Knowledge Centre
Providing rapid medical guidance and expert health insights for informational purposes. Our guides come from a Harley Street clinic, but they do not replace a professional consultation.
Kidney infection: symptoms, treatment and how serious it is
Written by Mr Stephen Lingam · Reviewed by Dr Mohammad Bakhtiar (GMC 4694470) on 25/09/2026
Key takeaways
- A kidney infection (pyelonephritis) is usually a bladder infection that has travelled up to one or both kidneys.
- The difference you feel is being unwell all over: fever, shivering, nausea and pain in your back or side, on top of stinging when you pass urine.
- It needs antibiotics promptly, usually for 7 to 14 days, and a urine culture taken before the first dose guides the choice.
- Most people feel better within about 2 weeks.
- A same-day GP appointment at 117a Harley Street is £150, and renal ultrasound is available on site for infections that keep coming back.
It began as cystitis: stinging, rushing to the toilet, cloudy urine. Then, over a few hours, you started shivering, the temperature climbed and a deep ache settled in your back, just under the ribs on one side.
That change is the classic sign of a kidney infection. Bacteria have moved from the bladder up to the kidney, and your whole body is now reacting, not just the bladder. It is more serious than simple cystitis and needs antibiotics promptly, but with the right treatment most people recover fully.
If that sounds like you, a same-day GP appointment at our Harley Street clinic is £150. Urine tests and any prescription are charged separately.
Kidney infection symptoms
Symptoms tend to come on quickly, often over a few hours, and you usually feel noticeably unwell. That is the main difference from a bladder infection, where the discomfort stays mostly in the bladder.
Bladder infection or kidney infection?
| Bladder infection (cystitis) | Kidney infection (pyelonephritis) | |
|---|---|---|
| Urine symptoms | Stinging, frequency, urgency, cloudy or strong-smelling urine | Often the same, as the infection usually starts in the bladder |
| Pain | Low tummy, above the pubic bone | Back or side, usually one side, sometimes the groin |
| Temperature | Usually normal | High temperature, shivering or chills |
| How you feel | Uncomfortable but otherwise well | Generally unwell, often with nausea or vomiting |
Kidney infection symptoms are the same in women and men. In older adults they can be harder to spot. Confusion, agitation or a sudden decline in how someone is managing may be the main signs, sometimes with no fever at all.
Is a kidney infection serious?
It is a proper infection and needs treating the same day, but it responds well to antibiotics. Left untreated, it can lead to sepsis, a kidney abscess or lasting kidney damage, which is why waiting it out is never the plan.
Get urgent help if you have a high temperature (38°C or above) with shivering, severe pain in your back, side or groin, blood in your urine, vomiting that stops you keeping fluids down, confusion or drowsiness, or you have not passed urine for more than 12 hours. If you feel very unwell, call 999 or go to A&E. For a same-day assessment with us, call 0207 499 1991.
What causes a kidney infection?
E. coli, a bacterium that normally lives in the bowel, causes most kidney infections. If it gets into the urinary tract and is not cleared, it can climb from the bladder to the kidneys.
They are more common in women, partly because a shorter urethra lets bacteria reach the bladder more easily. The risk is also higher with recurrent UTIs, pregnancy (the growing womb can press on the tubes from the kidneys and slow urine flow), kidney stones, a structural difference in the urinary tract, a weakened immune system or a urinary catheter.
Men get kidney infections less often. When they do, it can point to a prostate problem or a blockage to urine flow, which is worth investigating once the infection is treated.
How is a kidney infection tested and treated?
Diagnosis rests on your symptoms, an examination and a urine test. NICE recommends sending a midstream urine sample for culture before the first antibiotic dose. The culture identifies the bacteria and which antibiotics will work, so treatment can be switched quickly if the first choice is not right.
Tests that may be used
| Test | What it shows | When |
|---|---|---|
| Urine dipstick | Signs of infection, such as white cells and nitrites | At the appointment |
| Urine culture | The exact bacteria and which antibiotics work | Before antibiotics start |
| Blood tests | Infection markers and kidney function | If you are very unwell or not improving |
| Renal ultrasound | Stones, blockage or a structural cause | Repeat infections, men, or slow recovery |
Treatment is a course of antibiotics, usually for 7 to 14 days, chosen to match local resistance patterns and then your culture result. Cefalexin is a common first choice. Most people are treated at home with tablets, paracetamol for pain and fever, and plenty of fluids. Finish the course even once you feel better.
NICE advises hospital care if there are signs of sepsis, and considering it if you cannot keep fluids or tablets down, are pregnant or are at higher risk of complications. In hospital, antibiotics and fluids go through a drip, and most people go home after 3 to 7 days.
How long does a kidney infection take to get better?
Most people start to feel better within 48 hours of the right antibiotic, and feel well again after about 2 weeks. It can take up to 2 weeks before you feel ready to return to work, and older people or those with other health conditions may take longer. If you are not improving after 48 hours, contact the doctor, as the antibiotic may need changing once the culture result is back.
Stopping it happening again
Preventing kidney infections mostly means preventing UTIs or catching them early. Drink enough to keep your urine pale, do not hold on for long periods, wipe front to back and pass urine after sex. If you are prone to recurrent UTIs, the doctor may suggest low-dose preventive antibiotics, D-mannose, or antibiotics taken after sex, depending on the pattern.
If you have had two or more kidney infections, a renal ultrasound and a urology opinion are recommended. We can do the ultrasound on site and arrange a urology referral if a structural cause is suspected.
Seeing a doctor at 117a Harley Street
We see suspected kidney infections on the same day. A GP consultation (£150) covers the clinical assessment. Urine tests, including culture, and any prescription are charged separately. Blood tests and imaging, including renal ultrasound, can be arranged here at the clinic.
If you are not sure whether stinging and frequency are a UTI or something else, our guide to UTI or STI testing explains which urine test fits which symptoms.
Sources
- Pyelonephritis should be suspected when UTI symptoms are accompanied by fever, loin pain, or systemic illness. Prompt antibiotic treatment is required to prevent complications including sepsis. — NICE Guideline NG109: Pyelonephritis (acute): antimicrobial prescribing[Link]
- Women are significantly more likely to develop UTIs and kidney infections than men due to a shorter urethra. Approximately 1 in 2 women will experience a UTI in their lifetime. — NHS: Kidney infection[Link]
- Urine culture should be performed before starting antibiotics in suspected pyelonephritis to guide treatment if first-line therapy fails. — NICE Clinical Knowledge Summary: UTI (lower): women[Link]
- Obtain a midstream urine sample before antibiotics are taken and send for culture and susceptibility testing. Refer to hospital if there are signs of a more serious illness such as sepsis; consider referral if significantly dehydrated or unable to take oral fluids and medicines, or pregnant. — NICE guideline NG111: Pyelonephritis (acute): antimicrobial prescribing[Link]
- Antibiotic tablets are usually taken for 7 to 14 days. You will usually feel better after about 2 weeks, and it can take up to 2 weeks before you feel able to return to work. Most people treated in hospital go home after 3 to 7 days. — NHS: Kidney infection[Link]
Mr Stephen Lingam
Managing Director
Executive leadership
Frequently asked questions
What is the difference between a UTI and a kidney infection?
A UTI usually means a bladder infection (cystitis). A kidney infection happens when bacteria travel up from the bladder to one or both kidneys, and it brings fever, back or side pain and nausea that a simple bladder infection does not.
Can a kidney infection go away without antibiotics?
No. Kidney infections need antibiotics, because without them the infection can lead to sepsis, a kidney abscess or permanent kidney damage. If you suspect one, see a doctor the same day.
How long does a kidney infection take to clear?
Most people feel better within 48 hours of the right antibiotic, and the course usually runs 7 to 14 days. Feeling fully well can take about 2 weeks, and if you are not improving after 48 hours, contact your doctor.
When should I go to hospital with a kidney infection?
Call 999 or go to A&E if you feel very unwell, are confused or drowsy, cannot keep fluids down or have not passed urine for more than 12 hours. Pregnancy or a long-term health condition can also mean treatment in hospital.
Can men get kidney infections?
Yes, though less often than women. In men it can point to a prostate problem or a blockage to urine flow, so further tests such as a renal ultrasound are often worthwhile.
Why do I keep getting kidney infections?
Recurrent kidney infections usually start as UTIs that keep coming back or are not fully cleared. Kidney stones, a structural difference, a catheter, a weakened immune system or anything blocking urine flow add to the risk, and after two or more episodes a renal ultrasound and specialist referral are recommended.
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