Residual urine: why we need to measure ?￼

Last updated: March 6, 2024

Lots of patients at urology office always asked me “why does doctor need us to do residual urine (RU) measurement?” I will explain you more clearly in this article. An abnormal voiding urination can be possibly causes from tons of etiologies, and one suspicious cause is, the inability of urinary bladder to complete or emptying the urine which can lead to a urine stasis inside the bladder after finish urinating.

In the past, we do residual urine measurement by intermittent urethral catheterization which is quite too invasive.

In the past, Residual urine measurement seems to be something that looks scary and harmfully, because we have to insert the urethral catheter when patient finish voluntary voiding, and then we have to measure it in milliliters. But currently, the ultrasonography becomes a basic screening tool in urinary tract system’s surveillance which is also can measure RU when the patients complete their urination. After routinely done the ultrasound workup, the patient will be encouraging to pass urine in restroom then repeat the ultrasound again for measuring RU virtually. The radiologist will report RU in milliliters same as conventional investigation.

Now a day, residual urine measurement is done by ultrasonography which is a lot more convenience.

Anyway, the cons of ultrasonographic RU measurement is inaccuracy. Inaccurate possibility RU measurements from ultrasonography are being shown below.

Overestimate calculating RU from not instantly measure RU once finish urinating – The ideal is RU measurement should be done within 10 minutes after urinating and as we all know that the urine production is continuously produced all the time.
Limitation from ultrasonography itself – Ultrasonography is an operator-dependent, the mathematic calculator must be filled in the program by Radiologist’s free-hand measurement.

Well, let’s discuss on the normal value of Residual urine.

Residual urine in Adult

RU < 50 mL consider as adequate bladder emptying
RU > 200 mL consider as inadequate bladder emptying

Residual urine in Children

RU > 20 mL consider as inadequate bladder emptying

Normal residual urine in children should less than 20 mL.

Residual urine in Elderly

RU 50 – 100 mL consider as normal bladder emptying

Less than 100 mL Residual urine in elderly is considered as normal.

Once High RU is reported, various kind of causes must be assessed – Neurogenic, Mechanical, Medication-related, infectious or inflammatory. Then the treatment will be tailor-made among individuals. There was a reported that RU > 100 mL has a risk of urinary tract infection.

RU is a valuable additional clinical clue in making an abnormal urination diagnosis. If you have question, you can ask or discuss with your trusted urologist, and I’m glad and honor if you text me for this kind of stuff. Take care!

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Medically written & reviewed by: Dr. Soarawee Weerasopone (Dr. Pom) — Board-Certified Urologist, Bangkok Hospital Headquarters. International Fellow: Baylor College of Medicine (USA) · Juntendo University (Japan) · Chang Gung Memorial Hospital (Taiwan).

Dr. Soarawee Weerasopone

Dr. Soarawee Weerasopone (Dr. Pom) is a board-certified urologist at Bangkok Hospital Headquarters, specializing in Men’s Health, Robotic Surgery (Da Vinci System), and Kidney Stone treatment. He has completed international fellowships at Baylor College of Medicine (USA), Juntendo University Hospital (Japan), and Chang Gung Memorial Hospital (Taiwan). All medical content on this site is written and reviewed by Dr. Soarawee based on his clinical experience and international training.