What is a kidney infection?

An infection is where germs, such as bacteria, get into the body and multiply, causing harmful effects. If this happens in the kidney, this is called a kidney infection, or pyelonephritis. This type of infection usually starts in the bladder but spreads to the kidney.

Urinary tract infections (UTIs) are grouped. Upper urinary tract infections are infections of the kidneys and ureters, the tubes that carry urine away from the kidneys. Lower urinary tract infections are infections of the bladder and urethra, the tube that carries urine away from the bladder. Upper urinary tract infections are less common than lower urinary tract infections but are usually more serious.

This page is focused on kidney infection that happens suddenly, called an acute infection. This is different to chronic kidney infection where repeated or ongoing infections cause damage to the kidneys.

What are the symptoms of kidney infection?

Symptoms of a kidney infection often begin suddenly within a few hours.
In adults these symptoms can include:

Pain in the lower back and sides
High temperature
Feeling sick
Being sick
Diarrhoea
Blood in your urine
Symptoms of bladder infection, such as pain when weeing, and needing to wee more often
Muscle aches and pains
Flu-like symptoms

Some people may just feel generally unwell with a kidney infection which can be accompanied by the above symptoms or not.

A kidney infection in older people can also cause confusion.

In children, symptoms of a kidney infection can include:

High temperature
Needing to wee more often
Pain or problems weeing
Being sick
Pain in the stomach or sides
Lack of appetite or interest in food
Problems controlling weeing, such as wetting the bed or themselves

In very young children, less than 3 months old, symptoms can include:

High temperature
Being sick
Tiredness or being irritable
Lack of appetite or interest in food
If they have chronic kidney infection, not growing and gaining weight as expected for their age and biological sex

You should see your GP if:

You are experiencing pain, burning or stinging when you wee
You need to wee more urgently and often
Your urine is dark or cloudy
Symptoms of a kidney infection are not improving with treatment
You are feeling sick

You should ask for an urgent GP appointment or call 111 if you suspect you have a kidney infection and:

Your temperature is very high
You feel hot, cold or shivery
You have a very low (below 36 degree) temperature
You have pain in your back or lower belly
You have blood in your urine
You haven’t been for a wee all day
You are being sick
You are pregnant

You should call 999 or immediately go to Accident and Emergency if:

You are confused
You are drowsy
You have difficulty speaking

If left untreated a kidney infection can lead to sepsis, a serious and life-threatening reaction of the body to infection. For information about sepsis visit the NHS website.

It is important you seek prompt medical attention if you think you have a kidney infection or any of the above symptoms.

If you have a kidney transplant and you think you have a kidney infection (or are worried about your health), always seek medical advice from your transplant healthcare team.

If you are on dialysis and you think you have a kidney infection (or are worried about your health), always seek medical advice from your dialysis healthcare team.

Kidney infection diagnosis

If your doctor thinks you may have a kidney infection, they will ask you for a urine sample to do a dipstick test. This involves dipping a special testing strip into the urine sample, which will reveal whether an infection is likely.

To confirm whether there is an infection, and if so, identify the bacteria causing the infection, your urine sample will be sent to a laboratory for further tests. By identifying the type of bacteria this helps your doctor choose the most effective antibiotics to work against the bacteria to get rid of the infection. As laboratory testing can take several days treatments are usually started immediately whilst waiting for the results.

For some patients, further tests may be done to investigate the cause of your kidney infection. For example, if your doctor thinks you may have a problem with your kidneys or a kidney stone, you have had multiple kidney infections, or are a biological male, as they less commonly have kidney infections than females. Further tests can include medical imaging, such as an ultrasound scan.

In some cases, your doctor will also do a blood test to look for signs of infection.

What causes kidney infections?

Most cases of kidney infection are caused by the spread of an infection from the bladder. Bacteria causing the infection can spread through the tubes which transport urine from the kidney to the bladder, called the ureters. Most commonly the bacteria which cause a kidney infection are those living in the gut. These bacteria enter the urinary tract via the urethra, the tube where urine leaves the body. Occasionally bacteria get into the kidney directly from the bloodstream due to sepsis which can cause abscesses.

Sometimes, infections which spread to the kidneys don’t start in the bladder. This infection may be caused by a problem with the kidney, such as a kidney stone. A kidney stone can block the flow of urine causing a build-up of waste and bacteria.

Usually, just one kidney is affected by a kidney infection instead of both.

How to treat kidney infections

In many cases if you have a kidney infection your doctor will give you medicines which you can take at home which are:

Antibiotics: these tablets kill the bacteria or prevent them from spreading to try and get rid of the infection.

Paracetamol: may be used to relieve pain and to decrease a high temperature. Other types of pain relief medicines, called NSAIDs which include Ibuprofen, are usually avoided as these can be harmful to the kidney if taken too often.

In some cases, you may need to be treated in a hospital. For example, if the kidney infection is very severe or you are vulnerable to infection, or the infection is not responding well to treatment. In hospital antibiotics can be given into a vein allowing them to work quicker.

Kidney infection recovery time

Most people will start to feel better around two weeks after starting treatment and usually make a full recovery.

People who are more likely to take longer to recover and have complications from a kidney infection include people that:

Need hospital treatment

Have an immune system that works less well (due to a medical condition or certain medications)

Have existing kidney problems or a kidney stone

Have less well managed diabetes

Are over 65

Are pregnant

References

National Cancer Institute. NCI Dictionary of Cancer Terms: Infection [online]. National Institutes of Health. [Accessed 06 January 2026]. Available from: Definition of infection - NCI Dictionary of Cancer Terms - NCI

Tidy C. Pyelonephritis [online]. Patient.info, Navigate Health Ltd: London, UK; Jan 2022 [Accessed 06 January 2026]. Available from: Pyelonephritis and acute pyelonephritis

Hazel, T. Kidney infection [online]. Patient.info, Navigate Health Ltd: London, UK; Aug 2024 [Accessed 06 January 2026]. Available from: Kidney Infection: Symptoms, Causes, and Treatment

NHS [online]. Kidney infection. Nhs.uk, England; Feb 2025 [Accessed 06 January 2026]. Available from: Kidney infection - NHS

NHS [online]. Symptoms of sepsis. Nhs.uk, England; Sep 2022 [Accessed 06 January 2026]. Available from: Symptoms of sepsis - NHS

Tidy C. Antibiotics [online]. Patient.info, Navigate Health Ltd: London, UK; Jul 2024 [Accessed 06 January 2026]. Available from: Antibiotics: Types, Uses, and Side Effects

NHS [online]. Kidney stones. Nhs.uk, England; Nov 2022 [Accessed 07 January 2026]. Available from: Kidney stones - Symptoms - NHS

NICE [online]. What are the signs and symptoms of acute pyelonephritis? Nice.org, England; Dec 2024 [Accessed 01 April 2026] Available from: Signs and symptoms | Diagnosis | Pyelonephritis - acute | CKS | NICE

NICE [online]. Urinary tract infection – children. Nice.org, England; Apr 2024 [Accessed 01 April 2026] Available from: Urinary tract infection - children | Health topics A to Z | CKS | NICE

Harrogate urology [online]. What's the difference between a lower and upper urinary tract infection? England; Jun 2020 [Accessed 01 April 2026] Available from: What's the difference between a lower and upper urinary tract infection? - Harrogate Urology

Great Ormond Street Hospital for Children [online]. Septicaemia. NHS Foundation Trust, England; Jul 2011 [Accessed 01 April 2026] Available from: Septicaemia | Great Ormond Street Hospital

Reviewed April 2026 by Professor Simon Davies