Technique

• Should be performed when the patient's bladder is adequately filled and there is a normal desire to void

• Ideally, patients ought to have completed a FVC prior to flow tests

• Patients are asked to drink at least a liter of fluid at home prior to attending hospital. In addition, intra-test hydration is maintained by asking patients to drink a further liter of water during the test. Flow rates following a catheter fill are less physiological than free flow rates

• Due to the variability in FR traces, the patient is asked to perform three flow rates, each of a voided volume similar to those on the frequency volume chart

• Voiding should occur in privacy, with male patients standing up and females voiding sitting down on a commode

• In adults, a minimal voided volume of 150 mL is required to allow meaningful analysis of the FR trace, while a voided volume of >600 mL may result in erroneous results secondary to bladder overdistension and detrusor decompensation. In such cases the uroflowmetry must be repeated. Patients with repeated voids less than 150 mL are likely to have bladder outflow obstruction in about 70% of cases, if pressure-flow tests are performed

• Ultrasound estimation of bladder residual volume must be performed following micturition to measure residual urine volume

Most electronic uroflowmeters provide a paper tracer (paper speed 0.25 cm/s), which can be kept with the patient records for comparison at a later stage. Uroflowmeters usually have an accuracy of >95% and use a variety of principles in order to calculate flow rates, including—

1. Rotating disk method: urine falls onto a disk spinning at a constant speed. Urine slows down the speed of rotation. The power required by the motor to keep the disk spinning at a constant speed is proportional to the flow rate

2. Electronic dipstick (capacitance) method: urine collects in a chamber mounted with a dipstick capacitor. The change in capacitance due to urine in the chamber is indicative of the flow rate

3. Gravimetric method: this simply involves the calculation of urine flow rate by measuring the weight of voided urine

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