Uroflowmetry
Uroflowmetry records the rate at which urine leaves the body during a single void, and it plots the flow against time. It is a simple, non-invasive screening test for voiding problems, and the first test used for men with lower urinary tract symptoms, for children with wetting or voiding dysfunction, and for patients after surgery on the outlet.
The patient voids into a funnel connected to a flow meter. The flow meter works by weighing the urine, by spinning disc, or by capacitance, and it gives the flow rate in mL per second. The main results are the peak flow rate (Qmax), the average flow rate (Qave), the voided volume, the time to peak flow, the flow time and the shape of the curve. The test takes about 1 minute and should be done after a normal desire to void, with a voided volume of at least 150 mL.
A flow test is a measure of the result of the interaction of the detrusor and the outlet. A low flow can be caused by outlet obstruction or by a weak detrusor, and uroflowmetry alone cannot tell these apart. Used with a post-void residual, it identifies about 70 to 80% of men with obstruction when the peak flow is under 10 mL per second, and about 40% when it is 10 to 15 mL per second. Combined with symptom scores, it helps to decide who needs further tests.
Indications.
- Men with lower urinary tract symptoms (hesitancy, weak stream, intermittency, straining): before and after treatment.
- Suspected urethral stricture, bladder neck obstruction, or obstruction by an enlarged prostate.
- Women with voiding difficulty, after surgery for incontinence or prolapse, or with suspected urethral obstruction.
- Children with daytime wetting, recurrent infection, or suspected voiding dysfunction, and boys after surgery for hypospadias or posterior urethral valves.
- Neurological disease with bladder involvement.
- Monitoring after urethral dilatation, prostate surgery or stricture repair.
Not indicated alone for diagnosing the cause of the symptoms or for planning surgery in cases where urodynamic data are needed.
Contraindications and precautions.
- No absolute contraindications.
- Voided volume under 150 mL gives unreliable results, and the test is repeated with a fuller bladder.
- Patients who are unable to void in front of the device or in a strange place: privacy and a familiar position help.
- Active urinary infection, in which the flow pattern may be abnormal because of pain, which is noted on the report.
Preparation and technique.
The patient arrives with a comfortably full bladder and is asked not to void for 2 to 3 hours beforehand, but not to hold urine until they are in discomfort. Fluid intake of about 500 mL to 1 litre in the hour before the test is usual. The patient voids in private in the usual position (standing for men, seated for women), without straining and without touching the funnel.
A post-void residual is measured by ultrasound straight after the void. At least two voids give a better view when the first result is abnormal, since a single void is variable, and a diary of home flow tests helps where available.
