Recognise / red flags: Fever with flank pain, urinary symptoms, vomiting or systemic illness suggests upper urinary-tract infection. An infected obstructed collecting system is an emergency requiring source control.
Investigate / stratify: Take an MSU for culture before antibiotics if practicable; evaluate sepsis, renal function and previous microbiology. Imaging is indicated when obstruction, stones, atypical disease or failure to improve is suspected.
Initial management: Offer antibiotic treatment guided by severity, cultures, resistance and local guidance; oral therapy if stable and able to tolerate, IV therapy if severely unwell or unable to take oral drugs, with review at 48 hours. Admit with sepsis; consider referral when pregnant, significantly dehydrated or at higher complication risk.
Exam pitfall / safety: Do not apply a blanket “7 days of antibiotics” to all agents and populations: recommended treatment duration varies by medicine, pregnancy and clinical circumstances. Avoid routine fluoroquinolones when safer recommended options are suitable.

Source checked 2026-10-30: NICE NG111. Focused teaching summary only; historical material below has not had a full dose-by-dose review. Use current BNF, local guidelines and specialist advice for individual patients.

🦠 Acute Pyelonephritis is a bacterial infection of the renal parenchyma and collecting system. It represents the severe end of the UTI spectrum and may progress to urosepsis if untreated. ⚠️ Can lead to renal scarring (especially in recurrent or severe cases). Prompt treatment is essential.

🧭 Approach to Suspected UTI: symptoms + severity determine everything
💧 Dysuria + frequency ONLY → Likely Lower UTI → 3-day antibiotics (if female) 🔥 Fever + flank pain ± vomiting → Pyelonephritis → Send MSU + bloods→ Start 7-day antibiotics→ Review at 48h ⚡ Red flags: hypotension, confusion, tachycardia, oliguria → Urosepsis→ 🚨 Emergency admission → IV antibiotics + fluids (Sepsis Six) 💎 Always ask: Could this be obstruction (stone)?, Is the patient pregnant?, Is this a male or recurrent infection? 🩻 If not improving at 48h → Imaging (USS/CT) → Exclude abscess or obstruction

🧭 Approach to Suspected UTI: symptoms + severity determine everything

💧 Dysuria + frequency ONLY → Likely Lower UTI → 3-day antibiotics (if female)
🔥 Fever + flank pain ± vomiting → Pyelonephritis → Send MSU + bloods→ Start 7-day antibiotics→ Review at 48h
⚡ Red flags: hypotension, confusion, tachycardia, oliguria → Urosepsis→ 🚨 Emergency admission → IV antibiotics + fluids (Sepsis Six)
💎 Always ask: Could this be obstruction (stone)?, Is the patient pregnant?, Is this a male or recurrent infection?
🩻 If not improving at 48h → Imaging (USS/CT) → Exclude abscess or obstruction

⚠️ Key rule: Flank pain + fever = treat as pyelonephritis, not simple UTI

🚀 Quick Guide (NICE-aligned)

👩 Lower UTI (women) → 3 days antibiotics
🔥 Pyelonephritis → 7 days antibiotics (review at 48 hours)
👨 Men, pregnancy, catheter → treat as complicated UTI
🚫 Do NOT treat asymptomatic bacteriuria (except in pregnancy)
👵 Do not rely on dipsticks alone in >65s - treat only if symptomatic
🤰 In pregnancy → always send urine for culture and treat asymptomatic bacteriuria

🧠 About

Common infection - ~1 in 3 women experience a UTI in their lifetime.
Usually due to ascending infection from bladder (e.g. E. coli).
Female anatomy (short urethra) predisposes to infection.
In men, UTIs are uncommon → require further evaluation.

📖 Key Definitions

Bacteriuria = bacteria in urine; Pyuria = WBCs in urine
Asymptomatic bacteriuria = >10⁵ CFU/mL without symptoms
UTI = bacteriuria + symptoms and/or inflammatory response

💧 Urinalysis

🧫 Nitrites → suggest Gram-negative organisms (specific)
🧪 Leucocyte esterase → sensitive but less specific
📬 Send MSU for culture in suspected pyelonephritis
⚖️ Always interpret results alongside clinical features

🦠 Aetiology

Ascending infection from perineal flora
Risk factors: obstruction (stones), reflux, catheter, diabetes, pregnancy, immunosuppression

🧫 Common Organisms

Escherichia coli (most common)
Proteus mirabilis (associated with stones)
Klebsiella spp.
Enterococcus faecalis
Staphylococcus saprophyticus (young women)
Staphylococcus aureus → consider haematogenous spread

🩺 Clinical Presentation

💧 Lower UTI symptoms: Dysuria, frequency, urgency, suprapubic pain
🔥 Pyelonephritis: Fever, rigors, flank pain, renal angle tenderness, nausea/vomiting
⚡ Urosepsis: Hypotension, tachycardia, confusion, oliguria

⚡ Red flags: Hypotension, confusion, severe pain, oliguria → suspect urosepsis → urgent admission and IV antibiotics

🔍 Investigations

🧪 Urinalysis and MSU culture (before antibiotics if possible)
🩸 FBC, CRP (inflammation)
💧 U&E (renal function, AKI)
🩸 Blood cultures if febrile or septic
🤰 Pregnancy test if applicable
🩻 Imaging (USS/CT) if no improvement at 48h or suspect obstruction/abscess

💥 Complications

🧫 Bacteraemia / sepsis
💣 Renal or perinephric abscess
🔥 Renal papillary necrosis
🪶 Chronic scarring → CKD (recurrent/severe cases)

💎 Clinical tip: Obstruction + infection (e.g. stone) = urological emergency due to high sepsis risk

💊 Management

🧴 Oral fluids, rest, analgesia (paracetamol ± NSAID if appropriate)
🏥 Admit if:
- Sepsis or haemodynamic instability ⚡
- Unable to tolerate oral antibiotics
- Pregnant
- Suspected obstruction (e.g. stone)
- No improvement after 48 hours
🫀 Manage sepsis using ABC approach + IV fluids + monitoring

⚖️ UTI vs Pyelonephritis vs Urosepsis

Feature	💧 Lower UTI (Cystitis)	🔥 Pyelonephritis	⚡ Urosepsis
Site	Bladder	Kidney (renal parenchyma)	Systemic infection from urinary source
Symptoms	Dysuria, frequency, urgency	Fever, flank pain, nausea/vomiting	Confusion, collapse, severe illness
Pain	Suprapubic	Flank pain, loin-to-groin	May be minimal or diffuse
Systemic features	Absent or mild	Fever, rigors	⚡ Hypotension, tachycardia, delirium
Urinalysis	Nitrites ± leucocytes	Nitrites + leucocytes	May be positive or non-specific
Blood tests	Usually not required	↑WCC, ↑CRP	↑Lactate, AKI, deranged U&E
Imaging	Not needed	If no improvement at 48h	Urgent imaging if obstruction suspected
Antibiotics	3 days (women)	7 days (review at 48h)	IV broad-spectrum immediately
Admission	No	If severe / unable PO	🚨 Always
Key danger	Progression	Sepsis	Multi-organ failure / death

💊 Antibiotic Choice in UTI – Key Principle Treatment depends on site of infection and drug penetration. 💧 Lower UTI (cystitis): Nitrofurantoin and Trimethoprim are appropriate → high urinary concentrations 🔥 Pyelonephritis: Requires antibiotics that penetrate renal tissue + bloodstream (e.g. cefalexin, fluoroquinolones) ❌ Do NOT use nitrofurantoin – poor renal tissue penetration Trimethoprim: Use only if low resistance risk or culture-guided 🎯 Clinical rule: Fever or flank pain = treat as pyelonephritis, NOT simple UTI

💉 Antibiotic Therapy (NICE-aligned)

Setting	Recommended Treatment
Oral (non-pregnant)	• Cefalexin 500 mg TDS for 7 days (first-line) • Ciprofloxacin 500 mg BD for 7 days (only if other recommended antibiotics are unsuitable – MHRA 2024) ⚠️ Co-amoxiclav ONLY if culture confirms sensitivity ⚠️ Fluoroquinolones: risk of tendon rupture, neuropathy, CNS effects
Pregnant (oral)	• Cefalexin 500 mg TDS for 7 days
Severe / IV	• Ceftriaxone OR IV ciprofloxacin • ± Gentamicin (monitor renal function) 🔄 Review at 48h → step down to oral if improving

🧾 Always review at 48 hours and adjust to culture results
🧪 Send urine culture before antibiotics where possible
🩺 Consider further investigation in men or recurrent infections

💡 Exam tip: Fever + flank pain + UTI symptoms = pyelonephritis, not simple cystitis → requires longer treatment

📚 Reference

NICE NG111 / CKS – Urinary tract infection (upper)

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Acute Pyelonephritis and Urosepsis (UTI) ✅

Acute pyelonephritis — infection versus obstruction