Urinary tract infections

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UTI Basics - Pee Under Pressure

  • Etiology: Primarily E. coli (>80%), followed by Klebsiella, Proteus, Enterococcus, S. saprophyticus (sexually active young women).
  • Pathophysiology: Ascending infection from urethra → bladder (cystitis) → kidneys (pyelonephritis). Hematogenous spread is rare.
  • Risk Factors: Female sex (short urethra), sexual activity, catheterization (biofilms!), urinary stasis (BPH, stones), diabetes, pregnancy.
  • Classification:
    • Uncomplicated: Healthy, non-pregnant, premenopausal female.
    • Complicated: All others (male, pregnant, obstruction, catheter, immunosuppressed).

Female urinary tract anatomy and proximity to rectum

⭐ Sterile pyuria (WBCs in urine without bacteria) suggests urethritis from Chlamydia or Neisseria, or TB cystitis.

Diagnosis - The Urine Detective

  • Urinalysis (UA): Initial screen.
    • Leukocyte Esterase: Suggests pyuria (WBCs).
    • Nitrites: Specific for gram-negative bacteria (e.g., E. coli).
  • Urine Culture: Gold standard for diagnosis & sensitivities.
    • Significant bacteriuria: $> extbf{10^5}$ CFU/mL.
  • Imaging (CT/Ultrasound): Reserved for complicated UTI, suspected obstruction, or pyelonephritis.

Urine Dipstick Test Results for UTI

Sterile Pyuria: Presence of WBCs in urine with a negative standard culture. Think Chlamydia trachomatis, Ureaplasma urealyticum, or renal tuberculosis.

Uncomplicated UTI - Simple & Swift

  • Definition: Acute cystitis in healthy, non-pregnant, premenopausal females. Excludes structural/functional urinary tract abnormalities.
  • Etiology: Predominantly E. coli (>80%). Others: Staphylococcus saprophyticus (especially in sexually active young women), Klebsiella, Proteus.
  • Clinical: Dysuria, frequency, urgency, suprapubic pain. Absence of systemic symptoms (fever, chills, flank pain) is key.
  • Diagnosis:
    • Urine dipstick: Positive for leukocyte esterase & nitrites.
    • Urinalysis: Pyuria (WBCs), bacteriuria.
    • Urine culture is definitive but often deferred.
  • Treatment (Short-course):
    • Nitrofurantoin: 100 mg BID for 5 days.
    • TMP-SMX: 1 DS tab BID for 3 days (if local E. coli resistance <20%).
    • Fosfomycin: 3 g single dose.

High-Yield: Positive nitrites on dipstick are highly specific for gram-negative bacteria (Enterobacteriaceae family, like E. coli), which can convert urinary nitrates to nitrites.

Complicated UTI - Kidney on Fire

UTI in the presence of factors that ↑ risk of failing therapy (e.g., male sex, obstruction, catheter, diabetes, pregnancy). Includes pyelonephritis & perinephric abscess.

  • Pyelonephritis: Infection of kidney parenchyma.
    • Triad: Fever, CVA tenderness, nausea/vomiting.
    • Urinalysis: WBC casts are pathognomonic.
  • Perinephric Abscess: Pus collection around the kidney.
    • Suspect if no improvement after 48-72h of IV antibiotics.
    • Diagnosis: CT or ultrasound.

⭐ WBC casts are fragile and disintegrate quickly; their presence in a fresh urine sample strongly suggests pyelonephritis.

High‑Yield Points - ⚡ Biggest Takeaways

  • E. coli is the most common uropathogen, causing >80% of UTIs.
  • Distinguish uncomplicated cystitis (healthy, non-pregnant women) from complicated UTIs (males, pregnancy, catheters).
  • Urinalysis showing leukocyte esterase and nitrites is key; urine culture is the gold standard.
  • Only treat asymptomatic bacteriuria in pregnant patients or prior to urologic procedures.
  • Pyelonephritis (fever, flank pain) requires broader antibiotics (e.g., fluoroquinolones) than simple cystitis.
  • CAUTI is the top nosocomial infection; remove the catheter promptly.

Practice Questions: Urinary tract infections

Test your understanding with these related questions

A 62-year-old man comes to the physician because of a 2-day history of fever, chills, and flank pain. Five days ago, he was catheterized for acute urinary retention. His temperature is 39.3°C (102.7°F). Physical examination shows right-sided costovertebral angle tenderness. Urine studies show numerous bacteria and WBC casts. Urine culture on blood agar grows mucoid, gray-white colonies. Urine culture on eosin methylene blue agar grows purple colonies with no metallic green sheen. Which of the following is the most likely causal pathogen?

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Flashcards: Urinary tract infections

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One risk factor for pyelonephritis is urinary tract _____

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One risk factor for pyelonephritis is urinary tract _____

obstruction (not infection)

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