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Most people with kidney infections look and feel very sick. The symptoms usually come on suddenly over a few hours or a day.

The main symptoms are:^{1 2}

• A fever
• Chills and shivering
• Pain and tenderness in your side, or on both sides
• Feeling nauseated
• Diarrhea
• Back pain.

Your kidneys help to make urine and pass it out of your body. So a kidney infection may mean you often need to urinate. You may also get pain when you urinate or generally have pain in your pelvis.

Your urine may also look cloudy or be a different color. It may smell bad or just smell stronger than usual.³

Older people with kidney infections sometimes have different symptoms. These could be:⁴

• A fever without other symptoms
• Pain in their side or back, along with a fever
• Confusion and sluggishness
• Loss of appetite.

Up to a third of older people with a kidney infection don't get a fever. About 2 in 10 have symptoms that mainly affect their intestines or their lungs.¹

Some people with a kidney infection get very few symptoms or even none at all. This is known as a silent kidney infection.¹

Infections in other parts of your body that deal with urine, such as your bladder or urethra, can also cause pain when you urinate or make you need to urinate more often. These parts of your body are called your lower urinary tract. It's possible to get a kidney infection and just have the symptoms of an infection in your lower urinary tract.

Some researchers think that up to 3 in 10 people who seem to have an infection in the lower part of their urinary tract actually have a silent kidney infection.^{1 5 6}

You are more likely to have a silent kidney infection if you:¹

• Have diabetes
• Are taking drugs that weaken your immune system
• Have had a kidney transplant
• Are pregnant
• Had urinary tract infections before you were 12
• Have had three or more urinary tract infections in the last year.

The symptoms of a kidney infection can be similar to some other illnesses. For example, pelvic inflammatory disease, an inflamed gall bladder, appendicitis and kidney stones can all cause similar symptoms.¹ To check if you really have a kidney infection, your doctor may ask you for a urine sample. Your urine can then be tested for signs of infection.

You will be asked to wash your genital area and collect a sample of urine halfway through urinating.⁷ This way of collecting urine helps stop bacteria from the skin around your genital area getting into the sample and confusing the test results.

There are two parts to the urine test:

• First, your urine is checked with a dipstick to look for pus.¹ Pus is a thick white or yellowish fluid that can form where you have an infection. It's made up of dead body tissue, bacteria and white blood cells.
• Next, any bacteria found in your urine are grown in a laboratory and tested to see which drug works best against them.^{1 8}

These may be the only tests that you need. If you have to be treated in the hospital, you'll probably also have a blood test to check whether the infection has spread to your bloodstream.¹

Some people need further tests, such as an X-ray, ultrasound or CT scan. Your doctor might want to check if you have a blockage or another problem with your bladder or the tubes that carry urine. You may need more tests if:⁸

• Your doctor isn't sure that you have a kidney infection
• You're very sick or have a weakened immune system
• You don't get better with antibiotics
• Your doctor thinks the infection has led to another health problem (a complication, such as scarring or pus in your kidneys).

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.