In this CKS topic, the following definitions apply:

• Bacteriuria: bacteria are present in the urine.
• Significant bacteriuria: the count of bacteria in the urine exceeds the threshold for diagnosing the presence of infection in the urine. The threshold depends on the age and sex of the person, and on the risk of contamination during collection of the urine sample. See Interpreting urine microscopy and culture results.
• Asymptomatic bacteriuria: there is significant bacteriuria without symptoms and signs of infection.
• Urinary tract infection (UTI): there is significant bacteriuria and characteristic symptoms and signs — see Symptoms and signs of cystitis.
• Lower UTI means infection of the bladder, the main concern of this CKS topic. The term lower UTI is used only with reference to UTI in women with an indwelling catheter, because in this situation the symptoms and signs of UTI are seldom typical of cystitis.
• Cystitis is used as a synonym for lower UTI (although technically it means inflammation of the bladder and there are rare non-infectious causes of cystitis, such as radiation and chemicals).
• Upper UTI includes pyelitis (infection of the proximal part of the ureters) and pyelonephritis (infection of the kidneys and the proximal part of the ureters).
• Recurrent UTI: repeated UTI, which may be due to relapse or reinfection.
• Relapse is recurrent UTI with the same strain of organism. Relapse is the likely cause if infection recurs within a short period (for example within 2 weeks) after treatment.
• Reinfection is recurrent UTI with a different strain or species of organism. Reinfection is the likely cause if UTI recurs more than 2 weeks after treatment.
• The number of recurrences that is regarded as clinically significant depends on the risks of infection and the impact of cystitis on the woman.
• Uncomplicated UTI: infection of the urinary tract by a usual pathogen (see Causes) in a person with a normal urinary tract and with normal kidney function.
• Complicated UTI: UTI when one or more factors are present that predispose the person to persistent infection, recurrent infection, or treatment failure. Examples include UTI with:
• Abnormal urinary tract (for example calculus, vesicoureteric reflux, reflux nephropathy, neurogenic bladder, indwelling catheter, urinary obstruction, recent instrumentation).
• Virulent organism (such as Staphylococcus aureus).
• Impaired host defences (for example poorly controlled diabetes mellitus, immunosuppressive treatment).
• Impaired renal function.
• Urethral syndrome or painful bladder syndrome: symptoms of cystitis in the absence of urinary tract infection. This syndrome is also called interstitial cystitis, bladder pain syndrome, and trigonitis.