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 about kidney infection

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.

Sources

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