Residual bladder urine volume refers to the amount of urine remaining in the bladder after the patient has urinated. Normal residual urine volume is about 0-10ml. When the residual urine volume is greater than 20 ml, it is called increased residual urine volume. Increased bladder residual urine volume is mainly seen in patients with lower urinary tract obstruction or bladder detrusor contraction dysfunction. Lower urinary tract obstruction diseases are mainly urethral strictures, urethral stones, prostatitis in young and middle-aged men, prostate enlargement in middle-aged and elderly men, or bladder neck obstruction in women. Bladder detrusor contractile dysfunction refers mainly to neurogenic bladder from various causes. The most accurate way to measure residual bladder urine volume is to catheterize after voiding, but this is an invasive method and ultrasonography is now often used.