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Urinary tract infection 

Urinary tract infection
Urinary tract infection

Charles Tomson

and Alison Armitage

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date: 08 December 2019

Urinary tract infection (UTI) is a common condition, accounting for 1 to 3% of all primary care consultations in the United Kingdom. It affects patients of both sexes and all ages. The commonest organism causing uncomplicated community-acquired bacterial UTI is Escherichia coli.

Aetiology and pathogenesis

The occurrence and course of a UTI is influenced by the integrity of the host defence and by bacterial virulence factors. Disruption of the highly specialized transitional cell epithelium which lines the urinary tract, incomplete bladder emptying, anatomical abnormalities, and the presence of a foreign body, such as a urinary catheter, can all contribute to disruption of the host defence and increase the likelihood of infection. Sexual intercourse, use of condoms, and use of spermicides all increase the risk, and genetic factors influence the susceptibility of some people, e.g. girls with the P1 blood group are at increased risk of acute pyelonephritis. Bacterial characteristics that determine their ability to cause infection include specific mechanisms to adhere to the uroepithelium (‘pili’ or ‘fimbrias’ in the case of certain E. coli), or adaptations allowing them to colonize foreign surfaces, such as a urinary catheter (proteus), and subsequently cause infection.

Clinical features and diagnosis

Presentation—UTI can present in a number of ways, but most commonly with some combination of dysuria, urgency, frequency, polyuria, suprapubic tenderness, and haematuria. Asymptomatic infection is common, especially in older people, but it is not justified to send a urine sample from an asymptomatic patient for culture, with the notable exceptions of pregnant women and prior to invasive urological surgery when treatment is mandatory.

Diagnosis—acute uncomplicated UTI can often be diagnosed on symptoms alone and submission of a sample for microbiological testing is unnecessary (exceptions to this rule include pregnancy and those patients with abnormal host defences). Current United Kingdom and European guidelines on the level of bacterial counts required to diagnose ‘significant’ infection are variable and should not be used as the sole determinant of whether antibiotic treatment should be initiated.

Differential diagnoses (‘culture negative syndromes’)—these include (1) chlamydial infection, which must be identified and treated to avoid long-term complications such as infertility; also (2) urethral syndrome and (3) painful bladder syndrome (interstitial cystitis), which significantly affect a patient’s quality of life and for which treatment is often unsuccessful.

Investigation—beyond microbiological testing, further investigation of women with uncomplicated UTI is seldom justified. In men, and those women with features indicating complicated infection, investigation for an underlying cause should be considered: diabetes must be excluded, and anatomical or functional abnormality of the urinary tract sought, as appropriate, by imaging, cystoscopy, and urinary flow studies.


Antibiotics—trimethoprim remains the first choice for community-acquired UTI in most areas. Complicated UTI (see below) is caused by a wider spectrum of organisms, and recommendations for treatment differ. Guidelines on specific antibiotic treatment and duration of treatment are available, but with increasing antibiotic resistance (including of E. coli to Trimethoprim), local microbiological advice should be taken into account when choosing antibiotic treatment.

Prevention of recurrent uncomplicated UTI—many clinicians advise patients with such recurrence to take measures to improve perineal hygiene, to empty the bladder after sexual intercourse, to maintain a high fluid intake, and (if vesicoureteric reflux is suspected) to practise double voiding, but the evidence that these measures are effective is weak. Long-term antibiotic prophylaxis reduces the rate of recurrent UTI, but at the risk of adverse effects. Nightly, thrice weekly, and postcoital prophylaxis have all been shown to be of benefit, but there is no evidence to support the use of rotating antibiotic prophylaxis regimen. Cranberry products and methenamine hippurate are effective in some patients. Oestrogens are not recommended for the routine prevention of recurrent infection in postmenopausal women, but may be of benefit in those with marked atrophic vaginitis. Vaccines against uropathogenic bacteria can prevent recurrent UTI, but these are not widely used in routine clinical practice in the United Kingdom.

Complicated urinary tract infections

Complicated UTIs are those occurring in a patient with abnormal host defence. It is uncommon for any man with an anatomically normal urinary tract to suffer a UTI. An important differential diagnosis in men is prostatitis—an umbrella term used to describe a disparate group of conditions, the treatment of which is often unsatisfactory.

Urethral catheterization—UTI occurs after 2% of in/out urethral catheterizations and after 10 to 30% of 5-day indwelling catheterization, and is nearly inevitable in patients with long-term indwelling catheters. This is an important cause of hospital-acquired infection, significantly increasing the risk of Gram-negative septicaemia and mortality. However, management of patients unable to empty their bladder fully for reasons such as prostatic outflow obstruction or neurogenic bladder dysfunction because of spinal cord injury is often difficult without medium- or long-term urinary catheterization. Use of prophylactic antibiotics to cover short-term catheter insertion may be justified, but this is not the case in patients with long-term catheters, although regular bladder washouts and methenamine may be of some benefit. Treatment of asymptomatic bacteriuria in patients with anatomically abnormal urinary tracts or with indwelling urinary catheters is unjustified and likely only to lead to the emergence of antibiotic-resistant urinary infection. Clean, intermittent self-catheterization should be considered as an alternative where possible.

Urinary tract stones—these are an important cause of recurrent and relapsing UTIs that are difficult or impossible to treat with antibiotic therapy alone, repeated courses of which often encourage the development of resistant organisms. Removal of stones is often difficult and requires repeated interventions. Identification of the stone type and prevention of formation of further stones is an important part of any treatment plan (see Chapter 21.14).

Anatomically abnormal kidneys—inherited renal abnormalities such as polycystic kidneys are often complicated by UTI, which can be difficult to treat if the infection involves a cyst that may be difficult to identify and can be sheltered from antibiotic penetration. Renal transplant recipients are at an increased risk of UTI due to a variety of factors, including the anatomy of the transplant kidney, postoperative catheterization, and immunosuppressive medication. Unusual viral organisms such as polyoma (BK) virus may cause infection in this group of patients.

Vesicoureteric reflux—the normal bladder prevents reflux of urine into the ureters during micturition. Congenital abnormalities of the vesicoureteric junction can allow this to occur, as can acquired abnormalities such as bladder outflow obstruction, which disrupts normal host defence against ascending infection and thus makes children (particularly girls) more prone to ascending UTI. Cortical defects (‘scars’) in the upper and lower poles of the kidneys are frequently found in such children. These may be caused by ascending infection causing acute pyelonephritis, but similar appearances can occur in the absence of UTI and are likely due to renal dysplasia, inherited along with abnormal insertion of the ureters into the bladder. Progressive kidney failure may occur in such patients, but it is not clear whether this is due to the late effects of renal dysplasia and congenital reduction in renal mass, or to the effects of scarring caused by ascending infection. Clinical trials comparing long-term prophylactic antibiotics for the first 5 years of life vs surgical ureteric reimplantation have shown similar incidence of symptomatic UTI in both treatment groups; whether either treatment reduces the risk of progressive kidney failure remains uncertain.

Pregnancy—there is a significantly increased risk of acute pyelonephritis in pregnant women with untreated bacteriuria, many of whom will be asymptomatic. Late pyelonephritis is associated with an increased incidence of preterm delivery and low birth weight, hence the need in pregnancy to screen for and treat UTI promptly with antibiotics.

Ascending UTI is rarely complicated by unusual conditions such as acute papillary necrosis or perinephric abscess. These can lead to destruction of renal parenchymal tissue and long-standing renal impairment, usually in the context of abnormal host defence such as diabetes or urinary tract obstruction. Malakoplakia is an extremely rare complication of bacterial UTI, characterized by destructive tumour-like granulomatous infiltrates in the urinary bladder, kidneys, and (occasionally) other organs. Other causes of UTI may need to be considered depending on the patient’s ethnic background and medical and travel history, e.g. fungal infections, tuberculosis, and schistosomiasis.

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