Post-void residual volume (PVR) is the amount of urine retained in the bladder after a voluntary void and functions as a diagnostic tool. A post-void residual volume helps in the evaluation of many disease processes, including but not limited to neurogenic bladder, cauda equina syndrome, urinary outlet obstruction, mechanical obstruction, medication-induced urinary retention, postoperative urinary retention, and urinary tract infections. Evaluating for post-void residual is typically performed using ultrasound, a bladder scanner, or with a urinary catheter.

A urine specimen is not always indicated but can aid in the diagnostic evaluation, especially if there is a concern for infection as the etiology of the elevated residual volume.  Urinary catheterization after voiding can provide both a post-void residual and a urine specimen. Urinary catheterization should be performed sterilely to prevent infection and to obtain an uncontaminated urine specimen. See below for urinary catheterization technique.

Post-void residual evaluation is by measuring the remaining urine in the bladder shortly after a voluntary void; this can be accomplished through ultrasound, bladder scan, or by directly measuring the urine volume drained by a urinary catheter.  Urinary catheterization is the gold standard for measuring the post-void residual.[1]

Measurement of post-void residual immediately after voiding is crucial for accurate measurement, with delays of as little as 10 minutes from bladder emptying to post-void residual measurement potentially causing clinically significant overestimation of post-void residual.[2]

Ultrasound (conventional or real-time ultrasound)

The conventional ultrasound is used to visualize the bladder directly, either transabdominally or transvaginally. It measures the bladder’s volume using the ultrasound machine's internal volume calculations or the mathematical equation seen below. Transvaginal ultrasound appears especially accurate for measuring low bladder volumes.[3]

Technique:

• For transabdominal bladder volume evaluation, the probe is placed over the suprapubic area while the patient is in the prone position. Bladder images are recorded in both the sagittal and transverse planes. The greatest transverse (width), and anteroposterior (depth) and superior-inferior (height) distances are recorded.[4]
• For transvaginal bladder volume evaluation, an intracavitary transvaginal probe with frequencies between 7 to 9 MHz is used. Ensure the probe is clean, has a probe cover, and has gel placed onto the end of the probe. Patient position is supine with the legs in stirrups or a pad under the pelvis. The probe is inserted into the vagina, and the bladder identification is in the sagittal plane. Measurements are from the longest anteroposterior dimension and the craniocaudal dimension. The probe is then rotated 90 degrees to measure the width or axial dimension.[5]

Most ultrasound machines have a function to automatically calculate volumes from the measurements used with the ultrasound calipers. If this function is not available the volumes can also be calculated using the prolate ellipsoid formula. Though there are multiple mathematical methods to calculate volume, the prolate ellipsoid is one accepted method, and the formula follows[6][7]:

•  Volume = length x width x height x 0.52. (Note, the prolate ellipsoid formula has multiple acceptable correction factors.)

The bladder is measured at its maximal transverse (width), longitudinal (length) and anterior-posterior (height) diameters. This method is recommended as the standard calculation because it is fast and easy.[7]

Limitations/Comparisons: 

Data on the accuracy of transabdominal ultrasound for determining post-void residual is mixed. Although most studies demonstrate high accuracy of post-void residual using transabdominal point-of-care ultrasound or bladder scanners with automated measurement of bladder volume,[8][9][10] other recent studies have brought their accuracy into question.[11][12]

The portable bladder ultrasound device (commonly known as a bladder scanner)

A portable bladder ultrasound device uses ultrasound to specifically measure the three-dimensional volume of urine in the bladder.  It is a non-invasive approach to identifying post-void residual.

Technique:

The patient lies supine, ultrasound gel is to be placed at the suprapubic area, and the probe is then placed on the gel and directed toward the patient's bladder. The button is depressed, initiating the machine's evaluation of the bladder's volume, which is then displayed for the operator to see. The process can be repeated to better align the bladder in the center of the display. The largest volume is recorded.[8] Different bladder scanner machines may have slightly different procedures, but the basics of the technique are similar across devices.

Limitations:

Bladder scanning is not suitable for use on patients with severe abdominal scars, prolapses of the uterus, or current pregnancy. [13] Abdominal ascites appears to cause falsely elevated measurements in some cases.[14] There is some evidence that bladder scanners are more accurate than two-dimensional ultrasound imaging.[4]

Urethral Catheterization

Urethral catheterization directly measures the post-void residual and is considered the gold standard for determining post-void residual, although there may be variable volumes present after catheterization, depending on the size and type of catheter used for drainage.[14] 

Technique:

The male and female anatomy requires different techniques with inserting a urethral catheter, but the overall procedure and sterile technique are similar. Perform hand hygiene and place the patient in position of accessibility; a female patient should be placed in the frog-leg position, sterile drapes should cover the genitals with the urethral meatus exposed. Don sterile gloves. The non-dominant hand should hold the penis at a 90-degree angle toward the ceiling, and for females, the labia should be separated and held to expose the urethral meatus. The urethral meatus and glans penis for males or perineum for females should undergo prepping with an antiseptic solution using sterile technique. Insert a well-lubricated catheter into the urethral meatus with the dominant, sterile-gloved hand, and observe for a spontaneous return of urine to confirm placement in the bladder. Once placement is confirmed, inflate the catheter balloon with sterile water or saline.[15] For in-out catheterization to obtain a post-void residual, allowing the bladder to drain completely into a graduated container and measure the volume, then remove the catheter; this should be performed shortly after a voluntary void to ensure an accurate post-void residual.

Complications[16][17]:

• Common traumatic complications include urethral trauma, false passages created in the urethra, and inadvertent balloon inflation in the urethra.
• The precursor of catheter-associated urinary tract infection is bacteria, which develops at an average rate of 3% to 10% per day of catheterization. The commonly studied incidence of infection relates to indwelling catheterization and not in-and-out catheterization, which would be used to identify a PVR or to obtain a sterile urine sample.  

Comparison of bladder scanner to urinary catheterization:

• Ching-Hwa Teng et al. found no statistical difference in PVR when measured using the bladder scanner or urinary catheterization. Additionally, the average time to perform the bladder scan was 45 seconds, whereas the average time to perform the urinary catheterization was 293 seconds.[13]
• Additionally, Christopher E. Kelly states that the portable bladder scanner is convenient, noninvasive, accurate, cost-effective and carries no risk of urethral trauma or urinary tract infection, and they are beneficial in a variety of populations including patients who have a neurologic disease, children and the elderly.[18]

A post-void residual measurement is a diagnostic tool used when there is a concern for underlying neurological disease/injury, bladder dysfunction, mechanical obstruction, infection or medication-induced urinary retention.

Symptoms of urinary retention and post-void residual can be indirect. A common presenting symptom of acute urinary retention is low abdominal pain and inability to pass urine,[19] although, in young children or patients with dementia, who provide limited history, agitation may be the only sign of bladder distension.

Patients with cauda equina syndrome may present with low back pain, saddle anesthesia, bilateral sciatica, motor weakness of the lower extremities, paraplegia, or incontinence.[20]

Young children may have urinary retention due to refusing to urinate because of the pain of urination caused by a UTI.

Underlying causes of elevated post-void residual include (but not limited to):

Neurogenic

• Cauda equina syndrome
• Spinal nerve impingements (due to trauma, neoplasm, disc herniation, infection or hematoma)
• Diabetic bladder neuropathy
• Spinal cord injury
• Cerebrovascular accident 
• Traumatic brain injury 
• Neurogenic bladder 
• Fowler syndrome (inability to relax bladder sphincters) 
• Vitamin B12 deficiency
• Alcoholism
• Multiple sclerosis 
• Parkinson disease 
• Bladder denervation from pelvic surgery injury
• Heavy metal poisoning
• Detrusor-sphincter dyssynergia
• Post-surgical urinary retention 

Mechanical

• Benign prostatic hypertrophy
• Phimosis and paraphimosis
• Prostate cancer
• Ureteral calculi
• Bladder calculi
• Urethral strictures
• Bladder neoplasm
• Fecal impaction
• Blood clots
• Cystocele
• Tumors
• Detrusor underactivity

Medication-related 

• Anticholinergics
• Antihistamines
• Tricyclic antidepressants
• Decongestants
• Cyclobenzaprine
• Diazepam
• NSAIDs
• Amphetamines
• Opioids 

Infectious and/or inflammatory

• Cystitis
• Prostatitis
• Herpes simplex
• Herpes zoster
• Local abscess
• Vulvovaginitis
• Young children with UTI may retain urine refusing to urinate because of pain

Congenital

• Hypospadias 

Anatomic

• Large bladder diverticula
• Vesicoureteral reflux (causing rapid bladder refilling after voiding)
• Posterior urethral valves