Urinary Tract Infection (UTI) - Comprehensive Review

1. Definition and Classification

1. Site of infection - lower tract (cystitis, urethritis) vs upper tract (pyelonephritis, renal abscess)
2. Tendency to recur - isolated vs recurrent
3. Presence/absence of complicating factors - uncomplicated vs complicated

Categories of UTI (Comprehensive Clinical Nephrology)

2. Epidemiology

• UTI is the most common bacterial infection overall
• More than half of all healthy women experience at least one symptomatic UTI in their lifetime
• Annual incidence in sexually active young females: ~0.5 per 1 person-year
• Acute uncomplicated cystitis recurs in 27-44% of healthy females
• Incidence of pyelonephritis in young females: ~3 per 1000 person-years
• Self-reported incidence in postmenopausal females: ~10% per year
• UTI is uncommon in men with a normal GU tract but increases after age 65 (primarily due to prostatic hypertrophy and prostatitis)
• In young men, incidence is 5-8 per 10,000 per year
• ~80% of hospital-acquired UTIs are a consequence of indwelling bladder catheters

• 1-2% of schoolgirls
• 3-5% of sexually active premenopausal women
• 10-20% of healthy postmenopausal women
• 40-50% of elderly women in nursing homes
• 50% of patients with neurogenic bladder (without indwelling catheter)
• Virtually 100% of patients with chronic indwelling catheters

3. Microbiology / Causative Organisms

• Escherichia coli - accounts for 80-85% of all community-acquired UTIs (the most common by far)
• Staphylococcus saprophyticus - second most common in young sexually active women (5-15%)
• Klebsiella pneumoniae
• Proteus mirabilis
• Enterococcus faecalis

• The Enterobacterales remain dominant, but:
• Pseudomonas aeruginosa - especially catheter-associated and nosocomial
• Enterococcus spp. - particularly in institutionalized or catheterized patients
• Staphylococcus aureus - including MRSA in healthcare settings
• Candida spp. - especially in immunocompromised, ICU, or catheterized patients
• Klebsiella, Proteus, Enterobacter, Serratia - especially after instrumentation

4. Pathogenesis

Route of Infection

Bacterial Virulence Factors (Goldman-Cecil Medicine)

• Type 1 fimbriae (FimH adhesin) - binds uroplakin receptors on urothelial cells; facilitates bladder invasion and formation of intracellular bacterial communities (IBCs)
• P fimbriae (Pap adhesin) - binds globoside receptors in the upper urinary tract; critical for pyelonephritis
• Alpha-hemolysin - damages urothelial cells and promotes iron acquisition
• Aerobactin / siderophores - iron-scavenging proteins enabling growth in iron-poor urine
• Capsular polysaccharides (K antigens) - resist phagocytosis and complement
• Lipopolysaccharide (LPS, O antigen) - triggers inflammatory cascade

Host Defense Mechanisms

• Mechanical flushing by voiding (most important)
• Urine properties: low pH, high osmolality, urea content inhibit growth
• Uromodulin - traps bacteria and promotes their excretion
• Bladder epithelium - secretes IgA, antimicrobial peptides (defensins), IL-6, IL-8
• Vaginal lactobacilli - maintain low vaginal pH, competitive colonization resistance
• Vesicoureteral junction - prevents urine reflux to kidneys

Pathogenesis of Complicated UTI (Comprehensive Clinical Nephrology)

• Causing obstruction or stasis of urine flow
• Facilitating ascent of bacteria by impairing normal host defenses
• Allowing bacterial persistence despite antimicrobial therapy

5. Risk Factors

Risk Factors for Uncomplicated UTI in Women

• Sexual intercourse - the most important modifiable risk factor in premenopausal women; frequency of intercourse is directly proportional to UTI risk
• Use of spermicides (especially with diaphragm) - disrupts vaginal flora, eradicates protective lactobacilli, facilitates E. coli colonization
• New sexual partner - brings new bacterial strains
• Prior UTI history - strongest predictor of recurrence
• First UTI before age 15
• Maternal history of UTI - suggests genetic susceptibility

Postmenopausal Women

• Estrogen deficiency causes:
  • Loss of vaginal lactobacilli
  • Increased vaginal pH
  • Increased periurethral colonization by uropathogens
• Urinary incontinence and incomplete bladder emptying
• Cystocele, uterine prolapse
• Reduced mobility and toileting ability

Host Factors Associated with Complicated UTI (Goldman-Cecil Table 263-1)

Additional Risk Factors

• Diabetes mellitus - impaired neutrophil function, glucosuria favors bacterial growth, autonomic neuropathy causes incomplete bladder emptying
• Pregnancy - progesterone-induced ureteral smooth muscle relaxation, mechanical compression of ureters (especially right ureter), bladder pressure, altered host immunity
• Male sex-specific: homosexual activity, uncircumcised penis (10x risk increase), HIV infection, insertive anal intercourse with a partner carrying uropathogens
• Urinary catheters - indwelling catheters confer ~5% daily risk of bacteriuria; risk of catheter-associated UTI (CAUTI) increases with duration
• Elderly patients - institutionalization, decreased mobility, incontinence, neurogenic bladder, multiple comorbidities, antibiotic-resistant flora
• Spinal cord injury (SCI) - UTI occurs in nearly 100% of SCI patients in long-term follow-up; risk factors include male gender, cervical-level injury, and condom catheter use
• Blood group secretor status - non-secretors of blood group antigens have more P-fimbriated E. coli binding receptors available

6. Clinical Features

Lower UTI (Cystitis)

• Dysuria (burning, pain on urination) - hallmark symptom
• Frequency - increased urinary frequency, often small volumes
• Urgency - sudden compelling need to void
• Suprapubic pain/tenderness
• Strangury (painful urge to urinate with little result)
• Hematuria (gross or microscopic) - in up to 40-60%
• Turbid or foul-smelling urine
• Absence of vaginal discharge or irritation - important to distinguish from vaginitis/STI

Upper UTI (Pyelonephritis)

• Fever (often >38.5°C), chills, rigors
• Flank/loin pain (unilateral or bilateral)
• Costovertebral angle (CVA) tenderness - classic finding
• Nausea and vomiting
• May have associated lower tract symptoms (dysuria, frequency)
• In pregnancy: right-sided predominance due to greater physiological dilation of the right ureter
• Severe cases progress to urosepsis with hypotension and organ dysfunction

Asymptomatic Bacteriuria

• By definition, no urinary symptoms
• Does not require treatment in most circumstances (see below)

Special Populations

7. Diagnosis

Urinalysis

• Pyuria (>10 WBC/hpf): the cornerstone of UTI diagnosis
• Leukocyte esterase (dipstick): sensitivity ~75-96%, specificity ~94-98% for pyuria
• Nitrites (dipstick): produced by nitrate-reducing bacteria (E. coli, Klebsiella, Proteus); sensitivity ~45-60%, specificity ~95-99%; highly specific but not sensitive
• Combination of positive leukocyte esterase + nitrites: strong predictor of UTI
• Hematuria: supports UTI but non-specific
• Bacteria on Gram stain: 1 organism/oil-immersion field = ~10^5 CFU/mL

Urine Culture

• Gold standard for diagnosis
• Threshold: ≥10^5 CFU/mL clean-catch mid-stream specimen - defines bacteriuria
• For cystitis: as few as 10^2 CFU/mL may be significant in symptomatic women with pyuria
• Catheter specimens: ≥10^2 CFU/mL is significant

• Complicated UTI (always)
• Pyelonephritis
• Recurrent UTI
• Failure of empiric therapy
• All men with UTI
• Pregnancy (including screening for ASB at 12-16 weeks)
• Immunocompromised patients

Imaging

• Renal ultrasound: first-line for suspected obstruction, abscess, or stones; identifies hydronephrosis
• CT scan (non-contrast): best for urolithiasis detection
• CT with contrast / CT urogram: for suspected perinephric abscess, emphysematous pyelonephritis, or xanthogranulomatous pyelonephritis
• Voiding cystourethrogram (VCUG): for VUR evaluation (mainly in children)
• Nuclear scintigraphy (DMSA scan): gold standard for detection of renal cortical scarring

• Pyelonephritis not improving after 48-72 hours of appropriate therapy
• Suspected obstruction or abscess
• Emphysematous pyelonephritis (air on imaging = urologic emergency)
• Recurrent UTIs in children or men

Blood Tests

• CBC: leukocytosis in pyelonephritis/urosepsis
• Serum creatinine/BUN: assess renal function
• Blood cultures: indicated in pyelonephritis, urosepsis, hospitalized patients (positive in ~20% of pyelonephritis cases)
• CRP, procalcitonin: elevated in upper tract infection; procalcitonin helps distinguish upper from lower tract infection

8. Management

A. Uncomplicated Cystitis in Women

• Fluoroquinolones (ciprofloxacin 250 mg BID x3 days, levofloxacin 250 mg daily x3 days): highly effective but reserve for complicated/upper tract infections to reduce resistance pressure
• Beta-lactams (cephalexin, amoxicillin-clavulanate): lower cure rates than TMP-SMX or fluoroquinolones but acceptable alternatives

B. Acute Uncomplicated Pyelonephritis in Women

• Ciprofloxacin 500 mg BID orally x 7 days (or 1000 mg XR daily x 7 days) - preferred if local fluoroquinolone susceptibility is confirmed
• TMP-SMX 160/800 mg BID x 14 days (if susceptibility confirmed)
• Amoxicillin-clavulanate 875/125 mg BID x 14 days (not first-line; reserve for specific organisms)

• IV fluoroquinolone (ciprofloxacin 400 mg q8-12h or levofloxacin 750 mg q24h)
• IV beta-lactams: ceftriaxone 1g q24h, ampicillin-sulbactam 3g q6h, ertapenem 1g q24h (for ESBL organisms)
• Switch to oral therapy after clinical improvement (typically 48-72h)
• Total duration: 7-14 days depending on clinical response

C. Complicated UTI

1. Remove or correct the underlying condition wherever possible (remove catheter, relieve obstruction, drain abscess, remove stone). Antibiotics alone may be insufficient without source control.
2. Obtain urine culture and sensitivities before initiating therapy
3. Empiric therapy for mild-moderate illness: oral fluoroquinolones (broadest spectrum, excellent urinary tissue penetration) - except moxifloxacin (insufficient urinary levels)
4. Parenteral therapy for severe illness or hospital-acquired infection: broader spectrum agents

• Piperacillin-tazobactam 4.5g q8h
• Cefepime 1-2g q8-12h
• Levofloxacin 750 mg q24h or ciprofloxacin 400 mg q8-12h
• Carbapenems (meropenem, ertapenem, imipenem) - for ESBL-producing organisms
• For difficult-to-treat (DTT) Pseudomonas: ceftazidime-avibactam, ceftolozane-tazobactam, cefiderocol, imipenem-cilastatin-relebactam
• If MRSA suspected: add vancomycin
• Aminoglycosides (amikacin 15 mg/kg/day): excellent urinary levels; useful for gram-negatives

D. Catheter-Associated UTI (CAUTI)

• Most important intervention: avoid unnecessary catheters; when needed, minimize duration
• Treat only when symptomatic (not asymptomatic bacteriuria)
• Remove or replace catheter if feasible before/during treatment
• Obtain culture before starting antibiotics
• Duration: 7 days for prompt responders; 10-14 days for delayed responders
• Do NOT prescribe antibiotics to asymptomatic catheterized patients (Choosing Wisely recommendation)

E. Asymptomatic Bacteriuria - When to Treat

• Pregnant women (Grade A recommendation - reduces risk of pyelonephritis, preterm birth, low birth weight)
• Before urologic procedures expected to cause mucosal bleeding
• Kidney transplant recipients (in the immediate post-transplant period)

• Non-pregnant women
• Elderly men and women (including institutionalized elderly)
• Diabetic women
• Patients with neurogenic bladder
• Patients with indwelling catheters
• Most other populations (treating only promotes resistant organisms without clinical benefit)

F. Recurrent UTI in Women

• Avoid spermicides
• Increase fluid intake
• Post-coital voiding
• D-mannose powder - competes with FimH adhesin of E. coli
• Cranberry products - limited evidence from RCTs; minimal benefit
• Topical vaginal estrogen in postmenopausal women - restores lactobacilli, reduces colonization; effective for reducing recurrence

9. UTI in Special Populations

Pregnancy

• Screen all pregnant women for ASB at 12-16 weeks gestation (USPSTF Grade A)
• Up to 30% of untreated ASB in pregnancy progresses to pyelonephritis
• Treatment reduces risk of preterm delivery and low birth weight
• Preferred antibiotics: cephalexin 500 mg BID x 3-7 days; nitrofurantoin 100 mg BID (avoid in 1st trimester - folate antagonism risk, and after 36 weeks - risk of neonatal hemolysis)
• Avoid fluoroquinolones (teratogenic in animals), TMP-SMX in first trimester and near term
• Duration: 7-10 days for lower UTI; 14 days for pyelonephritis
• Follow-up culture monthly after treatment
• Recurrent ASB: consider suppressive prophylaxis with cephalexin or nitrofurantoin throughout pregnancy
• Acute pyelonephritis in pregnancy: carry substantial maternal morbidity including sepsis, permanent renal injury, and premature labor; treat aggressively with IV ampicillin + gentamicin or IV cephalosporins

Men

• Any UTI in a young man should be assumed complicated until proven otherwise
• Obtain urine culture always
• Consider prostatitis as source of infection
• Prostatic source: treat with a fluoroquinolone or TMP-SMX for 4-6 weeks (prostatic penetration required)
• Investigate with imaging/cystoscopy for anatomic abnormalities
• Recurrence with same organism suggests prostatic focus

Elderly

• Higher prevalence of bacteriuria (often asymptomatic)
• Do NOT treat asymptomatic bacteriuria in elderly (no benefit, promotes resistance)
• Presentations atypical - exclude other causes of confusion/delirium before attributing to UTI
• Fluoroquinolones and nitrofurantoin may require dose adjustment for renal function

Spinal Cord Injury

• Treat only symptomatic bacteriuria
• Use antimicrobials with minimal impact on normal flora
• Treat for at least 5 days; 7-14 days for reinfection or relapse
• Repair structural and functional risk factors
• Hydrophilic catheters reduce CAUTI risk
• Do not use prophylactic antibiotics in catheterized patients (promotes resistance)
• Prophylaxis only in recurrent UTI when no underlying cause identified, especially if dilated upper tracts

10. Complications

• Perinephric abscess - collection of pus in the perinephric fat; treated with drainage (especially if ≥3 cm) + prolonged antibiotics (4-6 weeks)
• Emphysematous pyelonephritis - gas-forming infection, most common in diabetics; high mortality; often requires emergency nephrectomy
• Xanthogranulomatous pyelonephritis - chronic destructive infection with lipid-laden macrophages; usually requires nephrectomy
• Renal scarring - a small proportion of women with severe acute pyelonephritis develop scars; generally not associated with impaired renal function in uncomplicated cases
• Urosepsis - fatality rate ~10%; risk increases with advanced age, significant comorbidities, and inadequate initial treatment
• Chronic pyelonephritis - repeated infections and VUR leading to progressive renal scarring, especially in children

11. Prognosis

• Uncomplicated cystitis: excellent prognosis; symptoms resolve within 1-2 weeks even without treatment; no long-term renal or cardiovascular sequelae
• Acute uncomplicated pyelonephritis: good prognosis with appropriate treatment; small subset develop renal scars but without significant functional impairment
• Recurrent uncomplicated UTI: no long-term adverse outcomes (no hypertension, no renal impairment)
• Complicated UTI: long-term outcomes primarily determined by underlying condition rather than UTI itself
• Urosepsis: ~10% case fatality rate; worsened by older age and inadequate initial therapy

12. Prevention Summary

• General: adequate hydration, voiding after intercourse, front-to-back hygiene, avoid spermicides
• Postmenopausal women: topical vaginal estrogen
• Catheterized patients: minimize catheter use and duration, aseptic insertion technique, meatal cleansing with chlorhexidine, early removal
• Pregnant women: routine screening and treatment of ASB
• Recurrent UTI: consider prophylactic antibiotics (see regimens above)
• Pediatric VUR: prophylactic antibiotics to prevent UTI-related renal scarring in selected cases

• Comprehensive Clinical Nephrology, 7th Edition - Chapters 44 and 53
• Goldman-Cecil Medicine, International Edition - Chapter 263
• Rosen's Emergency Medicine - Chapter 32
• Campbell-Walsh-Wein Urology, 3-Volume Set