Post-void residual

The post-void residual refers to the volume of urine that remains in the urinary bladder after micturating. It may be estimated by visualization using ultrasound or directly measured with urethral catheterization

Clinical presentation

Use of this measurement is of some utility in several distinct groups of patients, including the following:

• in the presence of back pain or neurologic deficits evoking concern for a myelopathy
  • compression of the efferent sacral nerve roots (S2-4) may decrease one's ability to voluntarily contract the detrusor, resulting in an elevated post-void residual
• in the presence of lower urinary tract symptoms
  • particularly for etiologic differentiation of incontinence
• in the presence of an abnormal or worsening renal function 
  • elevated values suggestive of obstructive uropathy

Radiographic features

Ultrasound

The post-void residual may be formally measured using one of several proposed formulae, or (more commonly at the point of care) semi-quantified into abnormally elevated and normal.

Using a suprapubic transducer position the urinary bladder is located and centered on the screen; the width and anteroposterior dimensions are traditionally measured with the probe in a transverse orientation. After rotation of the probe marker cephalad to a sagittal plane the infero-superior height is obtained. An estimation of volume (and if performed immediately after micturition, a post-void residual) in milliliters may be calculated as follows ²:

• estimated bladder volume = 0.75 x (width) x (length) x (height)

Differential diagnosis

Several considerations in the differential diagnosis for an abnormally elevated (exceeding 100-200 mL in an adult or more than 10% of bladder capacity in a pediatric patient) post-void residual include:

• urethral stricture
• bladder outlet obstruction
  • benign prostatic hypertrophy
• uretherolithiasis
• hematuria with obstructing clots
• compressive myelopathy
  • spinal epidural abscess
  • transverse myelitis
• neurogenic bladder