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Conditions · Abdomen

Acute pyelonephritis !

Acute pyelonephritis is a bacterial kidney infection, usually rising from the bladder; it causes fever, flank pain and often urinary symptoms.

Updated 2026-10-29

Structures involved

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In biochemistry

Microorganisms that cause it

In the clinical exam

Why it happens

What it is

Bacterial infection of the kidney and renal pelvis, usually ascending from the bladder, causing fever, flank pain and costovertebral angle tenderness.

Who gets it

Much more common in women, especially young sexually active women and pregnant women. In Latin America resistance of E. coli to common oral antibiotics, including extended-spectrum beta-lactamase (ESBL) producing strains, is a growing concern.

Causes

  • Escherichia coli (most common)
  • Klebsiella, Proteus and other Enterobacterales
  • Enterococcus, Staphylococcus saprophyticus
  • Pseudomonas and resistant organisms in hospital-acquired or catheter-related infection
  • Rarely haematogenous spread (e.g., Staphylococcus aureus)

Risk factors

  • Female sex, sexual activity, spermicide use
  • Pregnancy
  • Diabetes and immunosuppression
  • Urinary obstruction: stones, prostatic enlargement, strictures
  • Vesicoureteral reflux, neurogenic bladder, urinary catheter or recent instrumentation
  • Previous urinary tract infections

Pathophysiology

Uropathogenic bacteria from the bowel colonise the periurethral area, ascend to the bladder and then up the ureter, helped by adhesins (fimbriae) and by reflux or obstruction. They invade the renal pelvis and parenchyma, causing an acute neutrophilic interstitial inflammation; obstruction or diabetes can lead to abscess, emphysematous infection or bacteraemia.

Classification

  • Uncomplicated (healthy non-pregnant women, normal urinary tract) vs complicated (men, pregnancy, obstruction, catheter, diabetes, immunosuppression)
  • With or without sepsis
  • Special forms: renal or perinephric abscess, emphysematous pyelonephritis, xanthogranulomatous pyelonephritis

Symptoms and signs

  • Fever and chills
  • Flank or back pain, usually on one side
  • Costovertebral angle tenderness
  • Nausea and vomiting
  • Frequency, urgency and pain on urination (may be absent)
  • Older adults may present with confusion or falls

Diagnosis

Clinical
Fever with flank pain and costovertebral angle tenderness, often after cystitis symptoms; assessment of severity, pregnancy and complicating factors.
Tests
Urinalysis: pyuria, bacteriuria, nitrites, sometimes white cell casts. Urine culture with sensitivities before antibiotics. Blood count, creatinine, CRP; blood cultures in severe or complicated cases. Pregnancy test in women of childbearing age.
Imaging
Not needed in uncomplicated cases that improve. Ultrasound or contrast CT if obstruction or abscess is suspected, in sepsis, or when there is no response after 48-72 hours; ultrasound in pregnancy.

Differential diagnosis

  • Kidney stones (renal colic)
  • Acute appendicitis, cholecystitis or diverticulitis
  • Pelvic inflammatory disease or ectopic pregnancy
  • Lower lobe pneumonia
  • Renal infarction
  • Musculoskeletal back pain

Goal of treatment

Eradicate the infection with antibiotics chosen by severity and local resistance, prevent sepsis and kidney damage, and correct any obstruction or underlying cause.

Treatment

Medical
Antibiotics adjusted to culture: oral fluoroquinolones, cephalosporins or other agents for mild cases when local resistance allows; parenteral cephalosporins, aminoglycosides or, for ESBL-producing organisms, carbapenems in severe cases.
Acute
Hospital admission for sepsis, vomiting, pregnancy, obstruction or failed outpatient treatment, with intravenous fluids, antipyretics and analgesia.
Procedure
Urgent drainage of an obstructed infected kidney (ureteral stent or nephrostomy) and percutaneous or surgical drainage of an abscess.
Follow-up
Reassessment at 48-72 hours; imaging if no improvement. In pregnancy, follow-up urine cultures, and evaluation for structural abnormalities in recurrent cases and in children.

Drugs for this condition

Complications

  • Sepsis and septic shock
  • Renal or perinephric abscess
  • Emphysematous pyelonephritis (mainly in diabetes)
  • Papillary necrosis
  • Renal scarring and chronic kidney disease (especially in children with reflux)
  • Preterm labour in pregnancy

Prognosis

With prompt, appropriate antibiotics most patients improve within a few days and recover fully. Outcomes are worse with obstruction, diabetes, older age, resistant organisms or delayed treatment.

Prevention

  • Screening and treatment of asymptomatic bacteriuria in pregnancy
  • Prompt treatment of cystitis
  • Avoiding unnecessary urinary catheters
  • Treating obstruction and urinary tract abnormalities

Test yourself

Think of the answer, then tap the question to see it.

Which organism is the most common cause of acute pyelonephritis?

Escherichia coli.

Which urinary finding suggests kidney involvement rather than isolated cystitis?

White cell casts, which form in the renal tubules.

When is imaging indicated in acute pyelonephritis?

With sepsis, suspected obstruction or abscess, or no clinical improvement after 48-72 hours of appropriate antibiotics.

Why is asymptomatic bacteriuria screened and treated in pregnancy?

Because in pregnancy it carries a high risk of progressing to pyelonephritis, which is linked to preterm labour.

Which severe form of pyelonephritis with gas in the kidney occurs mainly in patients with diabetes?

Emphysematous pyelonephritis.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: Acute Pyelonephritis (StatPearls (NCBI Bookshelf), 2024).

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Representative model, for educational purposes only.

Kidneys highlighted on the Durgical 3D model