# Residual urine

As residual urine is called the amount of urine ( urine ), according to a normal micturition in the bladder remains. The cause of residual urine build-up are usually obstructions to drainage, e.g. B. by an enlarged prostate or a neurological disorder of urination . Residual urine quantities that are more than 15% of the actual bladder capacity are considered pathological .

## Residual urine measurement

The residual urine can be determined using either ultrasound or a catheter . The sonographic procedure is more comfortable for the patient , since it does not require a catheter to be inserted into the urethra .

### Sonographic

#### Transabdominal

The residual urine is usually measured sonographically today. To do this, the patient is allowed to go to the toilet to empty the bladder . Then the size of the bladder and any residual urine it may contain is estimated. For the sonographic determination of the residual urine, the formula bladder volume in ml = H × W × D × 0.7 is used for transabdominal ultrasound (H = horizontal, W = width, D = depth; in cm):

The factor 0.7 is necessary because the bladder only appears circular when it is filled. The measurement accuracy decreases with volumes below 50 ml, the error rate here is around 21%. Recently, portable ultrasound devices whose software can automatically calculate the volume of the bladder have become increasingly popular. The accuracy of the devices could be improved to around 15% with a measuring range of 0–999 ml.

#### Transvaginal

The transvaginal ultrasound method also enables residual urine to be determined. The bubble is adjusted in the sagittal plane and the maximum diameter (H = horizontal, D = depth) is used to calculate the bubble volume V (in ml) using the formula V = 5.9 × H × D - 14.6 ' to determine.

### Using a catheter

Much more accurate results are obtained if, after the bladder has been emptied, the residual urine is determined using a catheter that is inserted transurethrally (through the urethra ). Since this procedure is much more uncomfortable for the patient, it is only carried out if an exact determination of the residual urine is necessary. At the same time, this method can also be used to use the removed urine for a urine culture in order to be able to detect any bacteria.

Immediately after emptying the bladder, a disposable catheter is inserted through the urethra while the patient is lying down . The end of the catheter is either in a collecting tray or a small sterile urine bag is attached (before insertion). The patient then stands up and the catheter is then slowly withdrawn so that the urine can also be drained from the deepest point of the bladder (urethral orifice). When determining the amount of residual urine, the urine in the catheter after its removal must also be taken into account.