standard / male | ~40–45 cmUse for all men, and for women when a female-length will not reach. |
|---|---|
| Female | ~20–26 cmNEVER in a man. A female-length catheter in a male urethra inflates the balloon in the urethra. |
Balloon
| Adult | 10 ml sterile waterWater, not saline — saline crystallises and can block the deflation channel. Not air. |
|---|---|
| Post-prostatectomy / urology | 30 mlOnly on urological instruction. Do not choose this yourself. |
| Inflate only when | Urine is drainingAnd in a man, only after advancing to the hilt. Balloon inflation in the urethra is a urological emergency. |
When not to try
| Pelvic trauma | StopBlood at the meatus, scrotal or perineal bruising, high-riding prostate — suspect urethral injury. Urology first. |
|---|---|
| Recent urological surgery | Ask firstA fresh anastomosis can be destroyed by a blind pass. |
| Two failed attempts | EscalateA third attempt by the same person swells the urethra and makes the next one harder for everybody. |
A bladder scan before catheterising changes the answer more often than people expect. An empty bladder in a patient who is not passing urine is not retention — it is anuria, and a catheter treats nothing while delaying the diagnosis.
In chronic retention, decompression can be followed by post-obstructive diuresis and haematuria; these patients need output monitored and U&Es checked, not discharge.
Sizes above are the common pattern, not a national standard — stock, three-way sizing and post-operative balloon volumes are set by your trust and its urology department. On a post-op patient, their instruction beats this table.
Source: EAUN catheterisation guideline; RCN catheter care; local urology policy