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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Upper UTI = infection of renal parenchyma. Presents with loin/flank pain, fever/rigors, nausea/vomiting ± lower UTI symptoms
- E. coli is the causative organism in ~80%. Always send MSU for MC&S BEFORE starting antibiotics
- Mild/moderate (outpatient): cefalexin 500 mg BD–TDS for 7–10 days (NICE NG111 first-line)
- Severe/systemically unwell: hospital admission, IV co-amoxiclav 1.2 g TDS or IV piperacillin-tazobactam. Switch to oral when improving
- Complications: renal abscess, perinephric abscess, urosepsis, AKI — USS if not improving by 48–72 hours
Overview
Acute pyelonephritis is a bacterial infection of the kidney, usually resulting from ascending infection from the lower urinary tract. It is characterised by renal parenchymal inflammation and may cause bacteraemia and sepsis. Risk factors include female sex, pregnancy, urinary tract obstruction (stones, BPH), vesicoureteric reflux, catheterisation, diabetes, and immunosuppression. Complicated pyelonephritis occurs in the presence of structural/functional urinary tract abnormalities, and carries a higher risk of treatment failure and complications.
Epidemiology
Pyelonephritis accounts for approximately 100,000 hospital admissions per year in the UK. It is 5 times more common in women than men. Incidence peaks in young sexually active women and during pregnancy. Approximately 20–30% of pyelonephritis cases develop bacteraemia. Mortality is low (~1%) in uncomplicated cases but rises significantly in urosepsis, elderly, or immunocompromised patients.
Clinical Features
Symptoms
Loin/flank pain (unilateral, may radiate to groin)
Fever and rigors (swinging pyrexia)
Nausea and vomiting
Lower urinary tract symptoms (dysuria, frequency, urgency) — may or may not be present
Malaise and anorexia
Confusion (especially in elderly)
Signs
Renal angle tenderness (costovertebral angle tenderness on percussion)
Fever (≥38°C)
Tachycardia and hypotension (sepsis)
Suprapubic tenderness (concurrent cystitis)
Investigations
First-line
MSU for MC&SESSENTIAL — send BEFORE starting antibiotics. Identifies organism and guides antibiotic choice. Pyuria + bacteriuria
Urine dipstickPositive nitrites and leucocytes support diagnosis. Haematuria may be present
BloodsFBC (leucocytosis), CRP (markedly raised), U&Es (AKI?), blood cultures if systemically unwell
Second-line
Blood culturesBefore IV antibiotics — positive in 20–30% of pyelonephritis
Renal tract USSIf not improving within 48–72 hours, recurrent episodes, or suspected obstruction/abscess. Not routinely required for first episode responding to treatment
Specialist
CT abdomen/pelvisIf suspected renal or perinephric abscess, emphysematous pyelonephritis, or obstructing stone with infection
NephrostomyUrgent drainage if obstructed infected kidney (pyonephrosis) — urological emergency
1
Mild/moderate (outpatient)
- First-line: cefalexin 500 mg BD–TDS oral for 7–10 days
- Alternative: co-amoxiclav 500/125 mg TDS for 7–10 days, or trimethoprim 200 mg BD for 14 days (if culture-guided)
- Advise adequate hydration, paracetamol for pain/fever
- Safety-net: return if worsening, not improving by 48 h, or unable to take oral fluids
2
Severe/systemically unwell (inpatient)
- Hospital admission
- IV antibiotics: co-amoxiclav 1.2 g TDS or piperacillin-tazobactam 4.5 g TDS (per local guidelines)
- IV fluids, analgesia, antiemetics
- Switch to oral antibiotics once improving for ≥24 h and tolerating oral intake (total 7–10 day course)
- Sepsis 6 if septic: blood cultures, lactate, IV antibiotics, IV fluids, urine output, oxygen
3
Complicated/obstructed
- Infected obstructed kidney (pyonephrosis) = urological emergency — requires urgent decompression (nephrostomy or ureteric stent)
- Renal abscess ≥3 cm: percutaneous drainage + IV antibiotics
- Emphysematous pyelonephritis (gas-forming infection in diabetics): life-threatening — IV antibiotics + drainage ± nephrectomy
Complications
- Urosepsis: Bacteraemia from renal infection → septic shock. Mortality 20–40% if not promptly treated
- Renal abscess: Collection within renal parenchyma — suspect if not improving after 48–72 h of appropriate antibiotics
- Perinephric abscess: Collection in the perinephric space — requires drainage
- Emphysematous pyelonephritis: Gas-forming infection, predominantly in diabetics — high mortality, requires emergency intervention
- AKI: From sepsis, obstruction, or direct parenchymal damage
- Chronic pyelonephritis: Recurrent infections can cause renal scarring and CKD, especially in childhood
UKMLA Exam Tips
- 1Infected obstructed kidney = EMERGENCY — must decompress with nephrostomy/stent. Antibiotics alone will not work
- 2Emphysematous pyelonephritis: diabetic patient + gas in renal parenchyma on CT — life-threatening
- 3Always send blood cultures AND MSU in pyelonephritis BEFORE starting antibiotics
- 4Pyelonephritis in pregnancy: admit for IV antibiotics — risk of preterm labour and maternal sepsis
- 5Distinguish from renal colic: pyelonephritis = fever + systemic illness. Renal colic = colicky loin-to-groin pain, no fever (unless infected stone)
practicetest your knowledge on pyelonephritisApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — renal and beyond.
open q-bank