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Pyelonephritis - acute

SNOMED: 36689008Updated 03/03/2026
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Exam Tips

  • Classic exam triad: fever + flank pain + nausea/vomiting, but any element may be absent early or in frail/immunocompromised patients.
  • Do not delay empiric antibiotics for dipstick testing when pyelonephritis is clinically suspected; obtain MSU/CSU culture first.
  • Dipsticks are less reliable in older adults (>65 years) and in catheterised patients; rely on clinical assessment and culture.
  • Escalate urgently for sepsis signs, inability to tolerate oral treatment, pregnancy, significant renal impairment, or no improvement by 48 hours.
  • Recurrent pyelonephritis needs specialist assessment for structural/functional urinary tract disease.

Definition

Acute pyelonephritis is a bacterial infection of the renal pelvis and kidney parenchyma, usually caused by ascent of organisms from the lower urinary tract. It presents as an upper UTI with systemic illness (for example fever, flank pain, nausea/vomiting) and requires prompt treatment to prevent sepsis and renal complications.

Pathophysiology

Most cases are ascending infections: uropathogens colonise the periurethral area, enter the bladder, then reflux or ascend via the ureter to the kidney, triggering interstitial inflammation and tubular injury. Escherichia coli is the dominant organism (about 60-80% in uncomplicated disease), with other Gram-negative organisms (Klebsiella, Proteus, Pseudomonas, Enterobacter) and occasional Gram-positive pathogens (for example Enterococcus faecalis, Staphylococcus saprophyticus, Staphylococcus aureus). Host/anatomical factors (obstruction, catheters, stones, reflux, diabetes, immunosuppression, pregnancy) increase bacterial persistence and risk of bacteraemia, abscess or emphysematous infection. See Figure: ascending UTI-to-kidney pathway on a renal tract anatomy diagram.

Risk Factors

  • Female sex and prior urinary tract infection
  • Age over 65 years
  • Pregnancy
  • Diabetes mellitus
  • Immunosuppression (including cancer therapy or HIV/AIDS)
  • Structural/functional urinary tract abnormality (for example vesicoureteric reflux, polycystic kidney disease, obstruction)
  • Renal stones or urinary tract foreign body (urinary/ureteric/nephrostomy catheter)
  • Chronic kidney disease or acute kidney injury
  • Recent instrumentation of urinary tract

Clinical Features

Symptoms

  • Flank/loin pain (often unilateral)
  • Feverishness, rigors, flu-like malaise, myalgia
  • Nausea and/or vomiting
  • Lower UTI symptoms may coexist: dysuria, frequency, urgency
  • Sometimes no lower urinary symptoms despite upper UTI

Signs

  • Temperature typically >=37.9 C (or <36 C may occur in older frail adults)
  • Costovertebral angle (renal angle) tenderness
  • Tachycardia and dehydration in more severe illness
  • Features of sepsis in complicated cases (hypotension, altered mental state, tachypnoea)

Investigations

Midstream urine (MSU) or catheter specimen urine (CSU) for microscopy, culture and sensitivities before antibiotics:Significant bacteriuria with a urinary pathogen; guides targeted therapy
Urine dipstick (adjunct only, not required for diagnosis):May show leukocytes/nitrites, but does not reliably exclude pyelonephritis; avoid reliance in age >65 or with indwelling catheter
Blood tests (FBC, CRP, U&E, creatinine, eGFR):Inflammatory response (raised WBC/CRP) and renal impairment if complicated
Blood cultures (if systemically unwell/sepsis):May identify bacteraemia and causative organism
Imaging (renal ultrasound first-line; CT KUB/CT abdomen if severe, atypical, or not improving by 48 hours):Hydronephrosis/obstruction, stones, renal or perinephric abscess, emphysematous change
Pregnancy test where relevant:Positive result changes antibiotic choice and threshold for admission

Management

Lifestyle Modifications

  • Encourage oral hydration and rest if managed in community
  • Safety-net clearly: urgent review for worsening symptoms, sepsis features, or no improvement within 48 hours
  • Analgesia/antipyretics as needed (for example paracetamol), and avoid dehydration

Pharmacological Treatment

Empiric oral antibiotics (adults >=16 years, non-pregnant, not severely unwell; tailor to culture/local resistance)

  • Cefalexin 500 mg two or three times daily for 7 days
  • Co-amoxiclav 625 mg three times daily for 7 days
  • Ciprofloxacin 500 mg twice daily for 7 days (only if appropriate)
  • Trimethoprim 200 mg twice daily for 14 days (if susceptibility likely/confirmed)

Send urine for culture before first dose. Reassess at 48 hours; switch to directed therapy once sensitivities available.

Pregnancy

  • Cefalexin 500 mg two or three times daily for 7 days (typical oral choice)

Lower threshold for hospital assessment/admission. Avoid teratogenic options; trimethoprim is generally avoided in first trimester unless specialist advice and folate strategy are used.

Initial IV therapy for severe/systemically unwell cases (hospital)

  • Ceftriaxone 1-2 g once daily IV
  • Cefuroxime 750 mg IV three to four times daily
  • Co-amoxiclav 1.2 g IV three times daily
  • Gentamicin 5-7 mg/kg IV once daily (local protocol)

Step down to oral therapy when clinically improved and cultures available; total duration commonly 7-10 days depending on agent and response.

Important contraindications and safety warnings

  • Fluoroquinolones (for example ciprofloxacin): avoid in pregnancy; caution with tendon disorders, aortic aneurysm risk, CNS effects, QT prolongation
  • Trimethoprim: caution/avoid in first trimester, severe folate deficiency, and with ACE inhibitors/ARBs/spironolactone due to hyperkalaemia risk
  • Beta-lactams (for example cefalexin/co-amoxiclav): avoid in true severe penicillin allergy; co-amoxiclav can cause cholestatic/hepatitic injury
  • Aminoglycosides (for example gentamicin): nephrotoxicity/ototoxicity risk, dose-adjust and monitor levels/renal function

Always check allergy status, renal function, interaction profile, and local antimicrobial guidance.

Surgical / Interventional

  • Urgent decompression for obstructed infected system (ureteric stent or percutaneous nephrostomy)
  • Drainage of renal/perinephric abscess if present
  • Definitive management of underlying cause (for example stone treatment) once infection controlled

Complications

  • Sepsis and septic shock
  • Renal or perinephric abscess
  • Renal parenchymal scarring
  • Recurrent upper/lower UTI
  • Acute kidney injury
  • Emphysematous pyelonephritis
  • Adverse pregnancy outcomes including preterm labour

Prognosis

With prompt appropriate antibiotics, most uncomplicated cases improve within days and recover fully over days to weeks. Prognosis is worse in older adults and in people with comorbidity, renal tract abnormalities, delayed treatment, or treatment failure, where risk of sepsis and structural renal complications is higher.

Sources & References

💊BNF Drug References(5)

✅NICE Guidelines(1)

📖Textbook References(20)

  • David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 874)[context]
  • David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 874)[context]
  • David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 874)[context]
  • David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 716)[context]
  • David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 1824)[context]
  • David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 872)[context]
  • David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 874)[context]
  • David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 872, 873)[context]
  • David Randall PhD MRCP (Editor), John Booth PhD MRCP (Editor), K - Kumar and Clark's Clinical Medicine (2025, American Elsevier Publishing Co.) - libgen.li.pdf(pp. 716)[context]
  • Emergencies in - Obstetrics and Gynaecology, Second Edition (Stergios K. Doumouchtsis, S. Arulkumaran) (Z-Library).pdf(pp. 361, 362)[context]
  • Emergencies in - Obstetrics and Gynaecology, Second Edition (Stergios K. Doumouchtsis, S. Arulkumaran) (Z-Library).pdf(pp. 50, 51)[context]
  • Emergencies in - Obstetrics and Gynaecology, Second Edition (Stergios K. Doumouchtsis, S. Arulkumaran) (Z-Library).pdf(pp. 362)[context]
  • Guyton and Hall Textbook of Medical Physiology (John E. Hall, Michael E. Hall) (Z-Library).pdf(pp. 428, 429)[context]
  • Guyton and Hall Textbook of Medical Physiology (John E. Hall, Michael E. Hall) (Z-Library).pdf(pp. 428)[context]
  • [Williams, Bailey and Love's Short Practice of Surgery] Norman Williams, Christopher Bulstrode, P Ronan O'Connell - Bailey & Love's Short Practice of Surgery 26E (2013, CRC Press) - libgen.li.pdf(pp. 1316)[context]
  • [Williams, Bailey and Love's Short Practice of Surgery] Norman Williams, Christopher Bulstrode, P Ronan O'Connell - Bailey & Love's Short Practice of Surgery 26E (2013, CRC Press) - libgen.li.pdf(pp. 1222)[context]
  • [Williams, Bailey and Love's Short Practice of Surgery] Norman Williams, Christopher Bulstrode, P Ronan O'Connell - Bailey & Love's Short Practice of Surgery 26E (2013, CRC Press) - libgen.li.pdf(pp. 1317)[context]
  • [Williams, Bailey and Love's Short Practice of Surgery] Norman Williams, Christopher Bulstrode, P Ronan O'Connell - Bailey & Love's Short Practice of Surgery 26E (2013, CRC Press) - libgen.li.pdf(pp. 1315, 1316)[context]
  • [Williams, Bailey and Love's Short Practice of Surgery] Norman Williams, Christopher Bulstrode, P Ronan O'Connell - Bailey & Love's Short Practice of Surgery 26E (2013, CRC Press) - libgen.li.pdf(pp. 1222)[context]
  • [Williams, Bailey and Love's Short Practice of Surgery] Norman Williams, Christopher Bulstrode, P Ronan O'Connell - Bailey & Love's Short Practice of Surgery 26E (2013, CRC Press) - libgen.li.pdf(pp. 1124)[context]

Sources

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