Etiology

Pathogens

• Escherichia coli: leading cause of UTI (approx. 80%)
  • Ten times more common in females (shorter urethras colonized by fecal microbiota).
• Staphylococcus saprophyticus: 2nd leading cause of UTI in sexually active women
• Klebsiella pneumoniae: 3rd leading cause of UTI
• Proteus mirabilis
  • Produces ammonia, giving the urine a pungent or irritating smell
  • Associated with struvite stone formation
• Nosocomial bacteria: Serratia marcescens, Enterococci spp., and Pseudomonas aeruginosa are associated with increased drug resistance.

Classification

Feature	Upper UTI (Pyelonephritis)	Lower UTI (Cystitis)
Location	Kidney / Renal Pelvis	Bladder / Urethra
Systemic Sx	Yes (Fever, Chills, Malaise)	No (Typically Afebrile)
Key Sign	CVA Tenderness, Flank Pain	Suprapubic Tenderness
Urinary Sx	May be present	Predominant (Dysuria, Frequency)
UA KEY FINDING	WBC Casts	NO WBC Casts
Severity	More Severe	Less Severe
Management	Often Inpatient, IV Antibiotics	Usually Outpatient, Oral Antibiotics

Clinical features

Diagnostics

Urinalysis

• Pyuria: presence of white blood cells (WBCs) in the urine
  • Positive leukocyte esterase: an enzyme produced by WBC
  • ≥ 5 WBC/HPF
• Bacteriuria: presence of bacteria in the urine
  • Positive urinary nitrites: indicate bacteria that convert nitrates to nitrites (most commonly gram-negative bacteria; e.g., E.coli, Klebsiella, Proteus mirabilis)
• Other findings
  • Leukocyte casts may indicate pyelonephritis.
  • Micro- or macroscopic hematuria may be present.
  • Alkaline urine (pH > 8) and struvite crystals in sediment: indicate urease-producing organisms (e.g., Proteus, Klebsiella, Staphylococcus saprophyticus)

Treatment

• Pregnancy may increase the risk of recurrent bacteriuria and UTIs.
• Treatment
  • Empiric antibiotics for ASB and lower UTI considered appropriate during pregnancy include:
    • Fosfomycin
    • Beta-lactam antibiotics for 5–7 days
      • Oral cephalosporins (e.g., cefpodoxime)
      • Aminopenicillins plus beta-lactamase inhibitors (e.g., amoxicillin/clavulanic acid)

Catheter-associated UTI (CAUTI)

• Definitions
  • Catheter-associated UTI (CAUTI): symptomatic UTI occurring in a patient with an indwelling urinary catheter OR within 48 hours after removal of a urinary catheter
  • Catheter-associated asymptomatic bacteriuria (CAASB): bacteriuria (≥ 105CFU/mL) without symptoms in a patient with an indwelling urinary catheter OR within 48 hours after removal of a urinary catheter
• Risk factors
  • Duration of catheter use, especially >48 hr (most important)
  • Female sex
  • Advanced age
  • Diabetes mellitus
• Treatment
  • Catheter removal or replacement
    • Remove if no longer necessary.
    • Replace if still necessary and present for > 2 weeks.
  • Antibiotic therapy
    • Guided by culture results and local resistance patterns
    • Duration: typically 7–14 days depending on the resolution of symptoms