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Urinary Tract Infection (UTI) — Nephrology Comprehensive Review

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1. Definitions & Classification

• ASB / pyelonephritis: ≥ 10⁵ organisms/mL
• Symptomatic cystitis: as few as 10²–10³ CFU/mL in the presence of pyuria and symptoms
• Complicated UTI: ≥ 10³ CFU/mL (females), ≥ 10⁴ CFU/mL (males)

Comprehensive Clinical Nephrology, 7th ed., Ch. 53

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2. Epidemiology

• Several million acute uncomplicated cystitis episodes annually in the US
• ~250,000 episodes of acute pyelonephritis per year
• Incidence of cystitis in sexually active young females: ~0.5 per person-year
• Recurrence occurs in 27–44% of healthy females
• Pyelonephritis in young females: ~3 per 1,000 person-years
• Postmenopausal females: ~10% per year symptomatic UTI
• Adult males <50 y: 5–8 per 10,000 annually (much lower than females)
• Catheter-associated UTI (CAUTI): the most common source of gram-negative bacteremia in hospitalized patients

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3. Pathogenesis

Route of Infection

• Ascending route (most common): Uropathogens from rectal/perineal flora colonize the urethra → bladder → ureter → renal parenchyma. Women are at higher risk due to a shorter urethra.
• Hematogenous route: Less common; occurs with organisms like Staphylococcus aureus or Candida in immunocompromised patients.

Microbial Virulence Factors

• Type 1 fimbriae (mannose-sensitive): mediate adhesion to bladder epithelium
• P fimbriae / Pap pili (mannose-resistant): critical for ascending to kidney; bind Gal-Gal moieties on uroepithelium — strains with P fimbriae are strongly associated with pyelonephritis
• Hemolysin, aerobactin, serum resistance: additional virulence factors of uropathogenic E. coli (UPEC)
• Toxin production and iron acquisition further facilitate upper tract infection

Bacterial Biofilm & Intracellular Reservoir

Host Defense Mechanisms

• Urine flow / voiding: flushes bacteria mechanically
• Urinary IgA, lactoferrin, Tamm-Horsfall protein (uromodulin): binding of type 1 fimbriated bacteria
• Urothelial exfoliation: sheds bacteria attached to luminal cells
• Neutrophil influx (pyuria): innate response to lipopolysaccharide (LPS) signaling via TLR4

Predisposing Conditions

• Female anatomy, sexual activity, spermicide use, lack of estrogen (postmenopausal)
• Urinary tract obstruction (stones, BPH, strictures)
• Vesicoureteral reflux (VUR)
• Diabetes mellitus (impaired neutrophil function, glucosuria)
• Neurogenic bladder, indwelling catheter
• Pregnancy (physiologic hydronephrosis/stasis; 70% of pyelonephritis cases have preceding bacteriuria)
• Immunosuppression, sickle cell disease, polycystic kidney disease

Comprehensive Clinical Nephrology, 7th ed., Ch. 53; Tintinalli's Emergency Medicine, Ch. 91

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4. Causative Organisms

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5. Symptoms & Clinical Presentation

Lower UTI (Cystitis)

• Dysuria (burning on urination)
• Urgency and frequency
• Strangury (slow, painful passage of urine drop by drop)
• Suprapubic discomfort / pressure
• Hematuria (in hemorrhagic cystitis)
• Malodorous / turbid urine
• Absence of fever (typically — fever suggests upper tract involvement)

Upper UTI (Pyelonephritis)

• Fever (≥38°C), rigors/chills
• Flank pain / costovertebral angle (CVA) tenderness
• Loin pain (unilateral or bilateral)
• Nausea, vomiting
• Systemic malaise, headache
• Lower urinary tract symptoms (in ~50–60% of cases)
• May progress to urosepsis: hypotension, tachycardia, confusion, multi-organ dysfunction

Complicated UTI — Additional Features

• Vague or nonspecific symptoms: fatigue, irritability, abdominal/back pain
• Presentations can be atypical, especially in elderly, diabetic, immunocompromised, or catheterized patients
• Occult renal or prostatic seeding common even when presenting as "cystitis"

Asymptomatic Bacteriuria (ASB)

• By definition: no symptoms. Important in pregnancy and pre-urological procedures; screen and treat; do not treat in most other populations

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6. UTI Classification / Grades (Severity & Complexity)

By Anatomical Level

By Complexity (Clinical Classification)

• Male sex
• Pregnancy
• Urinary catheter, stent, or nephrolithiasis
• Neurogenic bladder / spinal cord injury
• Diabetes mellitus, sickle cell disease
• Immunosuppression (chemotherapy, HIV, transplant)
• Polycystic kidney disease
• Urinary tract obstruction
• Structural genitourinary anomaly
• Advanced age / nursing home residency
• Recent GU instrumentation
• Known antibiotic resistance

Severe Upper Tract Patterns (Imaging-Defined)

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7. Preliminary (Clinical) Diagnosis

History

• Duration, character, and combination of urinary symptoms
• Sexual activity, contraception (spermicide use)
• Previous UTIs, antibiotics, culture results
• Comorbidities (diabetes, renal disease, immunosuppression)
• Catheter or recent instrumentation
• Fever/chills → suggests pyelonephritis

Physical Examination

• CVA tenderness (Murphy's kidney punch) → pyelonephritis
• Suprapubic tenderness → cystitis
• Pelvic/vaginal exam to exclude STI
• Urethral discharge → urethritis / STI
• Signs of sepsis: hypotension, tachycardia, fever, altered mental status

Urinalysis (Dipstick + Microscopy)

Urine Culture

• Gold standard
• When to obtain: All cases of pyelonephritis, complicated UTI, males, pregnancy, treatment failure, recurrent UTI, prior to urological procedures
• When culture may be omitted: Typical uncomplicated cystitis in young females (clinical diagnosis sufficient)
• Sensitivity & susceptibility guides targeted therapy

Blood Tests

• CBC with differential: leukocytosis with left shift in pyelonephritis/urosepsis
• CRP, procalcitonin (elevated in upper tract / systemic infection)
• Blood cultures (2 sets): obtain in pyelonephritis requiring admission, urosepsis
• BMP/renal function (creatinine, BUN, electrolytes) — especially in complicated/severe UTI
• Blood glucose (screen for undiagnosed diabetes)

Imaging

Standard admission: admitted patients require imaging if no clinical improvement within 48–72 h

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8. Treatment

A. Uncomplicated Cystitis (Young, Non-pregnant Female)

Fluoroquinolones should be reserved due to collateral damage (resistance, C. difficile, tendinopathy). Nitrofurantoin contraindicated if eGFR <30 mL/min.

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B. Uncomplicated Pyelonephritis (Outpatient)

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C. Complicated UTI (Including Inpatient Pyelonephritis)

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D. UTI in Pregnancy

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E. Asymptomatic Bacteriuria — Treat Only In:

1. Pregnancy (screen and treat all)
2. Pre-urological/cystoscopic procedure
3. Renal transplant recipients (controversial — only within first month post-transplant)

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F. Recurrent UTI — Prevention

• Behavioral modification: post-coital voiding, hydration, avoid spermicide
• Post-coital prophylaxis: TMP-SMX 40/200 mg or nitrofurantoin 50 mg within 2 hours
• Continuous low-dose prophylaxis: nitrofurantoin 50–100 mg nightly or TMP-SMX 40/200 mg nightly (6–12 months)
• Self-start therapy (patient-initiated): for compliant patients with classic symptoms
• Topical estrogen (postmenopausal): restores lactobacilli flora, reduces recurrences
• Cranberry products: modest evidence; proanthocyanidins inhibit fimbriated E. coli adhesion
• Treat underlying anatomic abnormality (VUR, obstruction, stones) if identified

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G. Catheter-Associated UTI (CAUTI)

• Remove catheter if clinically feasible — this alone resolves many infections
• 7–10 days for complicated CAUTI
• Avoid treating asymptomatic bacteriuria in catheterized patients
• Consider catheter change for symptomatic CAUTI before starting antibiotics

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9. Vesicoureteral Reflux (VUR) Grade & Nephrology Significance

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10. Complications

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Key Mnemonics & Quick-Reference

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Recent Evidence Update (PubMed 2024–2025)

• [PMID 39495518] — WikiGuidelines Group 2024 consensus (JAMA Network Open): comprehensive UTI guidelines for pediatrics and adults, emphasizing antibiotic stewardship and limiting fluoroquinolone use.
• [PMID 40228579] — 2025 meta-analysis (Clin Microbiol Infect): short-course antibiotics for pyelonephritis/complicated UTI comparable to long courses in RCTs — supports 5–7 day therapy where possible.
• [PMID 38916643] — Systematic review on catheter replacement in CAUTI: catheter removal/replacement at time of antibiotic initiation remains recommended.

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Pyelonephritis — Detailed MBBS Nephrology Notes

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1. Definition

• Acute pyelonephritis (APN): Acute suppurative bacterial infection of the kidney
• Chronic pyelonephritis (CPN): Repeated or persistent renal infection leading to progressive scarring, calyceal deformity, and eventual loss of renal function

Harrison's Principles of Internal Medicine, 22e, Ch. 140; Tintinalli's Emergency Medicine, Ch. 91

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2. Epidemiology

• ~250,000 episodes of acute pyelonephritis annually in the US
• Incidence in young females: ~3 per 1,000 person-years
• More common in pregnant women with urological abnormalities or diabetes
• Right kidney more commonly affected in pregnancy (greater physiologic ureteral dilation on right)
• ~70% of women who develop APN have preceding covert bacteriuria
• Up to 30% of women with untreated asymptomatic bacteriuria develop pyelonephritis

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3. Etiology / Causative Organisms

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4. Pathogenesis

A. Routes of Infection

Perineal/fecal flora → Urethra → Bladder → Ureter → Renal pelvis → Parenchyma

• Women are at higher risk due to shorter urethra (4 cm vs. 20 cm in males)
• Predisposed by: sexual activity, pregnancy (hydroureter/stasis), VUR, obstruction

• S. aureus, Candida seed the kidney from a distant bacteremic source
• Leads to cortical (not medullary) abscess formation initially
• Suspect in IV drug users, endocarditis, immunocompromised

B. Bacterial Virulence Factors

C. Host Risk Factors

• Female sex (anatomic)
• Vesicoureteral reflux (VUR) — allows ascent of infected urine to kidney
• Urinary obstruction (stones, BPH, strictures, tumor) — impairs flushing
• Pregnancy — progesterone-mediated ureteral dilation → stasis
• Diabetes mellitus — impaired neutrophil chemotaxis, glucosuria, autonomic neuropathy → incomplete bladder emptying
• Neurogenic bladder — incomplete voiding
• Indwelling catheters / urinary stents
• Immunosuppression (HIV, transplant, chemotherapy)
• Polycystic kidney disease — cysts as protected niches
• Previous UTI / renal scarring

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5. Pathology

A. Acute Pyelonephritis

• Kidney enlarged, swollen, congested
• Cortical surface shows multiple yellow-white abscesses (suppurative foci)
• Pelvis and calyces are hyperemic, may contain pus
• In severe cases: cortical and medullary involvement with necrotic foci

• Polymorphonuclear leukocyte (neutrophil) infiltration of the renal interstitium and tubules — the hallmark
• Tubular necrosis — tubules filled with neutrophils forming "neutrophilic tubular casts"
• Preservation of glomeruli in early stages (glomeruli are relatively resistant)
• Interstitial edema and vascular congestion
• WBC casts in tubular lumens — pathognomonic of upper UTI (renal parenchymal involvement)
• Granulomatous inflammation → suspect fungal (Aspergillus, Candida) or mycobacterial (TB) infection

B. Chronic Pyelonephritis

• Kidney diffusely contracted, scarred, and pitted
• Scars are "Y-shaped," flat, broad-based depressions with red-brown granular bases
• Scarring is preferentially polar (upper and lower poles — where intrarenal reflux is greatest)
• Calyceal blunting and dilation underlying cortical scars
• Parenchyma thin; loss of corticomedullary demarcation
• Asymmetric involvement (unlike symmetric diabetic nephropathy)

• Patchy interstitial fibrosis with lymphocytes, plasma cells, occasional PMNs
• Tubular atrophy and dilation — "thyroidization of the kidney" (colloid-like casts in atrophic tubules, resembling thyroid follicles)
• Periglomerular fibrosis → eventual glomerular obsolescence
• Vascular changes in advanced disease (intimal thickening)
• Compensatory hypertrophy of surviving nephrons

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6. Clinical Features

Acute Pyelonephritis

• High fever, rigors, hypotension → urosepsis (septic shock)
• Acute kidney injury (AKI) — especially in pregnancy (25% of gestational pyelonephritis), bilateral disease, obstruction
• Mental status changes in elderly

Chronic Pyelonephritis

• Asymptomatic until renal insufficiency develops
• History of recurrent febrile UTIs with fever, flank pain, dysuria
• Hypertension (common in advanced disease)
• Polyuria/nocturia (tubular dysfunction — impaired concentrating ability)
• Anemia of chronic kidney disease
• Signs and symptoms of CKD in advanced stages

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7. Diagnosis

Clinical Assessment

Urinalysis

Urine Culture (Gold Standard)

• Quantitative culture: ≥10⁵ CFU/mL of single uropathogen (classic threshold)
• Culture and sensitivity mandatory in all cases of pyelonephritis
• Collect mid-stream clean-catch specimen before antibiotics
• Sensitivity and susceptibility guide targeted therapy

Blood Investigations

Imaging

• Striated nephrogram — wedge-shaped, lobar hypodense areas extending from medulla to cortex (pathognomonic of focal bacterial nephritis)
• Perinephric stranding — inflammatory reaction
• Renal/perinephric abscess — ring-enhancing fluid collection
• Gas in renal tissue → emphysematous pyelonephritis
• Hydronephrosis → obstruction (stone/stricture)

• First-line in pregnancy, children, and bedside evaluation
• May be normal in uncomplicated APN (poor sensitivity ~30%)
• Shows: hydronephrosis, abscess, echogenic kidney, stones, reduced corticomedullary differentiation
• Cannot diagnose focal APN reliably

• Gold standard for detecting cortical scarring
• Shows photopenic (cold) areas = scars from past episodes
• Used in children post-febrile UTI to assess for reflux nephropathy

• Asymmetric, irregular, contracted kidneys
• Calyceal clubbing and blunting
• Cortical scars overlying dilated calyces
• Cortical thinning — preferentially at poles

• Detects and grades vesicoureteral reflux (VUR)
• Important in children with first febrile UTI and in adults with recurrent pyelonephritis

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8. Complicated Forms of Pyelonephritis

A. Acute Lobar Nephronia (Acute Bacterial Nephritis)

• Severe focal form of pyelonephritis without liquefaction/abscess
• CT: wedge-shaped hypodense mass, does not rim-enhance
• Treat with prolonged IV antibiotics (2–3 weeks); abscess may develop if undertreated

B. Renal Cortical Abscess

• Localized liquefactive necrosis within renal parenchyma
• Often from hematogenous route (S. aureus) in IVDU / endocarditis
• CT: ring-enhancing fluid-filled cavity
• Management: Antibiotics alone if <3 cm; percutaneous drainage if ≥3 cm; nephrectomy if nonfunctioning

C. Renal Corticomedullary Abscess

• From ascending route (complicated UTI, obstruction)
• CT: fluid-filled lesion crossing corticomedullary junction
• Percutaneous drainage + antibiotics

D. Perinephric Abscess

• Extension of infection through the renal capsule into Gerota's fascia
• May track into psoas muscle, diaphragm, or flank
• Hallmark: No improvement after 4–5 days of antibiotics for apparent pyelonephritis
• CT: collection with surrounding fat stranding in perirenal space
• Management: Percutaneous drainage (even small abscesses >3 cm) + prolonged antibiotics (4–6 weeks); nephrectomy if nonfunctioning kidney

E. Emphysematous Pyelonephritis (EPN)

• Life-threatening gas-forming infection of the renal parenchyma
• Organisms: E. coli, Klebsiella (facultative anaerobes fermenting glucose → gas)
• Occurs almost exclusively in poorly controlled diabetics
• CT (diagnostic): gas within renal parenchyma, perinephric space, or collecting system
• Mortality up to 40% without nephrectomy

• Management: Aggressive IV antibiotics + percutaneous nephrostomy as initial intervention; elective nephrectomy if required

F. Emphysematous Cystitis

• Gas within the bladder wall (not parenchyma)
• Also in diabetics; less severe than EPN
• CT: air in bladder wall
• Usually responds to IV antibiotics alone

G. Xanthogranulomatous Pyelonephritis (XGP)

• Chronic, destructive, suppurative granulomatous inflammation of the kidney
• Caused by: chronic obstruction (staghorn calculus) + chronic infection (Proteus mirabilis 38%, E. coli 33%)
• Virtually always unilateral; F:M = 2:1; middle-aged women
• Pathology: Replacement of renal parenchyma by lipid-laden foamy macrophages (xanthoma cells) + granulomatous tissue + areas of necrosis
• Extension into perinephric fat, psoas, diaphragm possible

• Clinical features: Fever, flank/abdominal pain, weight loss, anorexia, recurrent UTIs; all patients have renal calculi; nonfunctioning kidney
• CT: Enlarged kidney, multiple fluid-filled cavities replacing parenchyma, staghorn calculus, perinephric involvement — "bear paw sign"
• Differential: Renal cell carcinoma, renal TB, renal abscess
• Management: Nephrectomy (definitive); antibiotics have only secondary role. Partial nephrectomy if early/focal disease

H. Pyonephrosis

• Infected, obstructed collecting system (pus filling the hydronephrotic kidney)
• Medical emergency: urosepsis + renal destruction if untreated
• Ultrasound: hydronephrosis with echogenic debris
• Management: Emergency nephrostomy or ureteric stent to decompress + IV antibiotics; definitive treatment of obstruction thereafter

I. Renal Papillary Necrosis

• Ischemic necrosis of renal papillae complicating pyelonephritis
• Especially in: diabetes, sickle cell disease, analgesic nephropathy, obstruction
• Sloughed papillae may obstruct ureter → obstructive AKI
• Bilateral papillary necrosis → rapid rise in serum creatinine
• IVP/CT: "ring shadow" or "lobster claw" deformity of calyx

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9. Treatment

A. Uncomplicated Acute Pyelonephritis (Outpatient)

• Clinically stable, tolerating oral fluids, mild-moderate illness, no complicating factors

Consider initial parenteral dose (IV ceftriaxone 1 g or IM gentamicin) before discharge for reliable empirical coverage, especially if local fluoroquinolone resistance is a concern.

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B. Severe / Hospitalized Pyelonephritis (Inpatient IV Therapy)

• Unable to tolerate oral medications (vomiting)
• Urosepsis / hemodynamic instability
• Pregnancy
• Extremes of age (elderly, neonates)
• Immunocompromised / diabetics with severe illness
• Suspected obstruction or abscess
• No improvement after 48–72 h outpatient therapy

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C. Pyelonephritis in Pregnancy

• Admit all pregnant women (standard practice)
• IV cephalosporin (ceftriaxone) or ampicillin + gentamicin until afebrile → oral step-down
• Total course: 14 days
• Oral suppressive antibiotics until delivery to prevent recurrence (recurrence rate 6–8%)
• AKI in 25% — monitor renal function
• Avoid fluoroquinolones (cartilage toxicity, teratogenicity), TMP in 1st trimester

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D. Complicated Pyelonephritis / MDR Organisms

• Correct obstructing lesion (stone removal, stent, nephrostomy)
• Remove catheter/stent if possible
• Drain any abscess ≥3 cm (percutaneous under CT/US guidance)
• Nephrectomy for emphysematous PNeph (severe) or XGP

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E. Follow-up After Treatment

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10. Complications

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11. Chronic Pyelonephritis → Reflux Nephropathy

VUR (Grade III–V) + Recurrent UTI
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Intrarenal reflux → bacteria reach collecting ducts
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Inflammatory response → fibrosis → cortical scar
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Loss of nephrons → compensatory hypertrophy of remaining nephrons
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Hypertension + proteinuria + progressive CKD → ESRD

• Dilated atrophic tubules filled with eosinophilic, colloid-like casts
• Resembles thyroid follicles microscopically
• Surrounded by interstitial fibrosis and lymphocytic infiltration

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12. Key Differentials of Pyelonephritis

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13. MBBS Exam High-Yield Points

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