Common findings, determined by ultrasound of the bladder, include a slow rate of flow, intermittent flow, and a large amount of urine retained in the bladder after urination. A normal test result should be 20-25 mL/s peak flow rate. A post-void residual urine greater than 50 ml is a significant amount of urine and increases the potential for recurring urinary tract infections. In adults older than 60 years, 50-100 ml of residual urine may remain after each voiding because of the decreased contractility of the detrusor muscle. In chronic retention, ultrasound of the bladder may show massive increase in bladder capacity (normal capacity is 400-600 ml).Non-neurogenic chronic urinary retention does not have a standardized definition; however, urine volumes >300mL can be used as an informal indicator. Diagnosis of urinary retention is conducted over a period of 6 months, with 2 separate measurements of urine volume 6 months apart. Measurements should have a PVR (post-void residual) volume of >300mL.Determining the serum prostate-specific antigen (PSA) may help diagnose or rule out prostate cancer, though this is also raised in BPH and prostatitis. A TRUS biopsy of the prostate (trans-rectal ultra-sound guided) can distinguish between these prostate conditions. Serum urea and creatinine determinations may be necessary to rule out backflow kidney damage.
Causes
Bladder
Infection
Detrusor sphincter dyssynergia
Neurogenic bladder (commonly spinal cord damage, pelvic splanchic nerve damage, cauda equina syndrome,  pontine micturition or storage center lesions, demyelinating diseases or Parkinsons disease)
Iatrogenic (caused by medical treatment/procedure) scarring of the bladder neck (commonly from removal of indwelling catheters or cystoscopy operations)
Damage to the bladder

Prostate
Benign prostatic hyperplasia (BPH)
Prostate cancer and other pelvic malignancies
Prostatitis

Penile urethra
Congenital urethral valves
Phimosis or pinhole meatus
Circumcision
Obstruction in the urethra, for example a stricture (usually caused either by injury or STD), a metastasis or a precipitated pseudogout crystal in the urine
Pseudodyssynergia
STD lesions (gonorrhoea causes numerous strictures, leading to a "rosary bead" appearance, whereas chlamydia usually causes a single stricture)
Emasculation

Postoperative
Risk factors include

Age: Older people may have degeneration of neural pathways involved with bladder function and it can lead to an increased risk of postoperative urinary retention. The risk of postoperative urinary retention increases up to 2.11 fold for people older than 60 years. Medications: Anticholinergics and medications with anticholinergic properties, alpha-adrenergic agonists, opiates, nonsteroidal anti-inflammatories (NSAIDs), calcium-channel blockers and beta-adrenergic agonists, may increase the risk. Anesthesia: General anesthetics during surgery may cause bladder atony by acting as a smooth muscle relaxant. General anesthetics can directly interfere with autonomic regulation of detrusor tone and predispose people to bladder overdistention and subsequent retention. Spinal anesthesia results in a blockade of the micturition reflex. Spinal anesthesia shows a higher risk of postoperative urinary retention compared to general anesthesia. Benign prostatic hyperplasia: Men with benign prostatic hyperplasia are at an increased risk of acute urinary retention.