Uroflowmetry

In clinical practice Uroflowmetry (simple plotting of flow rate with time) has been found to offer reasonable sensitivity and specificity if carried out correctly.
Much of this work has been pioneered by Rollema ( Rollema HJ .PhD thesis University of Groningen 1981); he found that best results were gained from analysing an indirect time parameter related to how long the flow took to fall from its maximum to the point where 90% had been voided. However most flowmeters can not analyse the data in this way - apart from the commercial one with whom Rollema has links -but print certain standard variables. Among these is the maximum flow rate in ml per sec Q
max .

2 normal flow rates. The shape of the curve is unimodal i.e. monotonic increase, stable period,monotonic decrease. There are no artefacts from abdominal straining or "waggling" of the stream. Qmax is 19 in the first and 23 in the second. Since Qmax is found to depend on volume this is not surprising.

Some poor flows from an obstructed patient are shown below.

2 obstructed flow rates. There are no artefacts from abdominal straining or "waggling" of the stream. Qmax is 7 in the first and 6 in the second.

Although the above are very poor flows, there is a significant probability that they are a result of a poorly contracting bladder rather than and obstructed one.An example of such a cse can be seen in the pressure flow section.

Rollema found that Qmax could be used to give sensitivity and specificity of 90 %. These figures are definitely an upper limit - other investigators have not claimed such high values, particularly for sensitivity. Still, Uroflowmetry is an easy non invasive test, but of course care must be taken to use it in the correct patient group.

Like many clinical tests the value of Uroflowmetry depends very much on its positive predictive value.

The positive predictive value of a test is the percentage of patients with a positive test who are correctly diagnosed.

The negative predictive value of a test is the percentage of patients with a negative test who are correctly diagnosed.
.

The table shows the PPV of uroflowmetry using Rollema's best estimate for 3 prevalences

Prevalence PPV
50 % 90%
25% 70%
10% 50%

Clearly then Uroflowmtery can not be used to screen a low prevalance population.

In the case of benign prostatic hyperplasia (BPH), the prevalence of clinical disease ranges from about 10 % at age 50 to 35% at age 70. It is therefore important to select the correct patients for uroflowmetry. Symptoms, history, and other tests e.g. digital examination will aid the selection.

Flow Instrumentation

An array of techniques has been used to measure urine flow rate. Many depend on the recording of the volume of urine passed with the derivative being flow rate. For example the gravimetric method - passing the urine into a container which is continuously weighed. Most of these techniques suffer from the amount of processing that is required to eliminate artefacts from vibration or movement. Worth mention is the drop spectrometer, where it was noted from high speed photography that shortly after leaving the external meatus the urinary stream is broken up into drops. By an ingenious set of light beams and photomultipliers it is possible to measure flow rate with a very short time constant.

In this department the flowmeter favoured at present is the rotating disk mictiometer made by Dantec. Here the urine is directed onto a spinning disk whose rotational velocity is kept constant by a tachometer and feedback circuit. As fluid hits the disk, more electrical energy is required to maintain the constant angular velocity and measurement of the extra current can be used to give flow rate. Patients have to be carefully warned not to waggle the stream when voiding as this can produce artefacts. Imagine a man moves the stream from high up the funnel to low down and back up again. A bolus will arrive at the instrumentation and then there will be a gap.

The definitive way to diagnose bladder outlet obstruction is by a pressure flow study.