The urinary bladder is a hollow, muscular, expandable organ in the pelvis of adults, and in the lower abdomen of children younger than 6 years old. The bladder stores urine, so that urination can be infrequent and voluntary. When the bladder muscles contract, 2 sphincters (valves) open and the urine flows out of the body through the urethra. Bladder function is controlled by the interaction between the central nervous system and the organs of the lower urinary tract (bladder, urethra and pelvic floor muscles). If this control is damaged bladder function can be affected. Common bladder function problems include urinary incontinence (UI) and urinary retention (UR).

UI is when urine passes from the body without the person having control over it. Prevalence in the UK general population is estimated to range from 14% to 69% in women aged 15 years or older (Hunskaar et al. 2005). The Leicestershire MRC Incontinence Study found that 34.2% of women reported having occasional UI (see NICE's guideline on the management of urinary incontinence in women). The prevalence of UI in men aged 18 to 64 years is 3%, and increases to 7% to 10% in men older than 65 years (Royal College of Physicians 1995). Nocturnal enuresis (involuntary urination during sleep) is the most common form of UI in children. In a review of the best available evidence, Buckley et al. (2010) reported a night‑time UI prevalence of 6.8% to 16.4% in 7‑year‑old children, and a daytime UI prevalence of 3.2% to 9.0% in the same age group. It is important to note that this review does not clearly distinguish pathophysiological causes of UI from late toilet training.

UR occurs when the bladder does not empty properly. There are 2 main causes of UR; blockage in the flow of urine through the urethra, and weak bladder muscles. It is 10 times more common in men than in women, and is most prevalent in men older than 70, mainly because the prostate gland increases in size with age (Kuppusamy et al. 2011). In England, acute UR has an annual incidence of approximately 3/1000 men (Cathcart et al. 2006). In women, the incidence of UR can increase after childbirth (Mulder et al. 2012). Postpartum UR occurs in 10% to 15% of women (Chaurasia et al. 2013). UR often happens immediately after surgery. The combination of intravenous fluid therapy and anaesthesia may result in a full bladder with impaired nerve function, causing UR.

Investigations into bladder function problems typically include measuring the amount of urine left in the bladder after the person has urinated (known as bladder volume assessment). Bladder volume is usually assessed by urinary catheterisation after the person has completely emptied their bladder. Catheterisation is an invasive procedure that involves inserting a tube into the bladder either through the urethra or, less commonly, through a small opening in the lower abdomen, so that urine can drain out of the bladder. Catheterisation is used for diagnostic purposes to measure the post‑void residual (PVR) volume of urine in the bladder, and as a treatment to fully drain the bladder. Catheterisation can be intermittent or indwelling. Intermittent catheterisation is when the tube is left open‑ended and is removed immediately after all of the urine has been drained out of the bladder. Indwelling catheterisation is when the tube is attached to a drainage bag and left in place over a longer period of time, for example days or weeks.

Urinary catheterisation carries a risk of urethral trauma and urinary tract infection (UTI). UTIs account for an estimated 40% of all hospital‑acquired infections (NHS Quality Improvement Scotland 2010). About 80% of these UTIs are associated with catheterisation. These infections increase inpatient length of stay and cost of treatment and, in rare situations, may be life‑threatening.

Every year, over 1 million catheterisations are done in NHS hospitals (Nazarko 2010). Studies have shown that about 25% of these are unnecessary (Fakih et al. 2010; Rothfield et al. 2010). Catheterisations may be carried out unnecessarily because of a lack of guidelines on the indications for catheter placement (Bhatia et al. 2010).

Ultrasound scanners can be used to estimate the volume of urine in a bladder non‑invasively, and avoid unnecessary catheterisation. Extensive specialist training is needed for conventional ultrasound scanning and it therefore tends to be carried out by sonographers or doctors. An alternative, and increasingly common option, is portable bladder ultrasound scanning (PBUS). PBUS does not need to be done by people with extensive technical training because it automatically provides quantitative measurements of bladder volume.