Urinary retention is common, silent, and found with a scanner

Why it matters

The bladder is the organ the PACU manages by folklore: a void before discharge, or a wait until one happens. The sources on this day scanned bladders instead of waiting. At recovery-room discharge, 44% of unselected adults had more than 500 mL in the bladder, and 54% of those, with no symptoms, could not void within 30 min lamonerie-2004. On a PACU that measured volume on arrival and before discharge, retention defined as more than 600 mL with inability to void within 30 min occurred in 16% keita-2005.

The consequence of getting this wrong is either a distended, silent bladder sent home, or a patient held for an hour for a void that a scanner would have shown was not needed.

Mechanism

Micturition requires an intact sensory signal from a filling bladder, an intact spinal reflex, and cortical permission. Neuraxial anesthesia interrupts the first two for as long as the block lasts; opioids and anticholinergic drugs blunt detrusor contraction and the sensation of fullness; large volumes of intraoperative fluid fill the bladder faster than the recovering patient can empty it. The result is a bladder that fills without the patient knowing, which is why the sources find most distension asymptomatic and why the risk factors are the volume that went in, the block that stopped the signal, and the age that slowed the reflex.

Evidence

Keita 2005 followed 313 PACU patients with ultrasound bladder volume on entry and before discharge keita-2005. Retention, a bladder volume above 600 mL with inability to void within 30 min, occurred in 16% keita-2005. The full text was read on 2026-09-06 and gives three independent predictors on multivariate analysis, each with an odds ratio and a P value: intraoperative fluids of 750 mL or more, odds ratio 2.3, P = 0.02; age 50 years or more, odds ratio 2.4, P = 0.008; and a bladder volume on PACU entry of 270 mL or more, odds ratio 4.8, P = 0.0001 keita-2005. The paper reports no confidence intervals for any of the three, so the precision of those estimates is unknown.

Lamonerie 2004 scanned 177 adults at recovery-room discharge: 44% had a volume above 500 mL, and 54% of those, asymptomatic, could not void within 30 min lamonerie-2004. Age over 60 carried an odds ratio of 2.11 (95% CI 1.01 to 4.38), spinal anesthesia 3.97 (1.32 to 11.89), and surgery longer than 120 min 3.03 (1.39 to 6.61) lamonerie-2004.

Mulroy 2002 randomized low-risk ambulatory patients after short-acting spinal or epidural anesthesia between a standard pathway that required voiding and an accelerated pathway that did not mulroy-2002. The standard group was discharged at 153 (49) min; in the accelerated group 62 voided anyway, 46 were discharged with a bladder ultrasound below 400 mL, and 23 had more than 400 mL, of whom 20 voided within an hour and 3 were catheterized; the accelerated pathway was 22 min shorter (P=0.002) with no urinary problems or returns mulroy-2002.

What this does not show

Two of the three sources are cohorts, and the trial is small, single-institution and restricted to low-risk patients under 70 with short-acting neuraxial agents and no hernia, rectal or urological surgery keita-2005 lamonerie-2004 mulroy-2002. The risk factors are associations. The trial shows that a scanner can replace a void safely in a narrow population; it does not show that in every population, and it did not include peripheral nerve blocks.

Local practice differs. the bladder volume at which a PACU catheterizes, and whether a scan or a void is the discharge criterion, differ between institutions. Most units sit between 500 and 600 mL. A figure of 800 mL circulates alongside those and is usually misquoted. The trial behind it is now in the ledger: it compared catheterizing at 500 mL with catheterizing at 800 mL in inpatients having fast-track hip and knee arthroplasty under spinal anesthesia. Catheterization occurred in 114 of 354 patients at the 500 mL threshold against 49 of 367 at 800 mL, a relative risk of 0.4 (95% CI 0.3 to 0.6), with no difference in any secondary outcome including urinary tract infection at 2% in both arms [[bjerregaard-2016]. It is a catheterize-at threshold in that population, not evidence that an ambulatory patient can be left undrained at 800 mL. Know which number your unit uses.]he bladder-volume threshold for intervention differs between the sources, 600 mL in one and 500 mL in another, and the discharge gate in the trial was 400 mL; none of them establishes which number is right keita-2005 lamonerie-2004 mulroy-2002. Local practice differs. the bladder volume at which a PACU catheterizes, and whether a scan or a void is the discharge criterion, differ between institutions.

The retention risk after a peripheral nerve block, as distinct from a neuraxial one, was searched for again on 2026-09-06 and no study isolating peripheral blocks in a genuinely ambulatory population was found. The nearest is an inpatient knee arthroplasty series whose confidence interval is too wide to teach from and whose population is the wrong one. This page therefore says nothing about it rather than borrowing an inpatient number.

At the bedside

Scan the bladder on arrival and before discharge in the patients the sources identify: over 50, given 750 mL or more of fluid, after a spinal, or arriving with 270 mL or more already in the bladder keita-2005 lamonerie-2004. Do not wait for a symptom; most distended bladders do not produce one.

Use the volume, not the void, as the decision. A patient below the local threshold can leave without voiding; a patient above it is watched or catheterized mulroy-2002.

Write the volume in the discharge note, because the ward that receives the patient will otherwise start the folklore again.

Check yourself

Three items. Every option carries an explanation. Progress is not saved.