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Urinary tract infections

Educational resources for renal medicine

Licensed under a Creative Commons LicenseCreative Commons Attribution 4.0 International License.

A slightly more advanced learning resource for students and others on UTI, from EdREN, at the Renal Unit of the Royal Infirmary of Edinburgh. For basic info read the Edren textbook info on UTI.

Urinary tract infection (UTI) is extremely common. It is one of the most common reasons for consultation with general practitioners, and it is in primary care that the great majority of UTIs are dealt with. UTIs are common in hospital but are often a secondary problem. It is estimated that 2% of children have urinary tract infections, but that over a lifetime, 30% of women experience a symptomatic UTI.

There are many sources of information about both simple and complicated urinary infections, and UTI is discussed early in the Edinburgh curriculum as an example of infectious disease while learning basic microbiology. So here we draw attention to some salient facts.

Definitions and concepts 

Identifying 'real' urinary tract infection

Urine easily becomes contaminated as it is being collected, and microorganisms will grow and multiply in contaminated urine while it is being transported to the laboratory. It can be difficult to distinguish between contamination and real infection and frequently you need to assess the probability of significance. For example, if the patient has symptoms compatible with UTI, it is likely that any urinary abnormalities will be significant.

There are a number of ways of reducing the likelihood of contamination:

  • Good collection technique Further information in links from patient info page on cystitis.
  • Direct collection of urine by puncture of bladder or collecting system (rare, but you would expect such samples to be completely sterile).
  • Adding sodium borate to the container used to store urine to retard growth in transit ......
  • Rapid transport of samples to the laboratory, and storing at 4 degrees centigrade if there is likely to be delay.

And a number of indicators to help decide whether a positive culture is likely to reflect contamination or genuine infection:

  • Symptoms suggestive of UTI markedly increases the probability that it is a real infection.
  • The presence of white blood cells suggests infection
  • The presence of many squamous epithelial cells suggests contamination
  • A growth of a single organism suggests real infection, multiple organisms more likely to be contaminants
  • High numbers of bacteria are more likely to reflect real infection. Kass observed that more than 100 000 organisms per ml in asymptomatic individuals is more likely to reflect real (ie reproducible and prognostically significant) infection than lesser numbers. Note that this applies to normal individuals and not to those with symptoms, or with abnormal urinary tracts.

Predisposing factors to UTI

  • An anatomically abnormal urinary tract - almost any abnormality
  • Vesicoureteric reflux
  • Pregnancy (see below)
  • Residual urine in the bladder after micturition - bladder outflow obstruction caused by prostatic disease, neurological lesion, urethral valves, or other cause
  • Urine that favours bacterial growth - diabetes mellitus
  • The first year or so of life (males and females)
  • Female sex
  • Sexual intercourse in women
  • Post-menopausal atrophic vaginitis
  • Instrumentation of the urinary tract
  • Indwelling catheter or other foreign body within the urinary tract (eg ureteric stent)

Clinical presentations

Regular presentations (cystitis (further information from EdRenINFO), pyelonephritis, urethral syndrome, etc) are thoroughly dealt with in medical textbooks (eg Davidson's) so are not dealt with further here. However the subjects below are complex and controversial so they are discussed briefly.

Reflux nephropathy and chronic pyelonephritis

Reflux nephropathy is the preferred name for the chronic interstitial nephritis that occurs in association with vesicoureteric reflux (VUR). Vesicoureteric reflux in this condition is present in infancy and probably usually even in utero. The name implies reflux of urine into the ureters during micturition; severity is graded according to how far the reflux extends. Grade I reflux is minimal; in grade IV it passes into the kidney and is associated with dilated and clubbed renal calyces.


Vesico-ureteric reflux is closely associated with recurrent UTI in childhood, and was previously assumed that this relationship was responsible for the assocation of VUR with progressive renal damage. However modern imaging techniques have shown that the renal scars can be first seen in utero, in the absence of infection. Furthermore, epidemiological surveys and controlled trials of surgical intervention to correct reflux have found that efforts to reduce progression to renal failure by surgical or other means have not been effective.

Susceptibility to VUR has a genetic component, and it may be associated with congenital renal dysplasia and with other abnormalities of the urinary tract. It usually occurs from an apparently normal bladder, but it may be associated with outflow obstruction, usually caused by urethral valves. Regardless of other lesions it is associated with a susceptibility to UTI.

The association of VUR with renal failure may therefore at one extreme be incidental. However it has not been rigorously proven that infection does not play some part in the pathogenesis.

Reflux diminishes as the child grows, and usually disappears. It is rarely demonstrable in an adult with a scarred kidney.

Presentation: reflux produces few symptoms itself. Presentation with high blood pressure, minor proteinuria, or coincidentally discovered renal impairment is common. In women these are often picked up during pregnancy, which may be complicated by exacerbated proteinuria and hypertension. Symptoms from continuing UTIs may be present.

Management is probably becoming less aggressive than previously, unless there is any evidence of urinary tract obstruction. Ultrasound and radionuclide imaging are useful here.

Prognosis is largely determined by the same factors that are associated with poor outcomes in renal impairment of other causes:

  • Amount of renal damage
  • Proteinuria
  • Hypertension

Risks of future trouble are slight if scarring is unilateral or minor, renal function is normal, and there is neither proteinuria nor hypertension.

Further information (Reflux nephropathy at EdRenINFO)

Renal failure and UTI

Pyelonephritis may lead to renal failure in some circumstances. Acute severe pyelonephritis which is bilateral or affecting a single kidney is associated with acute renal impairment, and function may never return to normal after treatment in extreme cases. However this is extremely uncommon in the absence of urinary tract obstruction or other gross abnormality (eg renal transplantation). This may have been more common in the pre-antibiotic era however.

New renal scars hardly ever appear in children over the age of 5 years, so even in reflux nephropathy, a direct role for infection is quite difficult to support.

An alternative hypothesis for the progression of renal disease after the insults of reflux and infection in childhood has been persistence of atypical organisms in the kidney. However there is little evidence that this occurs.

These are good reasons to explain why the term 'chronic pyelonephritis' has fallen into disuse.


It would be useful to have some Edinburgh-specific statistics here, wouldn't it. For GP samples versus hospital. 


This is dealt with effectively in standard texts so not discussed here. See also the BNF and local formularies for recommended antibiotic regimens, that should take into account local patterns of isolates and sensitivities.

Urinary tract infection in pregnancy

Author: Dr F. Johnston

Physiological changes in the kidneys and urinary tract in pregnancy

There is dilatation of the urinary collecting systems. This is partly due to ureteral smooth muscle relaxation due to high levels of progesterone, but is mainly due to the pressure of the enlarging pregnant uterus. Thus the dilatation is maximal after 16 weeks (when the uterus is of sufficient size to cause this effect) and is greater in the right kidney (because the left ureter is partially protected by descending colon). Because of this tendency to urinary stasis, pyelonephritis is more common in pregnancy, usually after 16 weeks and on the right side more than left.

Renal plasma flow increases in very early pregnancy, and with it glomerular filtration rate also increases. These changes result in an increase of about 50% in creatinine clearance, and a fall in serum urea, creatinine and urate. It is important to be aware of these changes, because levels within the non-pregnant normal range may be abnormal in pregnancy.

Increased urinary frequency is usual. In early pregnancy this is attributed to increased vascularity around the bladder, in later pregnancy to pressure on the bladder by the large uterus. Stress incontinence is quite common in late pregnancy, again as a pressure effect. These symptoms sometimes complicate assessment of possible urinary tract problems and also amniotic fluid leakage.

UTI occurs in about 6% of pregnant women, though the prevalence varies considerably in different populations. Bacteriuria tends to be persistent if untreated, and conversely, women who are bacteruria negative are very unlikely to acquire asymptomatic bacteruria during pregnancy. Women who are bacteruria positive have around a 30% chance of developing symptomatic urinary tract infection during pregnancy. Because this may be pyelonephritis, women with bacteruria should be treated and then retested. The most common organism is E.coli. Ampicillin, amoxycillin and co-amoxiclav are all safe in pregnancy. Cephalosporins are safe but are not very well absorbed orally.

Acute pyelonephritis in pregnancy

This can be a very serious infection in pregnancy, causing the woman to be extremely unwell and predisposing to premature labour.

Lower urinary symptoms may be absent, but high pyrexia, rigors, cloudy urine and marked renal angle tenderness are characteristic.

Management includes hospital admission, MSU Gram stain and culture, blood cultures, and urea and electrolytes. Ultrasound examination of the kidneys is sometimes appropriate. Treatment, which should not wait for investigations, is with high oral/intravenous fluid intake, and full-dose intravenous antibiotics. Cephalosporins or penicillins are first choices. Antibiotic treatment needs to be continued for at least two weeks, and care should be taken with step down to oral drugs, particularly if cephalosporins are used.

After an episode of acute pyelonephritis a mid stream urine should always be cultured to ensure that bacteriuria is not continuing. Recurrence of pyelonephritis is a risk factor for premature labour. Recurrence of pyelonephritis is usually an indication for prophylactic antibiotics for the remainder of pregnancy. It may also mean that there is an underlying renal abnormality, and merits ultrasound examination of the kidneys during pregnancy, and specialist referral for investigation after pregnancy.

Further information

There is further information about UTIs all over the place, but from a student's point of view, the following are useful:

Urinalysis notes and links from EdRENEdRenINFO (intended for all readers) there are pages on:

 Some excellent NHS Clinical Knowledge Summaries on UTI listed at the OpenMed Renal curriculum

Urinary tract infection in adults from the NIDDK, USAOthers? ....... please suggest by emailing

Page last modified 17.05.2016, 09:59 by Emma Farrell. edrep and edren are produced by the Renal Unit at the Royal Infirmary of Edinburgh and Univ. Edinburgh. CAUTIONS and Contact us. Note that the information published here is primarily intended for education, not for clinical care.