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Urine infections are caused when bugs grow in the urine and cause inflammation or irritation in the lining of the bladder, the tube coming out of the bladder (the urethra), and sometimes also in the kidneys. Urine Infections are also known as Urinary Tract Infection (UTI).


A urine infection usually causes pain and stinging when urine is passed, especially as someone is just finishing passing urine. The pain may be worst where the urine come out at the tip of the penis or the vulva. The bladder is inflamed and may be painful, and there may be a desire to pass urine at frequent intervals, even if the bladder is not full of urine. These symptoms are called ‘cystitis’, and pain on passing urine is often called ‘dysuria’ by doctors and nurses. Sometimes urine infections can also cause bleeding into the urine, called haematuria (Click here for more information about haematuria). You may also notice a change in smell, but not always.

In some cases, a urine infection may occur without these symptoms, and in anyone with a serious infection where the cause is not immediately obvious, a sample of urine should be tested for infection. The opposite is also the case, sometimes people can have low levels of bugs in the urine without any problems. This is commoner in the elderly or when someone has a catheter tube to drain the urine from the bladder.


Sometimes the infection just affects the lining of the bladder, and causes symptoms of cystitis, as described in the last section. These infections are usually easy to treat, and sometimes even clear without antibiotic treatment. Sometimes, the infection becomes more severe, causing fever, sweats, and pain in the small of the back, just below the ribs. Doctors call this ‘acute pyelonephritis’. These infections always require antibiotic treatment, and sometimes require hospital admission. A very severe infection may spread to the bloodstream, causing septicaemia (‘blood poisoning’). In this case there is high fever, with sweats and uncontrollable shivering attacks (called ‘rigors’). If left untreated, septicaemia can be life-threatening, so should be treated urgently.


The bugs that cause these infections are normally bacteria, and one of the commonest is called E. coli. This is a bug found inside our bowel, and is also on our skin. There is a type of E. coli that can rarely cause a serious blood condition with kidney failure (haemolytic uraemic syndrome); this is not the type of E. coli that commonly causes urine infection.

Sometimes fungal infections can cause urine infections, but this is rare unless someone has diabetes or their immune system is suppressed, for example by transplant drugs. Even if someone has thrush in their genital area, it is rare for this to get into the urine. Viral infections in the urine are even rarer, and are occasionally seen in people with transplants.

Bugs do not normally grow in the urine, and if infections occur, there is often some predisposing factor. Some of these are:-

  1. Urine infections are much commoner in women than in men. This is because the tube leading out of the bladder (the urethra) is much shorter in women than in men, so that it is easier for bugs to travel upwards into the bladder. This can be a particular problem after sexual intercourse, especially if the bladder is not emptied soon after intercourse.
  2. Any abnormality in the structure of the kidneys or bladder may allow urine infections to develop, particularly if it causes incomplete drainage of urine into the bladder or incomplete emptying of the bladder. Normally the body defends itself against urine infection by completely emptying the urinary system of urine, so there is nowhere for the bugs to hide. However, if the some urine is left behind, and contains small numbers of bugs, these can hang on for long enough to get an infection started before the bladder is next emptied. Conditions that cause incomplete emptying of the urinary system include:
  • Kidney stones
  • Enlargement of the prostate gland in men
  • Conditions that can affect the nerve supply to the bladder, such as spina bifida, multiple sclerosis, or spinal injury
  • Reflux nephropathy. This is a condition where urine passes backwards up to the kidneys when the bladder contracts, instead of all being passed to the outside. It is often associated with kidney damage and scarring, and can run in families. If people in a family have reflux nephropathy and kidney failure, it is important to check related children very carefully for reflux if they get urine infections (Click here for information on reflux)
  • Diabetes can lead to urine infections, or indeed any other condition which reduces the resistance of the body to infection. If some gets a urine infection unexpectedly, the diagnosis of previously unsuspected diabetes should be considered.
  • A catheter draining the bladder makes it almost inevitable that some bugs will get into the bladder, and they may cause infections. Therefore, as a general rule, when a catheter tube is necessary, it should be in place for as short a time as possible. If there is a question of having a catheter in place for long periods of time, careful consideration should be given to other types of treatment.
  • Dehydration by itself does not usually lead to a urine infection, but when combined with any other factor predisposing to infection, will make matters much worse. This is because urine flowing through the system 'flushes' out bugs.


If a urine infection is suspected, the first check is normally a visual inspection of the urine. Urine infection usually causes some cloudiness of the urine: if the urine is crystal clear, then infection is unlikely. Next is a dipstick test of the urine, which can be read immediately. If an infection is present, the dipstick will normally be positive for blood and protein. There are many causes of blood and protein in the urine which are not infections, so a positive dipstick test does not necessarily mean an infection is present. However, a negative dipstick test almost rules out infection. Some doctors use a dipstick which also tests for chemicals called nitrites. Nitrites are breakdown products of nitrogen waste in the urine. They are normally not present, but may appear if bugs are feeding on the nitrogen waste and breaking it down. There is also a dipstick test that can detect white blood cells (‘leucocytes’) in the urine; if this is positive, infection is very likely.

To confirm a urine infection, urine is sent to the laboratory to try and grow the bug, and then to determine its sensitivity to various antibiotics. For otherwise healthy people with 'one-off' urine infections, it may not be necessary to send the urine to the laboratory, the infection can be treated without laboratory tests. However, for some with kidney failure, or recurrent infection, urine should be sent to the laboratory.

If someone has recurrent urine infections, or infection affecting the kidneys, other tests may be needed, and in some cases referral to a specialist. An ultrasound scan (test in the X-ray department using sound waves) of the kidneys and bladder will be performed, to see what size and shape the kidneys are, whether they have any stones or structural abnormalities, and whether the bladder empties fully.


The first step is to increase the intake of fluid, to help flush infection out. There is a temptation to drink less fluid if it is painful to pass urine, but it makes things worse to get dehydrated. At least 2 litres of fluid a day (5 large glasses) should be taken; the more, the better. Some urine infections can be treated with these measures, and antibiotics may not be necessary.

If antibiotics are needed, the best choice is one to which the bug has been proven to be sensitive in the laboratory. However, it takes 2 days to get this result so in most cases it is best to start treatment right away, using the antibiotic which is most likely to work. The choice of antibiotic depends on which have worked or caused side effects in the past, and on the local guidelines for antibiotic use. If you have an allergy to an antibiotic, or have had side effects from antibiotics in the past, tell the person prescribing, do not wait to be asked or assume this information will be on your records.

Antibiotics are usually given for a course of 3-7 days, depending on the severity of the infection and whether there is underlying kidney disease or diabetes. In severe infections it may be necessary to give antibiotics and fluids through a drip into a vein, but this is not often needed.


After someone has had a urine infection, consideration should be given to preventing further attacks. Prevention normally includes increasing the intake of water, fruit juices or squashes to 5 large glasses a day.

Spermicidal gels can increase the risk of urine infections and should be avoided if possible. Most doctors advise patients to pay particular attention to hygiene in the genital area, and advise women to wipe from front to back after using the toilet as well as to empty the bladder after intercourse. If there is any possibility of abnormal bladder emptying, it is also worth trying ‘double voiding’ – attempting to empty the bladder again 5 minutes after passing urine, to make sure that the bladder is completely empty.

Some women who have been through the menopause become prone to urine infections as a result of ‘atrophic vaginitis’ – drying and cracking of the skin around the vagina. Treatment of this condition with oestrogen cream can reduce the risk of urine infection.

There is evidence that some people get protection from recurrent urine infections by drinking cranberry juice, which contains a substance that may help the bladder lining protect itself. Cranberry capsules are equally effective. However, it is likely that cranberry juice has to be drunk regularly, and certainly does not work in everyone. It is probably best in people with recurrent infections, who can see if it works for them by taking the juice on a daily basis over a period of time and seeing if their infection rate reduces. If it has no obvious effect, it is probably not worth continuing. Cranberry juice will probably not help treat an infection that is already under way. Cranberry should not be used by patients being treated with Warfarin.

A small number of people get recurrent urine infections despite a good fluid intake. A scan of the bladder and kidneys may then be necessary. If the scan shows no correctable cause such as a kidney stone or delay emptying the bladder, consideration may then be given to preventative (‘prophylactic’) antibiotics. All antibiotics can cause side-effects, including vaginal thrush, and some may encourage the growth of bugs in the bowel that are resistant to that antibiotic. However, for people with recurrent urine infections, a low dose of an antibiotic taken at night-time can greatly reduce the frequency of infections. It is worth continuing with preventative antibiotics for at least 6 months, or even a year before deciding whether the overall frequency of infections has been reduced. Some doctors advise taking a different antibiotic each month in the belief that this will discourage the emergence of antibiotic-resistant bugs, but there is no proof that this is of benefit.

Last reviewed September 2022
Next review September 2025

Reviewed by Dr Oshini Shivakumar 'Specialist Renal Registrar'

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The National Kidney Federation cannot accept responsibility for information provided. The above is for guidance only. Patients are advised to seek further information from their own doctor.