Flow rate testing is a simple, non-invasive test, which can provide useful clinical information, although with important limitations. It is an assessment of the volume passed in unit time and is often undertaken in conjunction with other measurements, most notably post-void residual urine volume (PVR) measurement.

Methods of flow rate assessment

The two most common flow rate measurement systems are:

1. □ Gravimetric: the weight of urine voided is measured over time; the flow rate is calculated from the rate of change in weight of urine.

2. □ Rotating disc: as the urinary stream falls on to a spinning disc, it increases the weight of the disc, so the motor has to increase power to keep the disc spinning. The flow rate is proportionate to the power needed to keep the disc spinning at the same rate.

Both approaches are widely employed in current commercial systems. They are prone to variations in reliability and need to be calibrated before use. Regular checks are needed at intervals specified by the manufacturer and should also be done if the equipment is moved or disturbed.

Preparation for the test

Patients should have completed a pre-test frequency volume chart, which will show the typical and maximum voided volume. The set-up for uroflowmetry has been described in chapter 2. The flow rate machine should be in a private area. The patient should be well hydrated and prepared to wait for as long as needed to obtain an adequate result.