Urinary Tract Infection (UTI)

Definition & Classification

Epidemiology

• UTI is the most common bacterial infection overall.
• >50% of all healthy women experience at least one symptomatic UTI in their lifetime; 2–10% have at least one episode per year.
• Uncommon in young men — any UTI in a man should be assumed complicated until proven otherwise.
• Prevalence of asymptomatic bacteriuria rises from ~1–2% in schoolgirls to 40–50% of elderly nursing home women.
• ~80% of hospital-acquired UTIs are catheter-associated.

Pathobiology

• Most uncomplicated UTIs arise from ascension of gut flora (predominantly E. coli) colonizing the vagina and periurethral mucosa, ascending into the bladder or kidney.
• Key virulence factors: FimH adhesin (cystitis), P fimbria (pyelonephritis via Galα1-4Galβ binding), toxins, iron-scavenging proteins.
• Adherence activates innate immunity → IL-6, IL-8 release → pyuria + systemic symptoms.
• Normal vaginal Lactobacillus flora maintains an acidic environment that suppresses uropathogens; spermicide use disrupts this and increases risk.
• Sexual intercourse is the strongest behavioral risk factor — 75–90% of episodes in premenopausal women are attributed to intercourse.
• Genetic risk factors: non-secretor ABH blood group status, polymorphisms in innate immunity genes, family history of recurrent UTI.

Common Causative Organisms

Clinical Manifestations

• Dysuria, urinary frequency, urgency, suprapubic discomfort
• No fever
• Rapid onset (within 24 hours)

• Fever, chills, flank pain/tenderness, costovertebral angle (CVA) tenderness
• May or may not have cystitis symptoms
• Bacteremia occurs in 10–30% of patients
• Elevated CRP (not seen in cystitis)

Diagnosis

• Pyuria (WBCs in urine) — present in most symptomatic UTI; absent pyuria has high negative predictive value
• Nitrite dipstick — sensitivity ~90% for gram-negative organisms; Pseudomonas, gram-positives, and fungi are nitrite-negative
• Leukocyte esterase — surrogate for pyuria
• Bacteriuria on Gram stain — >1 bacterium/HPF in uncentrifuged urine is significant

• Not required for uncomplicated cystitis with classic presentation and positive urinalysis
• Required for: pyelonephritis, complicated UTI, urosepsis, recent antibiotic use, persistent symptoms, recurrent/early-recurrence UTI
• Significant bacteriuria: ≥10⁵ CFU/mL (general); ≥10² CFU/mL for E. coli/S. saprophyticus in women with symptoms

• Not indicated in uncomplicated UTI managed outpatient
• CT with IV contrast — best modality for urosepsis, emphysematous pyelonephritis, renal abscess, nephrolithiasis
• Ultrasound — rapid assessment for obstruction, postvoid residual; poor sensitivity for intrarenal gas

Treatment

Uncomplicated Cystitis (Women)

• Nitrofurantoin 100 mg BID × 5 days (avoid if GFR <45 mL/min)
• TMP-SMX 160/800 mg BID × 3 days (only if local resistance <20%)
• Fosfomycin 3 g single dose
• Fluoroquinolones (e.g., ciprofloxacin) — effective but reserved due to collateral resistance; not first-line for uncomplicated cystitis

Acute Uncomplicated Pyelonephritis (Outpatient)

• Ciprofloxacin 500 mg PO BID × 7 days, or 1000 mg XR × 7 days (if local fluoroquinolone resistance <10%)
• TMP-SMX DS BID × 14 days (if susceptibility confirmed)
• Ceftriaxone 1g IV/IM single dose, followed by oral agent based on culture

Complicated UTI (Severe Symptoms — Inpatient)

• Meropenem 1 g IV q8h for 4–5 days → oral step-down guided by sensitivities
• Piperacillin-tazobactam 4.5 g IV q8h × 7 days
• Plazomicin 15 mg/kg IV once daily (for resistant organisms)
• Meropenem-vaborbactam (for KPC-producing organisms)
• Ceftazidime-avibactam / ceftolozane-tazobactam for difficult-to-treat Pseudomonas
• Duration: 7–14 days (up to 2 weeks for severe pyelonephritis)

In Pregnancy

• Asymptomatic bacteriuria must be screened (urine culture at 12–16 weeks or first prenatal visit) and treated — 30% progress to pyelonephritis if untreated
• Safe antibiotics: cephalexin 500 mg BID × 3–7 days, nitrofurantoin 100 mg BID × 3–7 days, amoxicillin 500 mg TID × 7 days, TMP-SMX (avoid in 3rd trimester)
• Acute pyelonephritis in pregnancy → hospitalize; IV ceftriaxone or ampicillin + gentamicin

Asymptomatic Bacteriuria (ASB)

• Pregnant women
• Before urologic procedures with mucosal trauma

Recurrent UTI Prevention

• Behavioral: post-coital voiding, increased fluid intake
• Cranberry products — systematic reviews show modest benefit in prophylaxis (PMID: 39030132)
• Low-dose antibiotic prophylaxis (daily or post-coital) for frequent recurrences
• Topical vaginal estrogen in postmenopausal women
• Identify and address structural abnormalities

Recent Evidence (2024–2026)

A 2024 WikiGuidelines consensus statement (PMID: 39495518, JAMA Network Open) provides updated guidance for UTI prevention, diagnosis, and management in both pediatrics and adults — useful for current practice alignment.

A 2024 systematic review and network meta-analysis (PMID: 39030132) found modest but consistent evidence that cranberry products (juice and tablets) reduce recurrent UTI risk.

2024 ACR updated appropriateness criteria for pediatric UTI imaging (PMID: 38823954).