After the body has completely eliminated the urine, when a urological ultrasound is performed, it is found that the urine in the bladder is greater than 50 ml or more, and this condition can be diagnosed as urinary retention. If the urine is greater than 20ml or more, this condition can be diagnosed as bladder residual urine. Usually this phenomenon is caused by obstruction of the urinary system, such as prostate hyperplasia or neurogenic bladder in elderly men, as well as urethral stenosis, posterior urethral valves, which can lead to obstruction of the process of urine discharge, or the bladder does not have independent contraction, resulting in the urine retained in the bladder, and can not be completely discharged. Therefore, ultrasound examination can be diagnosed as residual urine if the urine is more than 20ml; if it is more than 50ml, or even more than 100ml, it can be diagnosed as urinary retention. When this situation occurs, the patient should be treated in time to prevent urinary retention from progressing and causing hydronephrosis, which will damage the kidney function of human body.