In summary, testing needs clinical context to have clinical meaning. Accurate interpretation of urodynamics demands knowledge of the clinical situation and the circumstances of testing, otherwise one cannot be sure of the meaning of the results.

The tests themselves vary in complexity. Some are simple measurements of a urologic parameter. Others are more complex recreations or simulations of a specific clinical situation. The simpler tests are easier to perform and replicate. The more complex tests take more time, energy and resources but can yield a more complete answer. The price for this information however is the potential for more complex artifacts. In general, tests which are simple measurements are easy to do but provide less information. More complex simulations provide much more data and a more complete picture of the patient at the cost of greater complexity and difficulty in performance (see Fig. 2). Much of the art of using urodynamics comes from applying the appropriately level of complexity to solve the clinical question.

Simple urodynamic measurements include the uroflow and post-void residual. The former is nearly ubiquitous in all urology offices and measures how fast urine is voided from the bladder. As a screening test the uroflow is very helpful but it cannot easily differentiate between different pathophysiologies when it is abnormal. Is a poor uroflow result due to a weak detrusor or is there some outlet obstruction? Post void residual is typically obtained today using a small dedicated ultrasound device (BladderScan® [3]) although sterile catheterization is still used. Like the uroflow the post-void residual when normal is very helpful in that it suggest complete emptying is occurring. This conclusion can then be used to infer (assuming the absence of any other findings) that the bladder and urethra can empty effectively in an unimpeded normal fashion. Like the uroflow, when the post-void residual is abnormal it is unclear as to why. Is it due to a urethral stricture or prostatic enlargement? Could it be an areflexic hypoactive bladder? To further complicate interpretations, true ranges of normal values of post void residuals have not been completely validated. These simple parameter tests while useful, easy to perform and reliable cannot answer these more involved questions.

The next level of urodynamic testing encompasses the more advanced measurements which track two or more parameters. They are essentially small scale simulations. The cystometrogram (CMG) with or without electromyography, rectal or vaginal pressure monitoring, and urethral pressure tracking are examples. The CMG, often the unappreciated workhorse urodynamic test tracks two parameters—infused bladder volume and intravesical storage pressure. It is excellent at determining the relationship between these two parameters expressed as the bladder compliance (mathematically as ΔV/ΔP). It is however very poor in documenting the presence or absence of overactivity in the non-neurogenic patient. It is a paradox that the act of testing may confound the test. Superficially it may seem that the CMG would be ideal for this purpose since it seems to simulate the situation many patients with bladder overactivity (OAB) encounter— an unexpected sense of urgency during routine bladder filling. Yet, in practice the act of testing often yields a high false negative result, making it practically useless as a screening test for this purpose [4]. The solution to this paradox lies in the understanding that the act of testing does not in fact recreate a true simulation of the clinical circumstance. The typical overactive adult patient reports being caught off guard with a sudden feeling of urgency or even urge incontinence. During testing the patient is awake, catheterized and is now consciously focused on the bladder, its sensations and the pelvis in general. This is not the typical clinical scenario when the patient’s mind is busy with the daily routines of life; it is not focused on the bladder. The act of testing is creating an artificial situation. Rather than the more accurate depiction with a distracted patient, the test creates a misleading situation where the patient is completely focused on the bladder. During the test, this focus helps prevent the occurrence of the over activity. But this level of bladder focus is not possible practically for most patients who are trying to carry on with their lives. For this reason some researchers have advocated using continuous pressure monitoring with ambulatory urodynamics if testing is truly needed or simply treating symptomatically without using urodynamics if simple OAB is suspected (See chapter “Ambulatory Urodynamics”) [5, 6]. The highest level of urodynamic simulation combines several parameters and adds the patient’s cooperation to recreate the events and forces leading to the patient’s symptoms. Two common situations include the man who is having difficulty voiding and the woman with intermittent incontinence which may be related to stress. The best known of these are the pressure flow and stress incontinence tests which are done commonly with fluoroscopy. These tests track the infused bladder volume, intravesical storage and voiding pressures, coordination of the bladder neck and sphincter as well as urinary flow and leakage. The key factor is that at an appropriate point when the bladder is filled to the level that approximates the usual clinical situation, the patient is asked to volitionally void or bear down and try to recreate the leakage or obstruction. We can visualize and note at what volume and how much pressure (measured in cm H₂O) it takes to induce leakage or how much pressure it takes to drive urine across the obstructive prostatic hypertrophy. The study therefore not only should capture the event, but it also allows us to quantify the forces involved thereby revealing objective measurements which can be used to compare treatment results and efficacy.