Post-Void Residual Measurement

The post-void residual is the volume of urine left in the bladder after the patient has passed urine as completely as they can. It shows how well the bladder empties, and it identifies retention before it causes pain, infection, stones or kidney damage.

It is measured by ultrasound (a bladder scanner or a conventional ultrasound machine) or by catheterisation. The scan is done within 5 to 10 minutes of voiding, and takes about a minute. A portable bladder scanner uses automated three dimensional or multiplane measurements to give a volume.

With a conventional probe, volume is calculated from three bladder diameters using a simple formula (height x width x depth x about 0.52). Catheterisation gives an exact volume but is invasive, and it carries risks of infection, pain and urethral injury.

The accuracy of a bladder scan is within about 15 to 20% of the true volume, with the greatest error at small volumes, in women with pelvic masses or ascites, and in obese patients. Residual volume varies from one void to the next, and one measurement may mislead.

Chronic retention affects up to 10% of older men and a similar proportion of women with neurological disease or after pelvic surgery. In older men with lower urinary tract symptoms, about 10 to 15% have a residual of over 100 mL.

Indications.

• Lower urinary tract symptoms: hesitancy, weak stream, incomplete emptying, overflow incontinence, and recurrent urinary infection.
• Neurogenic bladder (spinal cord injury, multiple sclerosis, diabetes, spina bifida), including surveillance.
• After surgery, anaesthesia, or childbirth, with suspected retention.
• Medication that can impair emptying (anticholinergic, opioid, and some antidepressant and antipsychotic drugs).
• Hydronephrosis, raised creatinine, or recurrent infection of unknown cause.
• After surgery on the prostate, urethra or pelvic floor, and after incontinence procedures.
• Before starting treatment for overactive bladder, to exclude retention.
• Suspected urethral stricture and bladder outlet obstruction.

Not indicated for diagnosing the cause of the retention, which needs further tests.

Contraindications and precautions.

• No contraindication for ultrasound. Open wounds, dressings, or a catheter in place limit scanning.
• Catheterisation is avoided where there is urethral injury (blood at the meatus after trauma), and is done with care in a patient with a stricture or recent urethral surgery.
• Scans in women with a pelvic mass, large fibroids, ovarian cysts or ascites can give false high readings.
• Patients with a very full bladder or in distress are not made to wait for a scan.

Preparation and technique.

The patient is asked to void as completely as possible, in a comfortable position and privacy, and scanning is done straight after (within 10 minutes). The probe is placed on the suprapubic area with gel and aimed at the bladder, and the scanner outlines the bladder in several planes.

The scan is repeated if the reading seems doubtful, and the bladder shape and the position of the cursor are checked on the screen. When two or three measurements differ, the lowest, or the average, is used depending on the local routine, and a repeat on another day is done for borderline results.

Normal appearances.

In a healthy adult the residual is under 50 mL, and often almost nothing. A residual of 50 to 100 mL is borderline, particularly in older people, and may be normal for them. In children, residual urine should be under 10% of bladder capacity (or under 5 to 20 mL) after a normal void. The bladder wall is smooth, and the bladder has no stones or masses.

Interpretation.

• Under 50 mL: normal emptying.
• 50 to 100 mL: borderline, repeated for confirmation, and read with symptoms and flow rate.
• Over 100 mL: incomplete emptying. A value of 100 to 200 mL is a moderate residual, over 200 mL is significant, and over 300 mL is usually regarded as chronic retention.
• Over 500 mL with a distended bladder: acute or chronic retention. In chronic retention (painless, high volume), the bladder wall is trabeculated and there may be hydronephrosis, overflow incontinence, and renal impairment.
• Obstruction: prostate enlargement, stricture, bladder neck obstruction, or a pelvic organ prolapse.
• Detrusor weakness: neurological disease, diabetes, drugs, overdistension, or ageing.
• Drug effects: opioids, anticholinergics, calcium channel blockers, tricyclic antidepressants, and other drugs reduce bladder contractility.
• Residual urine is a risk for infection, stones, and renal damage, but the same volume carries different risk in different patients.

Critical values and findings needing immediate action.

• Painful retention with a distended bladder: urgent catheterisation, with measurement of the volume drained.
• Residual over 300 to 400 mL with hydronephrosis or raised creatinine: urgent relief of the obstruction by catheter, then investigation. Post-obstructive diuresis needs monitoring of fluid and electrolytes.
• Retention with fever, loin pain or sepsis: urgent treatment of the infection and drainage.
• High residual in a patient with a spinal injury or autonomic symptoms: bladder drainage, with care for autonomic dysreflexia.

Limitations and artefacts.

• The residual changes from void to void, with the time of the day, fluid intake, anxiety, and the setting. One reading in a rushed clinic may be falsely high.
• A scan done too long after voiding includes newly made urine and overestimates the residual.
• Scanners can mistake ovarian cysts, fibroids, ascites, or a distended uterus for urine, and they under read in obesity, in scars and after pelvic surgery. Bowel gas and a very small bladder reduce accuracy.
• A bladder diverticulum can hold urine that does not drain, and a normal scan can miss it.
• The scan gives the volume, not the cause.
• Catheter measurement gives the exact volume but carries risks of infection and trauma.

Special groups.

• Children: low volume, and the result is related to the age related bladder capacity.
• Pregnancy and the postpartum period: transient retention is common after delivery, particularly after epidural anaesthesia.
• Older men: a raised residual is common, and by itself is not an indication for surgery without symptoms or complications.
• Neurogenic bladder: regular measurement directs the catheterisation schedule.
• Post-operative patients: retention is frequent after spinal anaesthesia, pelvic or hernia surgery, so scanning is used to avoid unnecessary catheters.
• Regions: a portable bladder scanner is increasingly available, and a basic ultrasound machine with a measuring formula gives acceptable results. Where neither is available, an abdominal examination for a palpable bladder and the volume obtained at a single catheterisation are used.

Follow up tests.

• Repeat post-void residual measurement on another day.
• Uroflowmetry and urodynamic studies.
• Urinalysis, culture, serum creatinine, and electrolytes.
• Renal and bladder ultrasound for hydronephrosis, bladder wall thickness, stones and masses.
• Cystoscopy, prostate assessment, and urethral imaging.
• Review of medicines, and neurological assessment.
• Prostate specific antigen in men with obstructive symptoms.