The amount of residual urine in a normal person is very small, and is generally considered normal to be no more than 5ml. Residual urine volume is the volume of urine remaining in the bladder at the end of urination, reflecting the voiding function of the bladder, and can be measured by transabdominal ultrasound or catheterization, with ultrasound being generally discomfort-free and reproducible for patients, and catheterization being more accurate. If the amount of residual urine increases gradually, even more than 100ml can be diagnosed as urinary retention, which is a pathological state. Abnormal residual urine volume is usually seen in obstructive diseases of the urinary tract such as urethral stones, compression by periurethral masses, prostatic hyperplasia, and urinary incontinence. Patients with abnormal residual urine output are advised to go to the urology department of the hospital for a detailed examination to identify the cause and actively cooperate with the treatment.