Urinary Tract Infection: A High-Yield USMLE Review

Urinary tract infection is one of the most common bacterial infections encountered in outpatient, ED, and inpatient settings, spanning a spectrum from uncomplicated cystitis to pyelonephritis and life-threatening urosepsis. On the exam, you must recognize both the classic localized presentation and its atypical/systemic consequences, know the causative organisms, and select the correct diagnostic tests and empiric antibiotics. Its downstream effects — urosepsis, acute tubular necrosis from hypoperfusion, and delirium in vulnerable patients — are also frequently tested.

Presentation

• Lower UTI (cystitis): dysuria, urinary frequency, urgency, and suprapubic pain or tenderness, typically without fever
• Pyelonephritis: fever, chills, flank pain, and costovertebral angle (CVA) tenderness, often with nausea/vomiting and preceding lower-tract symptoms
• New-onset delirium or acute agitation, particularly in elderly patients and those with psychiatric illness, where behavioral change — not classic urinary symptoms — is the clue
• Septic shock from a urinary source (urosepsis), with hypotension requiring fluid resuscitation and vasopressors
• Recurrent episodes over time, a common chronic problem in women, particularly the elderly

Diagnosis

• Urinalysis: pyuria (>10 WBC/hpf) with positive leukocyte esterase; positive nitrites indicate nitrate-reducing Enterobacteriaceae (e.g., E. coli); hematuria and bacteriuria may also be present
• Urine culture (gold standard): in a symptomatic patient — including uncomplicated cystitis, which is by definition symptomatic — as few as ≥10^2 CFU/mL of a uropathogen is significant (Stamm criteria); the classic ≥10^5 CFU/mL threshold (Kass criterion) is used for asymptomatic bacteriuria screening and less clearly defined presentations
• Distinguish uncomplicated cystitis (healthy nonpregnant woman, normal tract) from complicated UTI (male sex, pregnancy, catheter, obstruction, immunosuppression, or structural/functional abnormality) and from pyelonephritis (fever, CVA tenderness, WBC casts on microscopy)
• In pyelonephritis or suspected urosepsis, obtain blood cultures, CBC, and metabolic panel; imaging (renal ultrasound or CT) if obstruction, abscess, or failure to improve is suspected
• When urosepsis progresses to ATN, urine studies show high urine sodium (>40 mEq/L), elevated FENa (>2%), and muddy brown granular casts on sediment

Management

• Uncomplicated cystitis: first-line oral therapy with nitrofurantoin, TMP-SMX (where resistance is low), or fosfomycin; fluoroquinolones are reserved due to resistance and side effects
• Pyelonephritis: oral fluoroquinolone for outpatient management, or IV antibiotics (e.g., ceftriaxone or a fluoroquinolone) for patients requiring admission, with tailoring to culture and susceptibility
• Urosepsis/septic shock: prompt fluid resuscitation, empiric broad-spectrum IV antibiotics, and vasopressors for refractory hypotension; obtain cultures before antibiotics when feasible
• Pregnancy: treat asymptomatic bacteriuria and use pregnancy-safe agents (e.g., beta-lactams such as cephalexin or amoxicillin-clavulanate; avoid fluoroquinolones)
• De-escalate and direct antibiotic therapy once culture and sensitivities return, and address contributing factors (obstruction, catheter removal/exchange)

High-yield

• E. coli is the most common cause of UTI in all populations; Staphylococcus saprophyticus is classic in sexually active young women; Proteus (urease-producing) causes struvite stones and alkaline urine; Klebsiella is another common Enterobacteriaceae
• Positive nitrites suggest Enterobacteriaceae (nitrate reducers); leukocyte esterase reflects pyuria
• WBC casts on urine microscopy localize infection to the kidney (pyelonephritis)
• In symptomatic women, colony counts as low as ≥10^2 CFU/mL of a uropathogen are diagnostic (Stamm criteria); ≥10^5 CFU/mL (Kass criterion) is the classic screening threshold for asymptomatic bacteriuria
• Treat and screen for asymptomatic bacteriuria only in pregnant women and before urologic procedures — not in most other patients
• A UTI can be the leading cause of new delirium/agitation in an elderly or psychiatric patient — work up the new clinical change as medical first
• Urosepsis with hypoperfusion is a classic setup for acute tubular necrosis (high urine Na, FENa >2%, muddy brown casts)

Pitfalls

• Treating asymptomatic bacteriuria in nonpregnant patients — this drives resistance and is not indicated outside pregnancy or pre-urologic procedures
• Assuming only ≥10^5 CFU/mL counts as a positive culture in a symptomatic patient — a lower count of a uropathogen (≥10^2 CFU/mL) is diagnostic in symptomatic cystitis, and the high threshold applies to asymptomatic screening
• Using nitrofurantoin for pyelonephritis — it does not achieve adequate renal tissue levels and is inappropriate for upper-tract infection
• Prescribing fluoroquinolones in pregnancy or as routine first-line cystitis therapy — avoid in pregnancy and reserve to limit resistance/toxicity
• Diagnostic overshadowing: attributing new agitation in a patient with psychiatric history to their illness rather than working up delirium from a UTI
• Mislabeling ATN from urosepsis: high urine sodium and FENa distinguish ATN from pre-renal azotemia, and muddy brown casts distinguish it from AIN (WBC casts) or glomerulonephritis (RBC casts)