Urinary Tract Infections for the USMLE Step 1
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Urinary Tract Infections for the USMLE Step 1 Exam
- Definition:
- Urinary tract infections (UTIs) involve microbial infection of the urinary tract, including the urethra, bladder, ureters, and kidneys. They are categorized into lower UTI (cystitis) and upper UTI (pyelonephritis) based on the location of the infection.
- Etiology:
- The primary causative agent is Escherichia coli (75-95% of cases). Other pathogens include:
- Staphylococcus saprophyticus (commonly in sexually active young women).
- Klebsiella pneumoniae, Proteus mirabilis, and Enterococcus species in complicated or healthcare-associated UTIs.
- Fungal infections, particularly from Candida, can occur in immunocompromised patients or those with catheters.
- Risk Factors:
- Anatomical factors: The shorter female urethra predisposes women to UTIs.
- Sexual activity: Increases the risk of infection due to bacterial introduction into the urethra.
- Postmenopausal women: Reduced estrogen leads to decreased lactobacilli and higher bacterial colonization.
- Urinary stasis: Conditions like benign prostatic hyperplasia (BPH), kidney stones, or strictures can obstruct urine flow.
- Diabetes: Hyperglycemia impairs immune defenses, increasing susceptibility to UTIs.
- Catheterization: Indwelling catheters serve as a nidus for infection.
- Clinical Features:
- Lower UTI (Cystitis):
- Dysuria: Burning or painful urination.
- Urinary frequency and urgency: Frequent urination with a strong urge, often with small volumes.
- Suprapubic discomfort: A sensation of pressure or pain over the bladder.
- Hematuria: Blood in the urine may be present.
- Upper UTI (Pyelonephritis):
- Fever and chills: Suggest systemic infection.
- Flank pain: Pain localized to the costovertebral angle (CVA).
- Nausea and vomiting: Gastrointestinal symptoms may occur with pyelonephritis.
- Urinary symptoms: Cystitis symptoms may also be present in pyelonephritis.
- Diagnosis:
- Urinalysis:
- Positive for leukocyte esterase (indicating pyuria) and nitrites (produced by gram-negative bacteria like E. coli).
- Microscopy: Pyuria (WBCs >5 per high-power field) and bacteriuria confirm infection.
- Urine culture:
- The gold standard for diagnosis, particularly in complicated or recurrent cases. A bacterial count of ≥10⁵ colony-forming units (CFU)/mL confirms UTI.
- Imaging:
- Not routinely required for uncomplicated UTIs but used in recurrent or complicated cases (e.g., ultrasound or CT scan to assess for kidney stones, abscesses, or obstruction).
- Management:
- Uncomplicated UTI:
- First-line antibiotics:
- Nitrofurantoin (5 days), Trimethoprim-sulfamethoxazole (TMP-SMX) (3 days), or fosfomycin (single dose) are commonly prescribed.
- Fluoroquinolones (e.g., ciprofloxacin) are reserved for resistant infections or contraindications to first-line agents.
- Symptomatic management: Phenazopyridine may provide relief for dysuria but should not be used long-term due to side effects.
- Complicated UTI:
- Requires longer antibiotic courses (7-14 days), with drug selection guided by urine culture results.
- Hospitalization and intravenous antibiotics (e.g., ceftriaxone, fluoroquinolones) may be necessary for pyelonephritis or systemic signs (fever, hypotension).
- Prevention in recurrent UTI:
- Behavioral modifications: Increased hydration, post-coital voiding, and avoiding spermicides.
- Prophylactic antibiotics: May be considered for patients with frequent UTIs.
- Complications:
- Acute pyelonephritis: Can lead to renal abscess formation or sepsis if untreated.
- Chronic pyelonephritis: Recurrent infections may result in renal scarring and chronic kidney disease (CKD).
- Sepsis: Urosepsis is a life-threatening complication of untreated pyelonephritis, especially in immunocompromised individuals.
Key Points
- Urinary tract infections are most commonly caused by Escherichia coli, with cystitis affecting the lower urinary tract and pyelonephritis affecting the kidneys.
- Symptoms of lower UTIs include dysuria, urinary frequency, and suprapubic pain, while pyelonephritis presents with fever, flank pain, and systemic symptoms.
- Diagnosis is based on urinalysis, showing positive nitrites and leukocyte esterase, and confirmed by urine culture.
- Uncomplicated UTIs are treated with short-course antibiotics (e.g., nitrofurantoin, TMP-SMX), while complicated infections require longer treatment and, in severe cases, hospitalization.
- Prevention strategies for recurrent UTIs include behavioral modifications and prophylactic antibiotics for select patients.