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Urinary Tract Infection (UTI) - A Comprehensive Review

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1. Definition and Classification

Key Definitions

• Comprehensive Clinical Nephrology, p. 4247; Bailey & Love, p. 1532

Classification

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2. Epidemiology

• Half of all women experience at least one UTI in their lifetime; up to 50% will have recurrent infection in the following 6-month period. - Bailey & Love, p. 1532
• The ratio of pyelonephritis to cystitis is 1:18 to 1:29 among women with recurrent infection.
• Highest pyelonephritis incidence: women aged 20-30 years.
• Hospitalization required for ~20% of nonpregnant women with pyelonephritis.
• Pyelonephritis complicates 1-2% of all pregnancies. - Brenner & Rector's, p. 1631
• ASB affects 2-9% of pregnant women; untreated, ~30% progress to pyelonephritis. - Comprehensive Clinical Nephrology, p. 4252

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3. Microbiology / Causative Organisms

Uncomplicated UTI

• E. coli - 85% of cases (most important pathogen)
• Staphylococcus saprophyticus
• Enterococcus faecalis
• Proteus mirabilis (particularly in diabetic women and urinary tract obstruction - raises urine pH, promotes struvite stones)
• Klebsiella spp.

Complicated UTI

• E. coli (50%)
• Enterococci
• Staphylococcus aureus
• Pseudomonas aeruginosa
• Bailey & Love, p. 1532-33; Comprehensive Clinical Nephrology, p. 4262-4264

Pyelonephritis

• E. coli isolated in 85-90% of women with acute uncomplicated pyelonephritis.
• Infecting strains characteristically produce P fimbria adhesin (Gal-Gal). - Brenner & Rector's, p. 1631

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4. Pathogenesis

Routes of Infection

1. Ascending (most common) - Contamination of the vaginal/periurethral area with uropathogenic organisms from the GI tract → adherence and migration into urethra and bladder. - Bailey & Love, p. 1532
2. Haematogenous spread - Seen with S. aureus and fungal infections.
3. Direct spread - From retroperitoneal abscess or inflammatory bowel disease.

Intracellular Bacterial Communities (IBCs)

Bacterial Virulence Factors vs. Host Defences

• Bailey & Love, Table 83.13

Role of Pregnancy in Pathogenesis

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5. Risk Factors

A. Host-Related (Genetic) Risk Factors

• Female sex - Short urethra (4 cm), proximity of urethra to anus and vagina
• Nonsecretor status of ABO blood-group antigens - Women with recurrent UTI are at least 3x more likely to be nonsecretors; these individuals express cell-surface glycosphingolipids on vaginal epithelium that bind uropathogenic E. coli more avidly.
• Genetic polymorphisms - Of IL-8 receptor CXCR1, TLRs, and TNF promoter
• Family history - Increased frequency of UTI in first-degree female relatives of women with recurrent infection
• Early age at first UTI - A major risk factor for recurrent cystitis in women of any age
• Brenner & Rector's, p. 1629

B. Behavioural Risk Factors (Premenopausal Women)

• Sexual intercourse - The single most important behavioral risk factor; 75-90% of episodes in young sexually active women are attributable to intercourse. Risk correlates with frequency of intercourse.
• Spermicide use - Independent risk factor; recurrent infection at least twice as high in spermicide users. Spermicides kill hydrogen peroxide-producing lactobacilli, raising vaginal pH and facilitating colonization with E. coli.
• New sexual partner
• History of previous UTI
• History of UTI in mother

Note: Popular myths debunked - type of underwear, bathing vs. showering, postcoital voiding, frequency of voiding, perineal hygiene practices, vaginal douching, and tampon use are NOT associated with increased risk. - Brenner & Rector's, p. 1629

C. Postmenopausal Women

• History of prior UTI at younger age - Strongest risk factor
• Estrogen deficiency - Alters vaginal flora (replacement of lactobacilli by potential uropathogens)
• Sexual intercourse is NOT an important contributor in this group

D. Risk Factors for UTI in Men (Uncommon)

• Intercourse with a female partner with recurrent UTI
• Uncircumcised status (intact foreskin)
• Anal intercourse
• Brenner & Rector's, p. 1630

E. Other Risk Factors

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6. Clinical Features

Lower UTI (Cystitis)

• Dysuria, urgency, frequency, stranguria (painful urination), hesitancy
• Suprapubic pain or tenderness
• Gross haematuria (common)
• No fever/systemic features in uncomplicated lower UTI

Upper UTI (Pyelonephritis)

• Costovertebral angle (loin) pain and tenderness
• Fever (often high), rigors
• Nausea, vomiting
• Variable lower urinary tract symptoms may be absent
• Renal angle tenderness on examination
• May progress to sepsis, AKI, DIC, ARDS (especially in pregnancy)
• Bailey & Love, p. 1532-33; Brenner & Rector's, p. 1631

Important Differential Diagnoses

• Lower tract symptoms: Sexually transmitted infections (gonorrhoea, chlamydia, herpes genitalis), vulvovaginal candidiasis, interstitial cystitis, vaginitis, ovarian torsion
• Upper tract/flank symptoms: Appendicitis, cholecystitis, pancreatitis, liver disease, nephrolithiasis, premature labour (in pregnancy)
• Rosen's Emergency Medicine, p. 301

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7. Investigation

Urinalysis (Dipstick)

• Leukocyte esterase - Screens for pyuria; pyuria is a consistent accompaniment of acute cystitis
• Nitrite test - Screens for bacteria (positive = bacteria reduce nitrate to nitrite); false negatives with Enterococcus spp. or short bladder dwell time
• Absence of pyuria suggests alternative diagnosis but does not rule out UTI with consistent clinical presentation. - Brenner & Rector's, p. 1630

Urine Microscopy, Culture and Sensitivity (MC&S)

• Standard threshold: ≥10⁵ CFU/mL for ASB and pyelonephritis
• Cystitis threshold: ≥10³ CFU/mL (or even lower with pyuria + symptoms)
• In uncomplicated cystitis in women with characteristic presentation, routine urine culture is NOT recommended - due to reliable clinical diagnosis, predictable microbiology, and prompt response to empirical therapy. - Brenner & Rector's, p. 1630
• Culture SHOULD be obtained when: atypical presentation, treatment failure, early recurrence (<1 month), complicated UTI, pregnancy, hospitalization, immunocompromise

Imaging

• Renal tract ultrasound - First-line for patients with rUTI to exclude anatomical pathology; detects obstruction, perinephric abscess
• Contrast-enhanced CT scan - Optimal for diagnosis; findings include renal enlargement, wedge-shaped areas of decreased attenuation, "striated nephrogram", inflammatory changes in Gerota fascia
• Cystoscopy - Reserved for atypical symptoms, haematuria, or suspicion of underlying pathology (bladder cancer, stones, fistula)
• Bailey & Love, p. 1532; Brenner & Rector's, p. 1632

Pregnancy Screening

• US Preventive Services Task Force (Grade A): Screen all pregnant women for ASB with urine culture at 12-16 weeks gestation (or first prenatal visit). Prenatal screening identifies ~95% of women at risk for subsequent bacteriuria. - Rosen's Emergency Medicine, p. 293

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8. Management

A. Acute Uncomplicated Cystitis

Fluoroquinolones should be reserved when first-line agents are not appropriate, due to resistance concerns and side effects.

• Brenner & Rector's The Kidney, Table 36.4

B. Recurrent UTI - Prevention Strategies

• Continuous low-dose prophylaxis - Nightly or every other night at bedtime; decreases recurrent episodes by ~95% while taken
• Post-coital prophylaxis - Single dose TMP/SMX or nitrofurantoin after intercourse (for sex-linked infections)
• Self-treatment (patient-initiated therapy) - 3-day course of TMP/SMX or ciprofloxacin; effective strategy for women who are travelling or have less frequent recurrences
• Initial prophylaxis course: 6 or 12 months. ~50% of women re-infect within 3 months of stopping; reinstitution for up to 2 years may be considered.
• Brenner & Rector's, p. 1631

• Brenner & Rector's, p. 1631-32; Bailey & Love, p. 1533

C. Acute Uncomplicated Pyelonephritis

• Pregnancy
• Haemodynamic instability / sepsis
• Uncertain GI absorption or poor oral compliance
• Need to exclude complicating factors (obstruction, abscess)
• Need to monitor or treat associated illnesses
• Brenner & Rector's, p. 1632

A useful strategy for emergency department management: Single parenteral dose of ceftriaxone 1 g or gentamicin 120 mg → then oral therapy once GI symptoms controlled. - Brenner & Rector's, p. 1632

D. Complicated UTI

• Remove/treat underlying cause if possible: remove catheter, drain obstruction, remove stones/stents
• Culture-guided therapy based on susceptibility results
• Broader spectrum antibiotics required; duration typically 7-14 days
• Renal or perirenal abscesses ≥3 cm: drainage + prolonged antibiotics (4-6 weeks) - Goldman-Cecil Medicine, p. 1098

• Quinolones: ciprofloxacin 400 mg IV q8-12h
• Aminoglycosides: amikacin 15 mg/kg/24h
• Antipseudomonal beta-lactams: piperacillin-tazobactam 4.5 g q8h
• Difficult-to-treat Pseudomonas: ceftazidime-avibactam, ceftolozane-tazobactam, imipenem-cilastatin-relebactam, cefiderocol

E. Catheter-Associated UTI (CA-UTI)

• Similar pathogenesis to standard UTI
• Avoid unnecessary catheterization; remove catheter as early as possible
• Treatment: culture-guided with catheter removal/replacement
• Duration: 7 days if prompt clinical response; 10-14 days for delayed response

F. UTI in Pregnancy

• Screen at 12-16 weeks with urine culture
• Treat if positive (even without symptoms), to reduce pyelonephritis by >80%
• Treatment duration: 7 days recommended
• Check urine culture after treatment to confirm eradication
• If persistent, consider prophylactic antibiotics throughout pregnancy

• Amoxicillin or cephalexin (first-line)
• Nitrofurantoin (avoid near term - risk of neonatal haemolysis)
• Trimethoprim (avoid in first trimester - antifolate effect; avoid in folate deficiency)
• TMP/SMX (avoid in third trimester)
• Comprehensive Clinical Nephrology, p. 4289; Rosen's Emergency Medicine, p. 303-305

• Admit to hospital; IV hydration, obstetric consultation, urine culture
• IV ceftriaxone preferred empirical regimen
• IV gentamicin as alternative (potential fetal ototoxicity, but not confirmed in large cohorts)
• Carbapenem for ESBL-producing organisms
• Treat for 7-14 days
• Monitor for sepsis, AKI, DIC, respiratory distress, preterm labour

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9. Special Populations

UTI in Males

• Uncommon in young men without risk factors; when it occurs, consider underlying structural abnormality, STI, prostatitis
• Any UTI in a male <50 years should prompt investigation (USS, MSU culture)
• Treat for 7-14 days (longer than women) due to higher risk of prostatitis

UTI in Children

• Key risk factors: female sex, high-grade hydronephrosis, intact foreskin (uncircumcised), VUR, ureteral dilation
• Grade of hydronephrosis predicts UTI risk: ~4% (grade 1) to ~40% (grade 4) - Campbell-Walsh-Wein, p. 1182
• Investigate with renal ultrasound; VCUG for suspected VUR
• Prophylactic antibiotics in high-risk children with VUR or high-grade hydronephrosis (role still debated)

UTI in Diabetes

• Higher risk of pyelonephritis, emphysematous pyelonephritis, renal abscess, papillary necrosis
• Young diabetic women are 15x more likely to be hospitalized for pyelonephritis vs. non-diabetics
• May be caused by Candida in addition to usual bacteria
• Good glycaemic control is important for resolution of infection

Genitourinary Tuberculosis (GU-TB) - Special Case

• Caused by Mycobacterium tuberculosis; can affect any part of the urinary tract
• Presents with fever, weight loss, night sweats, UTI symptoms, haematuria; often with sterile pyuria
• Diagnosis: 3 early morning urine samples for AFB/PCR; CT urogram
• Treatment: standard 4-drug anti-TB regimen (isoniazid, rifampicin, pyrazinamide, ethambutol)
• Severe bladder disease may require surgery (augmentation cystoplasty, cystectomy) after completing anti-TB therapy
• Bailey & Love, p. 1533

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10. Complications

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11. Summary Algorithm

Suspected UTI
      |
      ├── Lower tract symptoms only (no fever, no loin pain)
      │         → Uncomplicated cystitis? 
      │              Yes → Empirical short-course antibiotics (3-7 days)
      │              No (male, DM, catheter, structural) → Complicated UTI protocol
      │
      └── Upper tract symptoms (fever, loin pain)
                → Pyelonephritis
                     |
                     ├── Mild-moderate, tolerating oral → Outpatient oral FQ × 7-14 days
                     │
                     └── Severe / pregnant / haemodynamically unstable → Admit
                              IV antibiotics → switch to oral when tolerating
                              Imaging if treatment failure or complications

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Recent Evidence Update (PubMed, 2024-2026)

• WikiGuidelines Consensus (JAMA Network Open, 2024) [PMID 39495518]: Comprehensive consensus guidelines for UTI prevention, diagnosis, and management across pediatric and adult populations - one of the most current evidence-based summaries available.
• Short vs. long treatment for pyelonephritis (Clin Microbiol Infect, 2025) [PMID 40228579]: Living systematic review/meta-analysis of RCTs on antibiotic duration for pyelonephritis and complicated UTI - relevant to optimising treatment duration.
• Cranberry products (Eur Urol Focus, 2024) [PMID 39030132]: Network meta-analysis showing limited evidence for cranberry tablets/juice in UTI prevention, consistent with textbook data.

These recent publications are consistent with classical textbook teachings; no major contradictory evidence identified.

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