Kidney Infection (Pyelonephritis)

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A kidney infection can cause an unpleasant illness which is sometimes serious. Treatment includes medicines called antibiotics and painkillers. If the kidney was previously healthy then you are likely to make a full recovery. Complications occur in some cases, such as kidney damage or blood infection (septicaemia).

This leaflet only deals with sudden-onset (acute) kidney infection in adults. The medical term for this is acute pyelonephritis. There are separate leaflets on related topics, called Cystitis in Women (bladder infection), Urine Infection in Pregnancy, Urine Infection in Men, Urine Infection in Children, and Urinary Tract Infection in Older People. Also, this leaflet does not deal with chronic pyelonephritis (a condition where the kidney is scarred as a result of repeated infections).

There are two kidneys, one on each side of the abdomen. They make urine which drains down the ureters into the bladder. Urine is stored in the bladder. It is passed out through a the tube from the bladder (the urethra) from time to time.

Urinary tract

Most kidney infections develop as a complication of a bladder infection (cystitis). Germs (bacteria) causing cystitis can sometimes travel up to infect a kidney. The bacteria are usually those which live in the bowel. They can travel from the back passage (anus), up the tube which passes out urine from the bladder (the urethra), into the bladder and cause a bladder infection. This infection can then travel up to cause a kidney infection. (Note: cystitis is common, and most people with cystitis do not develop a kidney infection.)

Some kidney infections develop without a bladder infection. This is sometimes due to a problem in the kidney. For example, people are more prone to kidney infections if they have a kidney stone or an abnormality of the kidney.

It is usually only one kidney that develops an infection. A kidney infection can occur at any age. It is much more common in women. This is because women are more at risk of developing a bladder infection (which can spread to the kidneys). In women, the urethra is closer to the anus which makes it easier for bacteria from the bowel to travel up the urethra. The urethra is also shorter in women than in men.

Kidney infections are also more common in children and during pregnancy. (The leaflets mentioned above discuss urine infection in children and urine infection in pregnancy.)

Symptoms usually develop quickly, over a few hours or so, and may include:

  • Pain in a loin (side of the tummy (abdomen) or back area over the kidney).
  • High temperature (fever) which may cause shivers.
  • Feeling sick (nausea), being sick (vomiting) and/or diarrhoea.
  • Blood in the urine.
  • There will also usually be symptoms of a bladder infection - for example, pain on passing urine, and going to the toilet often.

Not all of the symptoms may develop, and sometimes a kidney infection can just cause vague symptoms. For example, just feeling generally unwell but not being able to say why.

In elderly people a kidney infection may cause confusion. This is why a urine test is often done when an elderly person suddenly becomes confused or appears generally unwell.

A simple test that your doctor can do is called a dipstick urine test. This involves testing a sample of your urine with a special testing strip to look for signs of infection. This test will show if a kidney infection is likely or not.

However, it is important to confirm the diagnosis and also to find out which germ (bacterium) is causing the infection. This will enable a doctor to prescribe the correct antibiotic. For this, your doctor will need to send your urine sample to the laboratory so that the bacterium can be identified and tests can be done to see which antibiotics will kill it. As this may take some days for the results, treatment is usually started straightaway (see below).

A urine sample may be the only test that is needed if you are a woman who is otherwise healthy, but develop cystitis which progresses to a kidney infection. Further tests may be advised in some situations - for example, if you have a kidney stone or a kidney abnormality is suspected. Tests are also usually advised if you are a man or if you have recurring kidney infections.

  • Antibiotics will usually clear the infection. An antibiotic is usually prescribed straightaway if a kidney infection is suspected, even before the result of the urine test is known. Some germs (bacteria) are resistant to some antibiotics. Therefore, sometimes a change of antibiotic may be needed if the urine test shows a bacterium which is resistant to the initial antibiotic. The course of antibiotics is for 7-14 days, depending on which one is used.
  • Painkillers such as paracetamol can ease pain and reduce a high temperature (fever). Stronger painkillers may be needed if the pain is more severe. Non-steroidal anti-inflammatory painkillers such as ibuprofen are not usually recommended, as they may cause an increased risk of problems with the working of the kidney during a kidney infection.
  • Lots of fluid should be taken to prevent dehydration.

In many cases, the infection is not too severe, treatment can be taken at home and the infection will clear with a course of antibiotic tablets. If treatment is to be home-based, a doctor should be called if the symptoms are not improving after 48 hours, or the person is feeling more unwell.

However, some people need to be admitted to hospital - for example, if:

  • the infection is severe; or
  • does not settle quickly with antibiotics; or
  • they are otherwise ill or frail; or
  • they are at risk of a lack of fluid in the body (dehydration) or are unable to keep antibiotic tablets down because they are being sick (vomiting) a lot; or
  • they have underlying kidney problems.

In hospital, antibiotics can be given straight into the vein for a more immediate effect. If the person is dehydrated, they may also need a drip (where fluid is put straight into a vein).

Most people who develop a kidney infection make a full recovery if treatment is given promptly. Possible complications which occur in a small number of cases include:

  • Sometimes germs (bacteria) from a kidney infection get into the bloodstream, particularly if treatment is delayed. This may cause blood poisoning (septicaemia). This can be serious or even life-threatening.
  • A kidney abscess can (rarely) develop. This is a collection of pus that forms within the kidney.
  • The infection can sometimes cause some permanent damage to kidney tissues.

These complications are uncommon but may be more likely if:

  • You become severely ill with the kidney infection.
  • You already have a problem with your kidneys, such as polycystic kidney disease, reflux or kidney failure.
  • You have kidney stones.
  • Your immune system is suppressed - for example, if you have cancer, if you are taking medication such as steroids or chemotherapy, or if you have AIDS.
  • You have poorly controlled diabetes.
  • You are an older person (over the age of 65 years).

Emphysematous pyelonephritis is also a rare complication. In this condition the kidney tissues are rapidly destroyed by the infection and the bacteria can release toxic gases which can build up in the kidneys. You become very unwell if you develop this complication. This complication seems mostly to affect people who have poorly controlled diabetes.

As most kidney infections are caused by germs (bacteria) travelling up from a bladder infection, the same things that can help to reduce your chances of bladder infection should reduce your chances of kidney infection. Some of the things that may help include the following:

  • Don't hold on to urine when you feel the need to pass urine. Go to the toilet promptly.
  • Make sure you drink plenty of fluids every day so that you are well hydrated.
  • Treat any constipation promptly, as constipation can increase your chances of a bladder or kidney infection. (See separate leaflet called Constipation in Adults for more detail.)
  • Empty your bladder after having sex.
  • Always make sure that you wipe from front to back after going to the toilet to pass poo (faeces, stools, or motions).
Original Author:
Dr Tim Kenny
Current Version:
Peer Reviewer:
Dr Adrian Bonsall
Document ID:
4441 (v39)
Last Checked:
Next Review:
The Information Standard - certified member
See also: Pyelonephritis written for professionals
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