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Urinary Tract Infections for the ABIM

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Urinary Tract Infections for the American Board of Internal Medicine Exam
  • Definition:
    • A urinary tract infection (UTI) refers to the microbial infection of any part of the urinary tract, including the urethra, bladder, ureters, and kidneys. It is classified into upper and lower UTIs based on the site of infection:
    • Lower UTI (Cystitis): Infection limited to the bladder and urethra.
    • Upper UTI (Pyelonephritis): Infection that involves the kidneys and may lead to more severe complications.
  • Etiology:
    • Pathogens:
    • The most common causative pathogen of UTIs is Escherichia coli, responsible for 75-95% of cases. Other common pathogens include:
    • Staphylococcus saprophyticus (in sexually active young women).
    • Klebsiella pneumoniae, Proteus mirabilis, and Enterococcus species, especially in patients with structural urinary tract abnormalities or catheter use.
    • Fungal UTIs (primarily caused by Candida species) are common in immunocompromised individuals or patients with indwelling catheters.
  • Risk Factors:
    • Anatomical Factors:
    • Shorter urethra in women, allowing easier bacterial entry.
    • Sexual Activity:
    • Sexual intercourse increases the risk of UTI, particularly in women.
    • Postmenopausal Women:
    • Reduced estrogen leads to decreased vaginal flora and increased susceptibility to bacterial colonization.
    • Catheterization:
    • Indwelling catheters provide a direct route for bacteria into the bladder, significantly increasing UTI risk.
    • Obstruction and Structural Abnormalities:
    • Conditions like benign prostatic hyperplasia (BPH), ureteral strictures, kidney stones, or vesicoureteral reflux can prevent proper urinary drainage and increase infection risk.
    • Diabetes:
    • Hyperglycemia impairs immune response and increases UTI risk.
    • Immunocompromised Patients:
    • Patients with HIV, cancer, or chronic corticosteroid use are at higher risk for complicated UTIs.
  • Classification:
    • Uncomplicated UTI:
    • Occurs in otherwise healthy individuals with normal urinary tract anatomy. Most cases are uncomplicated and respond well to short-course antibiotics.
    • Complicated UTI:
    • Associated with factors that increase the risk of treatment failure or recurrent infection, such as urinary tract abnormalities, catheter use, immunosuppression, or male sex.
  • Clinical Features:
    • Lower UTI (Cystitis):
    • Dysuria: Painful or burning sensation with urination.
    • Urinary Frequency and Urgency: Frequent urge to urinate with small volumes of urine.
    • Suprapubic Pain: Discomfort or pressure in the lower abdomen.
    • Hematuria: Blood in the urine may be seen.
    • Absence of systemic symptoms: Cystitis typically lacks fever or chills.
Cystitis urine output
    • Upper UTI (Pyelonephritis):
    • Fever and Chills: Systemic signs of infection.
    • Flank Pain: Pain in the costovertebral angle, often unilateral.
    • Nausea and Vomiting: Common with more severe infection.
    • Urinary Symptoms: Dysuria, frequency, and urgency may also be present.
    • Signs of Sepsis: Severe cases may present with hypotension and confusion, especially in elderly or immunocompromised patients.
  • Diagnosis:
    • Urinalysis:
    • A positive urine dipstick for nitrites (indicating the presence of Gram-negative bacteria) or leukocyte esterase (indicating white blood cells in the urine) is highly suggestive of a UTI.
    • Microscopic Analysis: Pyuria (white blood cells >5 per high power field) and bacteriuria on microscopy further confirm the diagnosis.
    • Urine Culture:
    • Gold standard for diagnosis, especially in complicated or recurrent UTIs. A colony count ≥10⁵ CFU/mL of a single pathogen is diagnostic in most cases.
    • In complicated UTIs or pyelonephritis, urine culture is essential for identifying the organism and determining antibiotic susceptibility.
    • Imaging:
    • Not routinely needed in uncomplicated UTIs but is indicated in patients with recurrent infections or suspected structural abnormalities. Renal ultrasound or CT scan can help identify urinary obstructions or abscesses.
  • Management:
    • Uncomplicated UTI:
    • Antibiotics:
    • First-line treatments include short-course oral antibiotics such as:
    • Trimethoprim-sulfamethoxazole (TMP-SMX) for 3 days.
    • Nitrofurantoin for 5 days.
    • Fosfomycin as a single dose.
    • Fluoroquinolones (e.g., ciprofloxacin) are reserved for resistant cases or patients with contraindications to other agents.
    • Symptomatic Management:
    • Phenazopyridine (urinary analgesic) may be used for symptomatic relief of dysuria but should not exceed 2-3 days of use.
    • Hydration:
    • Patients should be encouraged to maintain adequate fluid intake to help flush bacteria from the urinary tract.
    • Complicated UTI:
    • Longer courses of antibiotics (7-14 days) are required, and the choice of antibiotic is guided by urine culture results.
    • Hospitalization may be necessary for severe cases, especially pyelonephritis with systemic signs (e.g., fever, sepsis) or in immunocompromised patients.
    • Intravenous (IV) antibiotics, such as ceftriaxone or fluoroquinolones, are used in severe cases until the patient improves and can transition to oral therapy.
    • Recurrent UTI:
    • Defined as two or more UTIs within six months or three within a year. Prophylactic antibiotics may be considered for patients with recurrent infections.
    • Behavioral Modifications:
    • Increased fluid intake, post-coital voiding, and avoiding spermicides may help prevent recurrence.
    • Pyelonephritis:
    • Empiric therapy with broad-spectrum antibiotics (e.g., IV ceftriaxone or oral fluoroquinolones) is started immediately, adjusted based on culture results.
    • Hospitalization is indicated for patients with severe illness, intractable vomiting, or signs of sepsis.
  • Prevention:
    • Behavioral Modifications:
    • Adequate hydration, frequent voiding, post-coital voiding, and wiping front to back are recommended to reduce UTI risk in women.
    • Prophylactic Antibiotics:
    • Indicated for patients with recurrent UTIs, particularly those related to sexual activity. Options include low-dose daily antibiotics or post-coital antibiotics.
  • Complications:
    • Acute Kidney Injury:
    • Severe pyelonephritis can lead to renal parenchymal damage and acute kidney injury (AKI).
    • Sepsis:
    • Untreated pyelonephritis, especially in immunocompromised patients, may progress to urosepsis, leading to septic shock and multiorgan failure.
    • Renal Abscess:
    • Abscess formation in the kidney is a rare but serious complication of inadequately treated upper UTIs.
    • Chronic Pyelonephritis:
    • Recurrent infections can cause renal scarring, leading to chronic kidney disease (CKD).
Key Points
  • Urinary tract infections are primarily caused by E. coli, with cystitis affecting the lower urinary tract and pyelonephritis involving the upper urinary tract.
  • Common symptoms of lower UTI include dysuria, frequency, and suprapubic pain, while upper UTI presents with fever, flank pain, and systemic signs.
  • Diagnosis is based on urinalysis and confirmed by urine culture. Imaging is used for complicated or recurrent cases.
  • Management of uncomplicated UTIs includes short-course antibiotics (e.g., TMP-SMX, nitrofurantoin). Complicated UTIs require longer treatment and may necessitate hospitalization.
  • Preventive strategies for recurrent UTIs include behavioral modifications, adequate hydration, and, in some cases, prophylactic antibiotics.

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