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Urinary Tract Infection (UTI) — Comprehensive Overview

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Definition

Critical distinction: The mere presence of bacteria in the urine without symptoms = asymptomatic bacteriuria (ASB) — this is NOT a UTI and generally should NOT be treated (except in pregnancy and pre-urologic procedure settings).

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Classification

• ASB/pyelonephritis: ≥10⁵ organisms/mL
• Cystitis (with pyuria + symptoms): as few as 10² organisms/mL
• CAUTI: ≥10³ CFU/mL

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Epidemiology

• UTI is one of the leading reasons antibiotics are prescribed globally, both in hospitals and the community
• Far more common in women than men — the majority of clinical research involves young adult women
• ASB affects 2–9% of all pregnant women; higher in lower socioeconomic groups, older age, higher parity, women with urinary tract abnormalities, diabetics
• 1–2% of pregnancies complicated by acute cystitis; ~1% by pyelonephritis
• ~70% of women who develop acute pyelonephritis in pregnancy had preceding covert bacteriuria
• Treating ASB in pregnancy reduces pyelonephritis incidence by >80%

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Pathogenesis

Organism Factors

• Virulence is critical: Uropathogenic E. coli (UPEC) possesses fimbriae/pili that allow adhesion to uroepithelial cells, enabling ascent from the perineum up the urethra to the bladder, and further to the kidneys
• Type 1 fimbriae bind to uroplakin on bladder epithelium; P-fimbriae bind to renal tubular epithelium

Host Factors

• Female anatomy: short urethra close to the perineum/anus = easiest access for pathogens
• Genetic background: women who don't express antibody to E. coli O antigen may have chronic colonization
• Behavioral factors: sexual activity, spermicide use, catheterization
• Tissue-specific receptors for bacterial adhesins
• Underlying disease: diabetes mellitus, structural urinary abnormalities, neurogenic bladder, immunosuppression

Environmental Factors

• Vaginal ecology (altered by antibiotics, spermicides, menopause)
• Anatomy/urinary retention: pregnancy causes ureteral dilation (especially right), poor bladder emptying, progesterone-induced smooth muscle relaxation → urinary stasis
• Medical devices: catheters, stents, stones

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Causative Organisms

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Clinical Presentations

1. Acute Uncomplicated Cystitis

• Dysuria (burning/pain on urination)
• Urinary frequency and urgency
• Suprapubic pain/tenderness
• Hematuria (gross or microscopic)
• No fever, no systemic signs

2. Acute Pyelonephritis (Upper UTI)

• Fever, rigors, chills
• Flank pain / costovertebral angle (CVA) tenderness
• Nausea, vomiting, malaise
• May have associated lower urinary symptoms
• Can progress to urosepsis

3. Complicated UTI

• Any UTI with structural/functional abnormality, obstruction, immunocompromise, or catheterization
• Higher risk of treatment failure; broader-spectrum antibiotics often needed
• Includes pyelonephritis, renal abscess, emphysematous pyelonephritis

4. Recurrent UTI

• Relapse: same organism within 2 weeks — suggests inadequately treated upper tract infection
• Reinfection: different organism or same organism after >2 weeks — most common type in women

5. Catheter-Associated UTI (CAUTI)

• Bacteriuria is inevitable with catheterization; therefore, bacteriuria + pyuria alone is NOT sufficient for diagnosis
• Must have localizing urinary symptoms or unexplained fever, AND other causes systematically ruled out
• Threshold: ≥10³ CFU/mL

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Diagnosis

History

Urinalysis (Dipstick)

Urine Culture

• Gold standard for diagnosis
• Required in: complicated UTI, pyelonephritis, pregnancy, recurrent UTI, treatment failure, atypical presentations
• Not routinely required for uncomplicated cystitis in women with classic symptoms
• Midstream clean-catch specimen preferred; catheter specimen in hospitalized patients

Microscopy

• Pyuria (>10 WBC/hpf) present in nearly all symptomatic UTIs
• Bacteriuria on Gram stain of unspun urine → correlates with ≥10⁵ CFU/mL

Imaging

• Not routine for uncomplicated UTI
• CT scan (non-contrast preferred for stones; with contrast for renal abscess, emphysematous pyelonephritis)
• Ultrasound to evaluate hydronephrosis, obstruction
• Voiding cystourethrogram (VCUG) for vesicoureteral reflux (VUR) evaluation

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Management

Acute Uncomplicated Cystitis (Non-Pregnant Women)

Acute Pyelonephritis

• Ciprofloxacin 500 mg PO BID × 7 days (if susceptibility confirmed)
• TMP-SMX 160/800 mg PO BID × 14 days
• Oral beta-lactam × 10–14 days if susceptible

• IV fluoroquinolone or aminoglycoside (± ampicillin for enterococcus coverage)
• Antipseudomonal beta-lactam (piperacillin-tazobactam, ceftriaxone) for complicated/hospital-acquired
• Duration: 7–14 days total; up to 2 weeks for severe cases

Complicated UTI / Pseudomonas UTI

• Remove or replace catheters/stents/stones when possible
• 7–14 days antibiotic treatment; up to 4–6 weeks for renal/perirenal abscess
• Drain abscesses ≥3 cm
• Options: ciprofloxacin IV, levofloxacin, aminoglycosides, antipseudomonal beta-lactams (pip-tazo, carbapenems), ceftazidime-avibactam, ceftolozane-tazobactam for MDR organisms — Goldman-Cecil Medicine International Edition

UTI in Pregnancy

• Screen all pregnant women at 12–16 weeks with urine culture (USPSTF Grade A)
• Treat ASB in pregnancy (up to 30% develop pyelonephritis if untreated)
• Safe antibiotics in pregnancy:
  • Cephalexin 500 mg BID × 3–7 days
  • Amoxicillin 500 mg TID × 7 days
  • Nitrofurantoin 100 mg BID × 3–7 days (avoid in 1st trimester — possible birth defects; avoid near term — neonatal hemolysis)
• Avoid: Fluoroquinolones, tetracyclines throughout pregnancy; TMP-SMX in 1st trimester (folate antagonist) and 3rd trimester (kernicterus risk)
• Pyelonephritis in pregnancy: hospitalize, IV antibiotics (ampicillin + gentamicin), aggressive management — risk of premature labor, maternal sepsis, permanent renal injury
• Right-sided predominance (physiologic right ureteral dilation > left) — Rosen's Emergency Medicine; Tintinalli's Emergency Medicine; Comprehensive Clinical Nephrology 7th Ed.

Recurrent UTI Prevention

• Behavioral: post-coital voiding, adequate hydration, avoid spermicides
• Cranberry products (modest evidence — recent systematic review/meta-analysis, PMID 39030132)
• Antibiotic prophylaxis options:
  • Post-coital single dose (TMP-SMX, nitrofurantoin)
  • Daily low-dose prophylaxis (TMP-SMX 40/200 mg, nitrofurantoin 50–100 mg)
  • Patient-initiated therapy (for women who can self-diagnose reliably)
• Topical vaginal estrogen in postmenopausal women (restores lactobacillus-dominant flora)

CAUTI Prevention & Management

• Remove or change catheter as soon as possible
• Avoid treating ASB in catheterized patients (Choosing Wisely recommendation — AUA)
• If catheter cannot be removed, treat CAUTI with systemic antibiotics for 7–14 days
• Use catheter only when necessary; use hydrophilic catheters for intermittent catheterization (especially in spinal cord injury)

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Complications

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Differential Diagnosis of UTI Symptoms

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Special Populations

Spinal Cord Injury (SCI)

• UTI occurred in 100% of SCI patients with 40–50 year follow-up
• Risk factors: male sex, cervical injury level, condom catheter use
• Treat only symptomatic bacteriuria (National Institute consensus)
• Use antibiotics with minimal impact on normal flora; treat 5–14 days
• Do NOT use prophylactic antibiotics for indwelling or intermittent catheterization routinely — Campbell Walsh Wein Urology

Renal Transplant Recipients

• UTI common complication; hospitalization in 16.8% of patients within first 3 years
• Risk factors: obstructive uropathy, VUR, bladder abnormalities, ureteral stents
• TMP-SMX prophylaxis for P. jiroveci may reduce UTI incidence
• Avoid treating ASB without pyuria (risk of selecting resistant organisms)

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Recent Guidelines (2024)

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Summary Table

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