Renal & Urology

Acute Pyelonephritis

Acute pyelonephritis is an upper urinary tract infection involving the renal pelvis and parenchyma; fever, loin pain and systemic upset are the key clues, while sepsis or an obstructed infected system makes it an emergency.

In a nutshell

Think kidney infection when fever or systemic upset accompanies loin pain or urinary symptoms. Culture urine before antibiotics when safe, assess for sepsis and obstruction, use the current NICE antibiotic table, and escalate early in pregnancy, childhood, complicated disease or poor response.

Classic presentation

A patient with dysuria who develops fever, rigors, vomiting and unilateral loin pain with costovertebral-angle tenderness: send urine culture, assess severity and start appropriate upper-UTI antibiotics.

Key points

Fever, rigors and loin pain suggest upper UTI; dysuria may be absent, especially in older or vulnerable patients.
Send urine for culture and susceptibility before antibiotics when safe; if high-risk sepsis is present, do not delay treatment for samples.
Current NICE first-choice oral treatment for a stable non-pregnant adult is cefalexin; co-amoxiclav and trimethoprim require susceptibility, and ciprofloxacin is reserved for when other first-choice antibiotics are unsuitable.
An infected obstructed kidney is a source-control emergency: antibiotics must be accompanied by urgent urological decompression.
Admit or obtain specialist advice for sepsis, vomiting or dehydration, pregnancy, age under 16, obstruction, renal impairment, immunosuppression or significant comorbidity.
Do not use nitrofurantoin or another lower-UTI-only agent as treatment for renal parenchymal infection.
No improvement within 48 hours is a trigger to review cultures, consider imaging and look for obstruction, abscess, resistant infection or an alternative diagnosis.

First-line investigation

Urine dipstick plus urine culture and susceptibility, with sepsis bloods and renal-function assessment when systemically unwell or complicated.

Management

Assess severity and source-control risk

Perform ABCDE assessment, record observations and assess for sepsis, dehydration, acute kidney injury, anuria, pregnancy and obstruction; admit or escalate when outpatient treatment is unsafe.^1,2,4

Culture and treat the upper-tract infection

Send urine culture before antibiotics when safe, then use the current NICE table and local susceptibility information; cefalexin is the first-choice oral adult option, while ciprofloxacin is reserved for when other first-choice antibiotics are unsuitable.^1,8,9,7

Drain an infected obstruction

Fever or sepsis with obstruction, anuria, renal failure or a stone requires urgent urology and interventional-radiology discussion for ureteric stent or percutaneous nephrostomy; antibiotics alone are not adequate source control.^5,2,6

Reassess response and investigate complications

Review response by 48 hours, step down intravenous therapy when possible, and investigate obstruction, abscess, resistant infection or another diagnosis if the patient is not improving; give explicit safety-netting.^1,2,4

Exam traps

Pyelonephritis can occur without dysuria; systemic and loin features are more important than a complete cystitis history.
Do not treat a suspected infected obstruction with antibiotics alone: urgent decompression is the key source-control step.
Ciprofloxacin is not an unqualified first-line option in the current NICE table; check MHRA restrictions and use it only when other first-choice options are unsuitable.
Nitrofurantoin is a lower-UTI drug and should not replace an antibiotic that treats renal parenchymal infection.
Pregnancy and paediatric upper UTI need the specific NICE/BNF pathway; do not transpose an adult regimen or disposition.
Failure to improve within 48 hours is not a reason simply to repeat the same antibiotic: look for resistant organisms, obstruction, abscess or an alternative diagnosis.

Illustrations

Pyonephrosis with perirenal extension on CTAxial and coronal CT showing a destroyed pyonephrotic right kidney with perirenal phlegmon extending into the psoas and posterior abdominal wall.Malihy Z et al., Access Microbiology 2025, CC-BY-4.0 · CC-BY-4.0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.