Quick answer
What is kidney infection?
A kidney infection (pyelonephritis) is a bacterial infection of the kidneys — usually when a bladder infection spreads upward. Symptoms include fever, flank pain (side of back), nausea, and urinary symptoms. Needs prompt antibiotics — often longer course than simple cystitis. Seek urgent care if very unwell, pregnant, or not improving on antibiotics — hospital IV treatment may be needed.
Kidney infection — pyelonephritis
A kidney infection (acute pyelonephritis) is bacterial infection of the kidneys — usually E. coli ascending from a bladder infection (UTI). It is more serious than cystitis — can cause sepsis if untreated and occasionally permanent kidney scarring.
Symptoms
Urinary (overlap with cystitis):
- pain or burning when urinating
- frequency and urgency
- cloudy or smelly urine
- blood in urine sometimes
Systemic — suggest kidney involvement:
- high fever and rigors (shivering)
- flank pain — aching side/back below ribs — one or both sides
- nausea and vomiting
- general malaise, fatigue
Severe:
- confusion (especially elderly)
- hypotension — sepsis
Men with UTI symptoms — always investigate — may include prostatitis.
Who is at risk
- women — shorter urethra
- pregnancy — higher risk and complications
- diabetes
- kidney stones — obstruction
- structural urinary tract abnormalities
- catheter use
- immunosuppression
Diagnosis
GP assessment:
- history and examination — costovertebral angle tenderness (pain tapping back over kidney)
- urine dipstick — nitrites, leukocytes, blood
- midstream urine culture — identifies bacteria and antibiotics sensitivity
- blood tests — CRP, WCC if systemically unwell
- pregnancy test in women of childbearing age
Imaging (ultrasound/CT) if:
- not improving on antibiotics
- recurrent pyelonephritis
- suspected abscess or stone
Treatment
Outpatient (moderate illness)
Oral antibiotics 7 to 14 days — local guidelines; examples:
- ciprofloxacin
- co-amoxiclav
- trimethoprim — only if sensitive on culture
Paracetamol/ibuprofen — fever and pain
Fluids — prevent dehydration
Inpatient (severe)
- IV antibiotics
- fluids
- monitoring for sepsis
Indications for hospital:
- vomiting unable to take oral antibiotics
- pregnancy
- sepsis signs
- no improvement 48 hours oral
- social factors — cannot manage at home
When to worry during recovery
Contact GP if:
- fever persists >48 hours after starting antibiotics
- worsening pain
- new confusion
- unable to urinate — obstruction
May indicate resistant organism, perinephric abscess, or stone.
Prevention of recurrence
- complete antibiotic courses for UTIs
- treat constipation — affects bladder emptying
- good hydration
- wipe front to back
- pass urine after sex
- investigate recurrent UTIs — ultrasound, urology
Cranberry products — modest evidence for prevention — not treatment.
Kidney infection in pregnancy
Always urgent — increased risk of premature labour and sepsis.
Safe antibiotics selected by specialist — never self-treat.
Complications (rare with prompt treatment)
- sepsis
- kidney abscess
- chronic pyelonephritis and scarring
- papillary necrosis — diabetics
Bladder infections are common; fever plus back pain upgrades urgency — same-day antibiotics prevent escalation to hospital admission.
Common questions
- What is the difference between a UTI and a kidney infection?
- Cystitis (bladder infection) causes pain urinating, frequency, and urgency — usually no high fever or back pain. Pyelonephritis (kidney infection) adds fever, chills, flank (side/back) pain, nausea, and feeling systemically unwell. Kidney infections need longer antibiotics and closer monitoring.
- How is a kidney infection treated?
- Oral antibiotics for 7 to 14 days if moderately unwell — often ciprofloxacin, co-amoxiclav, or others depending on local guidelines and allergies. Hospital admission with IV antibiotics if severe, pregnant, vomiting, or not responding. Urine culture guides choice when available.
- Can a kidney infection damage kidneys?
- Single treated episode rarely causes permanent damage in healthy people. Untreated or severe infection can scar kidneys — especially with repeated infections, obstruction (stones), or structural abnormalities. Follow-up if recurrent UTIs.
- How long does a kidney infection take to clear?
- Fever usually settles within 48 to 72 hours of starting correct antibiotics — finish full course even when better. Fatigue may linger a week. If not improving at 48 hours — contact GP; resistance or abscess possible.
- Can kidney infections recur?
- Yes — especially with kidney stones, incomplete bladder emptying, or anatomical problems. Women with recurrent UTIs may need investigation — ultrasound, urology referral. Prophylactic antibiotics sometimes used after specialist review.