UTI on one side

Overview

A 'UTI on one side' usually means a kidney infection, also called pyelonephritis (pie-low-nef-RYE-tis). This happens when an infection from your bladder travels up to one of your kidneys. The kidney becomes inflamed and swollen, causing pain on that side of your lower back or belly. Kidney infections are more serious than a simple bladder infection and need prompt medical care.

Kidney infections are less common than bladder infections, but they still happen to many people. About 1 in 1,000 people get a kidney infection each year. Women are more likely to get them than men.

Anyone can get a kidney infection, but it is most common in women, especially those who are sexually active, pregnant, or have had a bladder infection. Older adults, people with diabetes, and those with a weakened immune system are also at higher risk. Men with an enlarged prostate or a catheter (a tube to drain urine) are more prone to kidney infections too.

Symptoms

Common symptoms

• Pain on one side of your lower back or side (flank pain) that may spread to your groin or belly
• High fever (often over 38°C or 100.4°F) and chills
• Feeling sick (nausea) or vomiting
• Urge to urinate often, pain or burning when you urinate
• Foul-smelling, cloudy, or bloody urine

Symptoms in children

• Fever without an obvious cause
• Irritability or crying
• Loss of appetite
• Vomiting or diarrhea
• Pain in the belly or back

Symptoms in older adults

• Confusion or agitation (sudden changes in mental state)
• Low energy or weakness
• Loss of bladder control (wetting themselves)
• Fever might be lower or absent
• Fall or sudden decline in function

Causes

Main causes

• Bacteria (most often E. coli from the bowel) enter the urethra and travel up to the bladder and then to one kidney.
• Blockages in the urinary tract, like a kidney stone or an enlarged prostate, can trap bacteria and cause infection on one side.
• Using a catheter (a tube to drain urine) can introduce bacteria into the bladder and kidneys.

Risk factors

• Being female (shorter urethra makes it easier for bacteria to reach the bladder)
• Sexual activity (can push bacteria into the urethra)
• Pregnancy (hormones and pressure on the urinary tract)
• Diabetes (high blood sugar can weaken the immune system)
• Having a kidney stone or other blockage
• Having a weakened immune system (from HIV, chemotherapy, or long-term steroid use)
• Urinary tract surgery or recent hospital stay with a catheter

When to see a doctor

See a doctor urgently if:

• You have pain on one side of your back or side along with fever and chills
• You are vomiting and cannot keep down fluids or medications
• You have a known kidney stone and develop fever or chills

Book a routine appointment if:

• You have mild burning or frequency when urinating that lasts more than a day
• You have had a bladder infection in the past that is now causing back pain
• You are pregnant and have any UTI symptoms

Diagnosis

Your doctor will ask about your symptoms, check your temperature and blood pressure, and press on your back to see if the area is tender. They will usually test a sample of your urine to look for signs of infection.

Tests that may be done

• Urine dipstick test (a quick test that checks for white blood cells, nitrites, and blood in the urine)
• Urine culture (a lab test that grows the bacteria to confirm the infection and show which antibiotic will work)
• Blood tests (to check for bacteria in the blood and how your kidneys are working)
• Imaging, like an ultrasound or CT scan (if the doctor suspects a blockage, such as a kidney stone)

What to expect at your appointment

Your doctor may start antibiotics right away, even before the urine culture results are back. If you are very unwell (e.g., high fever, vomiting, or dehydration), you may need to go to the hospital for IV fluids and IV antibiotics. Otherwise, you can take oral antibiotics at home. The results of the urine culture may take 2–3 days, and your doctor may adjust the antibiotic based on those results.

Treatment

Kidney infections are treated with antibiotics to kill the bacteria. It is important to finish all the antibiotics, even if you feel better. Drinking plenty of water helps flush out the bacteria. Rest is also important while your body fights the infection.

Self-care at home

• Drink 6–8 glasses of water or other clear fluids a day (unless your doctor tells you otherwise)
• Rest at home – take time off work or school until your fever goes down
• Place a warm pad or hot water bottle on your painful side for comfort (wrapped in a towel to avoid burns)
• Take recommended pain relievers (ask your pharmacist which are safe with your other medicines)
• Wipe from front to back after using the toilet to avoid spreading bacteria

Medical treatments

Your doctor will prescribe a course of antibiotics taken by mouth. Usually the antibiotics are broad-spectrum and target common urinary bacteria. If you have severe symptoms or cannot keep pills down, you may need to go to hospital for antibiotics through a vein (IV). Sometimes a single IV antibiotic dose is enough, followed by pills. The type and length of antibiotic treatment depend on how severe the infection is and your overall health. Always finish the entire course as prescribed. Never stop early, even if you feel better.

When is surgery considered?

Surgery is rarely needed for a kidney infection. It might be considered if there is a blockage, such as a large kidney stone or a structural problem in the urinary tract that cannot be treated with medicine alone. In those cases, a urologist (a doctor who specializes in the urinary system) may perform a procedure to remove or bypass the blockage.

Living with this condition

Most people recover completely from a kidney infection within one to two weeks. During recovery, take it easy. Stay home from work or school until your fever is gone and you feel strong enough. Keep up with your antibiotics and fluids. If you have a follow-up appointment, be sure to go – your doctor may want to test your urine again to make sure the infection is gone.

Lifestyle tips

• Drink plenty of fluids, especially water, every day
• Urinate when you feel the urge – don't hold it in for long
• Empty your bladder completely each time you pee
• After using the toilet, always wipe from front to back
• Wash your genital area with mild soap and water daily

Diet and exercise

A balanced diet helps support your immune system. There is no specific diet for kidney infections, but staying hydrated is key. Avoid too much salt, as it can raise blood pressure and stress your kidneys. Gentle exercise, like walking, is fine once you feel better, but don't exercise while you have a fever or severe fatigue.

Mental health and emotional wellbeing

Having a kidney infection can be stressful and tiring. Pain and fever can make you feel anxious or down. It is normal to feel a little worried about your health. Talk to your doctor or a trusted friend about how you are feeling. If you have a history of anxiety or depression, let your doctor know so they can support you.

Prevention

You can reduce your risk of kidney infections by treating bladder infections early, drinking plenty of water, and practicing good bathroom habits. Staying hydrated, urinating after sex, and wiping front to back are simple steps that help. If you get repeated UTIs, your doctor may recommend additional preventive measures, such as low-dose antibiotics or taking a single antibiotic after sex.

Vaccines

There are no vaccines that prevent kidney infections specifically. However, getting vaccinated against flu and pneumonia can help you avoid infections that weaken your immune system.

Screening programmes

Routine screening for UTIs is not recommended for the general public. However, if you have risk factors such as diabetes or a history of UTIs, your doctor may check your urine at regular check-ups.

Complications

If left untreated

• Kidney damage (scarring that can reduce kidney function over time)
• Sepsis (a life-threatening body-wide infection that can cause organ failure)
• Abscess (a pocket of pus that forms in the kidney and may require drainage)
• High blood pressure (can develop if kidney function is permanently affected)

Long-term outlook

With prompt antibiotic treatment, the outlook is excellent. Most people recover fully with no lasting effects. Untreated kidney infections can cause serious problems, but the vast majority of people get better with medical care. If you have repeated kidney infections, a specialist can help you manage your health to protect your kidneys for the long term.

Find support

International organisations

• National Kidney Foundation ↗
• Urology Care Foundation ↗

Local organisations

• NHS UK ↗ · UK

Helplines

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