Residual urine determination by ultrasound (synonym: ultrasound-assisted residual urine determination; sonographic residual urine determination) is a diagnostic procedure in urology that can be used when urinary retention (urinary retention) in the bladder is suspected. In addition to its use as a routine measure in cases of suspected urinary retention, the diagnostic procedure plays a crucial role in the assessment of postoperative urinary dysfunction, which can occur, for example, during surgical procedures on the rectum. With the aid of sonographic imaging, residual urinary symptoms can be demonstrated with relative precision. During the examination, a volume well below 100 ml of urine is considered the threshold value. However, in the case of a positive finding, before additional measures are taken, other factors such as age must be included in the further procedure. Older patients may have a higher residual urine volume, which may, however, be tolerated under certain circumstances. As a guideline for older patients, a residual urine volume of up to 20% of the maximum bladder capacity is still within the acceptable range. Nevertheless, even in elderly patients, it must be kept in mind that an increased residual urine volume (>300 ml) in combination with other diseases, such as hypertension (high blood pressure) or diabetes mellitus, represents a significantly increased risk of urinary tract infection.
Indications (areas of application)
- Routine diagnostics – the use of sonographic residual urine determination should be performed at regular intervals before senior age, because the presence of residual urine is associated with impaired health.
- Benign prostatic hyperplasia (BPH) – the presence of benign prostatic hyperplasia (benign prostatic enlargement) often leads to urinary retention, because the transition area from the bladder to the urethra is massively narrowed.
- Diabetes mellitus – in addition to narrowing in the genitourinary tract, diabetes mellitus is also an indication, because diabetes can cause nerve damage, which can lead to a reduced ability to empty the urinary bladder.
The procedure
With the help of sonographic residual urine determination, which is part of routine diagnostics, approximately 90% of all elderly patients can be classified to the point where a primary conservative therapy attempt (without surgical intervention) can be started. Special invasive examinations beyond routine diagnostics are generally not necessary or indicated, since the risk of complications can be classified as much higher than with residual urine determination by ultrasound.
- Based on the fact that sonographic residual urine determination is not associated with any complications and can be performed in almost every patient, the procedure is the method of choice for noninvasive residual urine determination.
- However, it must be noted in sonographic residual urine determination that the accuracy of the measurement is variable. Depending on the filling volume of the urinary bladder, but also on the device used and the formula used to calculate the volume, the assessment of the residual urine volume varies. This is of particular importance when making comparative measurement, for example, in patients with a chronic disease.
- As a result of the available variables, comparative measurements should be performed if possible by a physician with a device. If discrepancies become apparent between the sonographically determined residual urine volume and the clinical symptoms, an additional determination of residual urine volume should be determined and compared using single-use catheterization.
- However, for routine examination, sonographic determination of residual urine is nearly optimal. As a consequence of the different variables, the determination of absolute residual urine thresholds has by no means excelled in clinical practice. Therapeutic orientation to only one measured value is also not to be regarded as expedient in the use of the procedure, since fluctuations in the measurement results can be detected even in a patient depending on the time of day. As a rule, a lower residual urine volume can be determined in the morning than in the evening. For this reason, relative residual urine values in relation to bladder capacity should be considered more useful.In addition, repeated measurements at different times of the day should be included for assessments of residual urine volume.
Methods of sonographic residual urine determination
Transvaginal sonography
- The use of transvaginal ultrasonography allows precise residual urine determination. For residual urine determination, the bladder is viewed in the sagittal plane (arrow-like – when looking vertically at the sagittal plane, a lateral view of the body is seen).
- To determine the volume of the bubble, the formula “bubble volume in ml = 5.9 × H × D – 14.6” is used. “H” and “D” in the formula describe the maximum diameter in the horizontal and depth.
Transabdominal sonography
- Transabdominal sonography is usually the more commonly used method for residual urine determination. Before residual urine determination, the patient is asked to go to the toilet and, if possible, to perform a complete emptying of the urinary bladder.
- After successful micturition, it is now possible to visualize and further quantify the size of the urinary bladder and at the same time the residual urine, if any. However, a milliliter accurate sonographic determination of residual urine is difficult to realize.
- For calculation, the formula “bladder volume in ml = H × W × D × 0.7” is used in ultrasound. In this formula, “H” describes the maximum diameter in the horizontal, “W” that of the maximum width and “D” the maximum diameter in depth.
- Of crucial importance for the interpretation of the results is the fact that the measurement accuracy is significantly reduced for measurement volumes below 50 ml, so that higher error rates may be found.
Note: The correlation between residual urine volume and bladder outlet obstruction (BOO; obstruction of urine flow at the transition from the bladder to the urethra) is only slight. Most often, residual urine is caused by detrusor underactivity (underactivity of the bladder muscle that controls emptying of the bladder).
