A kidney infection (pyelonephritis) is a painful and unpleasant illness caused by bacteria travelling from your bladder into one or both of your kidneys.
It's more serious than cystitis, a common infection of the bladder that makes urinating painful.
If treated promptly, a kidney infection doesn't cause serious harm, but will make you feel very unwell. If a kidney infection isn't treated, it can get worse and cause permanent kidney damage.
Symptoms of a kidney infection often come on within a few hours. You can feel feverish, shivery, sick and have a pain in your back or side.
When to see your GP
See your GP if you have a fever and persistent tummy, lower back or genital pain, or if you notice a change to your usual pattern of urination.
Most kidney infections need prompt treatment with antibiotics to stop the infection from damaging the kidneys or spreading to the bloodstream. You may also need painkillers.
If you're especially vulnerable to the effects of an infection – for example, if you have a pre-existing health condition or are pregnant, you may be admitted to hospital and treated with antibiotics through an intravenous drip.
After taking antibiotics, you should feel completely better after about two weeks.
In rare cases, a kidney infection can cause further problems. These include blood poisoning (sepsis) and a build-up of pus in the kidney called an abscess.
What causes a kidney infection?
A kidney infection usually happens when bacteria – often a type called E. coli – gets into the urethra (the tube which carries urine out of the body) and travels up through the bladder and into the kidneys.
Who's at risk?
Kidney infections are relatively rare. It's estimated that one in every 830 people develops a kidney infection each year in the UK.
They can happen at any age, but are much more common in women. In fact, women are six times more likely to get a kidney infection than men. This is because a woman's urethra is shorter, making it easier for bacteria to reach the kidneys.
Younger women are most at risk because they tend to be more sexually active, and having frequent sex increases the chances of getting a kidney infection.
Younger children are also vulnerable to developing kidney infections because they may be born with an abnormality of the urinary tract or have a condition called vesico-ureteric reflux, where there is a backflow of urine from the bladder up to the kidneys.
Can kidney infections be prevented?
You can reduce your chances of developing a kidney infection by keeping your bladder and urethra free from bacteria. This can include drinking plenty of fluids, keeping your genitals clean and treating any constipation.
Symptoms of kidney infection
The symptoms of a kidney infection usually develop quite quickly over a few hours or days.
Common symptoms include:
pain and discomfort in your side, lower back or around your genitals
high temperature (it may reach 39.5C or 103.1F)
shivering or chills
feeling very weak or tired
loss of appetite
feeling sick or being sick
You may have other symptoms if you also have cystitis or urethritis (an infection of the urethra). These additional symptoms may include:
pain or a burning sensation during urination
need to urinate frequently or urgently
feeling that you're unable to urinate fully
blood in your urine
cloudy or foul smelling urine
pain in your lower abdomen
Children with a kidney infection may also have additional symptoms, such as:
a lack of energy
poor feeding and/or vomiting
not growing at the expected rate
jaundice (yellowing of the skin and whites of the eyes)
blood in the urine
unpleasant smelling urine
When to seek medical advice
Contact your GP if you have a high temperature, persistent pain, or if you notice a change to your usual pattern of urination. Contact your GP immediately if you think your child may have a kidney infection.
If you have blood in your urine, you should always see your GP so the cause can be investigated.
Kidney infections require prompt treatment with antibiotics to help relieve symptoms and prevent complications developing.
Your GP can carry out some simple tests to help diagnose a kidney infection.
Causes of kidney infection
A kidney infection happens when bacteria infects your kidneys. The bacteria are usually a type called E. coli, which live in your bowel.
The bacteria get in through the opening of the urethra and move upwards through your urinary tract, first infecting your bladder and then your kidneys.
It's thought the bacteria can get into your urinary tract by accidentally spreading from your anus to your urethra. This can happen if you wipe your bottom after going to the toilet and the soiled toilet paper comes into contact with your genitals. It can also happen during sex.
In rare cases, a kidney infection can develop if bacteria or fungi infect the skin and the infection spreads through your bloodstream into your kidney. However, this type of infection usually only occurs in people with weakened immune systems.
Who's most likely to get a kidney infection?
Women and children are most at risk of developing a kidney infection, as well as other urinary tract infections (UTIs) such as cystitis.
In women, the urethra is closer to the anus than in men, making it easier for bacteria from the anus to enter the urethra accidentally. The female urethra is also much shorter than the male urethra (which runs through the penis). This makes it easier for bacteria to reach the bladder and move into the kidneys.
Other factors can also put you more at risk of developing a kidney infection, including:
having a condition that blocks, or obstructs, your urinary tract, such as kidney stones or an enlarged prostate – children with constipation can also be at an increased risk
being born with an abnormality in your urinary tract
having a condition that prevents you emptying your bladder fully, such as an injury to your spinal cord – this can allow bacteria in your bladder to multiply and spread
having a weakened immune system – for example, due to type 2 diabetes or as a side effect of chemotherapy
having an infection of the prostate gland called prostatitis – the infection can spread from the prostate gland into the kidneys
having a urinary catheter (a thin, flexible tube inserted into your bladder to drain away urine)
being female and sexually active – sexual intercourse can irritate the urethra and allow bacteria to travel into your bladder
being a man who has anal sex – bacteria can travel up the urethra into the bladder
being pregnant – this can cause physical changes that slow the flow of urine out of your body and make it easier for bacteria to spread to the kidneys
Diagnosing kidney infection
To work out if you have a kidney infection, your GP will ask you about your symptoms and your recent medical history.
They will usually also assess your general health by taking your temperature and measuring your blood pressure.
A urine test can help to establish whether you have a urinary tract infection (UTI). The test involves taking a small sample of urine and checking it to see if there are any bacteria in it.
You'll be given a container and told how to collect the urine, which you can do in the surgery or at home. If you do it at home, you'll need to label the container, seal it in a plastic bag and store it in the fridge. Ideally, hand it in to the surgery within four hours.
A urine test can't tell whether the infection – if you have one – is in your kidneys or another part of your urinary system, such as your bladder.
For your GP to be confident you have a kidney infection, you need to have a positive urine test plus certain symptoms, such as a fever or a pain in your side.
You may be referred to hospital for further testing if:
your symptoms fail to respond to treatment with antibiotics
your symptoms suddenly get worse
you have additional symptoms that aren't usually associated with a kidney infection
you're at risk of complications of a kidney infection
Children with recurrent UTIs will also be referred to hospital for further testing.
In these circumstances, scans can check your urinary tract for signs of problems. This may include:
a computer tomography (CT) scan – where a scanner takes a series of X-rays and a computer is used to assemble them into a detailed image of your urinary tract
an ultrasound scan – which uses sound waves to build an image of the inside of your body
an isotope scan – where a dye is injected into the bloodstream and a series of X-rays are taken
Treating kidney infection
Most people with a kidney infection can be treated at home with a course of antibiotics, and possibly painkillers as well.
See your GP if you have a fever and persistent tummy, lower back or genital pain, or if you notice a change to your usual pattern of urination.
All children with symptoms of a urinary tract infection (UTI) or kidney infection, including cystitis, should see their GP or out-of-hours emergency service.
If you're being treated at home, you'll usually be prescribed a course of antibiotic tablets or capsules that lasts between seven and 14 days.
For most people – apart from pregnant women – antibiotics called ciprofloxacin or Co-amoxiclav are recommended. However, other antibiotics may also be used.
Common side effects of ciprofloxacin include feeling sick and diarrhoea.
Co-amoxiclav can make the contraceptive pill and contraceptive patches less effective, so you may need to use another form of contraception during the course of treatment.
A 14-day course of an antibiotic called cefalexin is recommended for pregnant women.
Usually, you'll start to feel better quite soon after treatment starts and you should feel completely better after about two weeks.
If your symptoms show no sign of improvement 24 hours after treatment starts, contact your GP for advice.
Taking a painkiller such as paracetamol should help relieve symptoms of pain and a high temperature.
However, non-steroidal anti-inflammatories (NSAIDs) such as ibuprofen aren't usually recommended to relieve pain during a kidney infection. This is because they may increase the risk of further kidney problems.
If you have a kidney infection, try not to "hover" over the toilet seat when you go to the loo, because it can result in your bladder not being fully emptied.
It's also important to drink plenty of fluids, because this will help prevent dehydration and will help to flush out the bacteria from your kidneys. Aim to drink enough so that you're frequently passing pale-coloured urine.
Make sure that you get plenty of rest. A kidney infection can be physically draining, even if you're normally healthy and strong. It may take up to two weeks before you're fit enough to return to work.
Treatment at hospital
Your GP may refer you to hospital if you have an underlying problem with your urinary tract, which makes you vulnerable to kidney infections.
It's standard practice to further investigate all men with a kidney infection simply because the condition is much rarer in men. Only women who have had two or more kidney infections tend to be referred. Most children with a kidney infection will be treated in hospital.
Hospital treatment may also be needed if:
you're severely dehydrated
you're unable to swallow or keep down any fluids or medications
you have additional symptoms that suggest you may have blood poisoning, such as a rapid heartbeat and losing consciousness
you're pregnant and you also have a high temperature
you're particularly frail and your general health is poor
your symptoms fail to improve within 24 hours of starting treatment with antibiotics
you have a weakened immune system
you have a foreign body inside your urinary tract, such as a kidney stone or a urinary catheter
you have diabetes
you're over the age of 65
you have an underlying condition that affects the way your kidneys work, such as polycystic kidney disease or chronic kidney disease
If you're admitted to hospital with a kidney infection, you'll probably be attached to a drip so you can be given fluids to help keep you hydrated. Antibiotics can also be given through the drip.
You'll have regular blood and urine tests to monitor your health and how effectively the antibiotics are fighting off the infection.
Most people respond well to treatment. As long as there are no complications, they're usually well enough to leave hospital within three to seven days. Treatment will usually switch to tablets or capsules after you stop receiving antibiotics through a drip.
Complications of kidney infection
Most kidney infections are treated successfully without complications, although some people may develop further problems.
Complications of a kidney infection are rare, but you're more likely to develop them if you:
are a child
are over 65
have diabetes, chronic kidney disease or sickle cell anaemia
have had a kidney transplant (particularly in the first three months after the transplant)
have a weakened immune system
developed the kidney infection while in hospital
Some of the main complications of kidney infections are outlined below.
A kidney abscess is a rare, but serious, complication of a kidney infection, where pus develops inside the tissue of the kidney.
You're thought to be most at risk of developing a kidney abscess if you have diabetes.
The symptoms of a kidney abscess are similar to those of a kidney infection. The most common are:
a high temperature of 38C (100.4F) or above
loss of appetite
pain when passing urine
Kidney abscesses are potentially serious because the bacteria inside the abscess can spread to other parts of your body, such as your bloodstream or lungs, and can be fatal.
Smaller abscesses can usually be treated with antibiotics through a drip. Surgery is usually required for larger abscesses. This involves draining the pus out of the abscess using a needle that's inserted into the kidney.
Blood poisoning (also called sepsis) is another rare, but potentially fatal, complication of a kidney infection. It happens when bacteria spreads from the kidneys into the bloodstream. Once bacteria are in your blood, the infection can spread to any part of your body, including all of the major organs.
In someone with a kidney infection, the symptoms of blood poisoning include:
low blood pressure, which makes you feel dizzy when you stand up
confusion or disorientation
uncontrollable shaking or shivering
high temperature or lower body temperature than usual (under 36C, or 96.8F)
Blood poisoning is a medical emergency that usually requires admission to a hospital intensive care unit (ICU) while antibiotics are used to fight the infection.
If you're taking certain medications for diabetes, such as metformin or angiotensin-converting enzyme (ACE) inhibitors, they may be temporarily withdrawn until you recover. This is because they can cause kidney damage during an episode of blood poisoning.
Another rare, but potentially fatal, complication of a kidney infection is a condition called emphysematous pyelonephritis (EPN).
EPN is a severe infection, where the tissues of the kidneys are rapidly destroyed and the bacteria causing the infection begin to release a toxic gas, which builds up inside the kidneys.
The exact cause of EPN is unclear, but almost all cases are in people with diabetes.
The usual treatment is emergency surgery to remove some, or all, of the affected kidney. It's possible to live a full and active life with only one kidney.
In very rare cases, a kidney infection can cause severe kidney damage that results in kidney failure. This is when the kidneys stop working properly.
Kidney failure is potentially fatal, but it can be treated with dialysis or a kidney transplant.
A kidney infection can also cause other complications, including high blood pressure (hypertension) or premature labour and birth.
Preventing kidney infection
The best way to prevent a kidney infection is to keep your bladder and urethra free from bacteria.
These self-help tips explain how you can do this.
Drink plenty of liquids
Drinking plenty of liquids, particularly water, will help to wash bacteria from your bladder and urinary tract.
Drinking cranberry juice or taking cranberry extracts may also help prevent urinary tract infections (UTIs). However, you should avoid cranberry juice or extracts if you're taking warfarin, a medicine used to prevent blood clots. Cranberry juice can make the effects of warfarin more potent, so there's a risk of excessive bleeding.
To help keep your urinary tract free from bacteria:
go to the toilet as soon as you feel the need to urinate, rather than holding it in
wipe from front to back after going to the toilet
practice good hygiene by washing your genitals every day and before having sex
empty your bladder after having sex
if you're a woman, avoid "hovering" over a toilet seat as this position can often leave urine behind in the bladder
Constipation can increase your chances of developing a urinary tract infection (UTI), so try to treat any constipation promptly.
Recommended treatments for constipation include:
increasing the amount of fibre in your diet to 20-30g of fibre a day
using a mild laxative on a short-term basis
drinking plenty of fluids
See your GP if your symptoms don't improve after 14 days (or seven days for children with constipation).
Be careful with contraceptives
If you keep getting UTIs (more than three a year is considered high), avoid using spermicide-coated condoms or diaphragms. This is because spermicide can stimulate the production of bacteria.
Stick to lubricated condoms without spermicide, because unlubricated ones can irritate the urethra and make it more vulnerable to infection.