Kidney infection (pyelonephritis)

About kidney infection

You have two kidneys, which 'clean' your blood by filtering out water and waste products to make urine. Urine is stored in your bladder and then passed out of your body through your urethra.

Illustration showing the position of the kidneys and surrounding structures

Pyelonephritis is a bacterial infection of the kidneys. Usually, only one kidney is affected, and the infection can be either acute or chronic. Acute and chronic refer to how long the condition lasts for, rather than how severe it is. This factsheet will focus on acute kidney infection.

Symptoms of kidney infection

The symptoms of an acute kidney infection often develop quickly over a few hours or a day and may include:

  • a high temperature (over 38°C)
  • shivering and chills
  • feeling weak
  • pain in your lower back (it may be only on one side)
  • feeling sick
  • vomiting
  • diarrhoea

Around one in three people with acute kidney infection will also have symptoms of a bladder infection (cystitis or lower urinary tract infection, UTI). See our frequently asked questions for more information about UTIs.

Symptoms of a kidney infection may include:

  • a stinging or burning sensation when you pass urine
  • feeling that you need to urinate more often than usual
  • feeling that you need to urinate urgently even if you pass very little or no urine
  • urine that is cloudy, dark coloured or has a strong smell
  • blood in your urine
  • pain or aching in your abdomen (tummy)

These symptoms may be caused by problems other than a kidney infection. If you have any of these symptoms, see your GP for advice.

Complications of kidney infection

Acute kidney infection can sometimes cause complications including:

  • a pocket of pus (abscess) in your kidney
  • swelling (inflammation) of your kidney
  • blood poisoning (septicaemia) – this can happen if the bacteria that have caused your kidney infection get into your bloodstream

However, you’re unlikely to get complications if your kidneys worked normally before you developed the infection. You’re more likely to have complications, if you:

  • are pregnant
  • have type 1 or type 2 diabetes
  • are over 65
  • have persistent kidney infection
  • have a weakened immune system such as those who have HIV/AIDS (which reduces your ability to fight infection)
  • have a urinary catheter fitted (a tube that takes urine from your bladder out of your body)

Sometimes, a kidney infection can cause severe damage and result in kidney failure, but this is rare (it only happens to around three in every 10,000 people).

Causes of kidney infection

A kidney infection is usually caused by bacteria such as Escherichia coli (E. coli), which live in your bowel. The bacteria may get into your urinary tract by spreading from your anus to your urethra – if you wipe yourself towards the front after a bowel movement, for example. You can also spread the bacteria during sex. The bacteria can enter the urethra and travel up into your bladder and will usually cause a lower UTI or cystitis. The bacteria can then travel up one or both of the tubes that connect your bladder to your kidneys (ureters) and cause a kidney infection.

Women are at greater risk of having a UTI partly because, in women, the urethra is nearer the anus. This makes it easier for bacteria to get transferred from the surrounding skin into the urethra. The urethra is also much shorter in women than in men, so there is less distance for the infection to travel to the bladder.

Sometimes you can get a kidney infection without having a bladder infection. This can happen if your ureter or kidney is blocked, for example by a kidney stone or an enlarged prostate gland.

You’re more likely to develop a kidney infection, if you’re pregnant (see our frequently asked questions for more information), have diabetes or a weakened immune system, such as those people who have HIV/AIDS.

Diagnosis of kidney infection

Your GP will ask about your symptoms and examine you. He or she may also ask you about your medical history. You will be asked to provide a urine sample. Your GP will usually do a 'dipstick' test to confirm if you have a urinary infection. He or she may send your urine sample to a laboratory to find out which type of bacteria is causing the infection.

If your symptoms are severe or you’re pregnant, your GP may refer you to a hospital for further tests and treatment. Other tests may include:

  • a blood sample – this is usually only done if you have complications
  • scans (such as a CT scan) – to check for any problems or obstructions in your ureters or kidneys

Treatment of kidney infection


It’s important to drink enough fluids to prevent getting dehydrated; this is particularly important if you’re pregnant or over 65.


If you need pain relief, you can take over-the-counter painkillers such as paracetamol or ibuprofen. Always read the patient information leaflet that comes with your medicine and if you have any questions, ask your pharmacist for advice.

Your GP will usually prescribe antibiotics if he or she suspects that you have a kidney infection. It’s important to take the full course of antibiotics to get rid of the bacteria completely, even if your symptoms clear up before you finish the course. Your symptoms should start to improve within a day or so. But if they get worse or you don't feel any better within 48 hours of starting treatment, contact your GP.

Hospital treatment

You may need hospital treatment if you’re:

  • pregnant
  • unable to keep down fluids or medicines
  • dehydrated
  • not responding to antibiotic treatment
  • diagnosed with blood poisoning (septicaemia)

In hospital, you will have a drip inserted into a vein in your hand or arm to give you fluids and antibiotics.


You may need to have surgery if tests show that you have an obstruction (such as a kidney stone) in your ureters or kidneys.

Kidney infection and pregnancy

If you have a kidney infection during pregnancy, it may lead to complications, such as:

  • kidney damage
  • pre-eclampsia (high blood pressure in pregnancy)
  • premature labour
  • blood poisoning (septicaemia)

If your GP suspects that you may have a kidney infection and you’re pregnant, he or she may send you for tests and treatment in hospital.

Prevention of kidney infection

If you often get a kidney infection or a UTI, your doctor may advise you to take a low dose of antibiotics every day. How long you need to take them for will depend on your personal circumstances.

You can also develop a kidney infection from cystitis. Getting prompt treatment for cystitis or taking self-help measures to cure the condition will reduce the risk of it spreading to your kidneys.


Produced by Rachael Mayfield-Blake, Bupa Heath Information Team, May 2012.

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