In a 19-year-old female with three days of crusted lesions on both lips, low-grade fever (101 °F) and tachycardia (113 beats/min), mild myalgias, negative influenza and mononucleosis tests, and pending CBC, CMP, and herpes simplex virus (HSV) 1/2 testing, and no cervical lymphadenopathy or pharyngeal findings, could this be impetigo of the lips?

Clinical Assessment: Herpes Labialis vs. Impetigo

This presentation is most consistent with primary herpes simplex virus infection (likely HSV-1 gingivostomatitis or herpes labialis) rather than impetigo, and you should await the pending HSV testing while considering early antiviral therapy if symptoms worsen.

Why This Is Likely HSV Rather Than Impetigo

Clinical Features Favoring HSV

• Systemic symptoms with fever and tachycardia: Primary HSV-1 infection commonly presents with fever, mild myalgias, and systemic symptoms, particularly in young adults experiencing their first infection 1, 2, 3
• Bilateral lip involvement: HSV characteristically affects both upper and lower lips with crusting lesions that progress through vesicular stages 2, 4
• Three-day duration with crusting: The natural progression of HSV lesions evolves from papules to vesicles to pustulation, ulceration, and ultimately scabbing over several days 1, 2
• Age and demographics: At 19 years old, she falls within the demographic for delayed primary HSV-1 infection, which occurs in approximately 60% of adults and can present more severely when acquired later in life 5, 4

Clinical Features Against Impetigo

• Absence of lymphadenopathy: Impetigo typically causes regional lymphadenitis, which is notably absent in this patient 1
• Systemic fever and tachycardia: Impetigo guidelines specifically state that "systemic symptoms are usually absent" in uncomplicated impetigo 1
• Bilateral symmetric distribution: Impetigo lesions are "frequently multiple" but remain "well-localized" and typically occur on exposed areas after minor trauma or insect bites, not symmetrically on both lips 1
• Thick honey-colored crusts vs. thin crusts: While both conditions can crust, impetigo characteristically forms "thick crusts" from nonbullous lesions, whereas HSV forms thinner crusts after vesicle rupture 1

Diagnostic Approach

Immediate Clinical Differentiation

• Prodromal symptoms: Ask specifically about tingling, burning, or itching sensations before lesion appearance—this prodrome strongly suggests HSV 1, 2
• Lesion evolution: HSV progresses through distinct stages (papule → vesicle → pustule → ulcer → crust), while impetigo begins as papules that rapidly become vesicles then pustules with thick crusting 1
• Pain characteristics: HSV lesions are typically painful throughout their course, whereas impetigo may be less painful unless complicated 1

Laboratory Confirmation

• Await pending HSV 1&2 testing: This will provide definitive diagnosis, as clinical examination alone cannot reliably distinguish HSV from other conditions 5, 2
• If HSV testing is serology only: Note that type-specific serologic assays are useful but may not be positive yet in primary infection; PCR or viral culture from lesion swabs would be more immediately diagnostic 5, 4
• CBC/CMP utility: These will help rule out other systemic processes but are unlikely to differentiate between HSV and impetigo 2

Management Pending Confirmation

If HSV Is Confirmed or Highly Suspected

• Early antiviral therapy is critical: Since peak viral titers occur in the first 24 hours after lesion onset, treatment should be initiated as soon as possible for optimal benefit 1
• Oral antivirals are preferred: For primary HSV infection with systemic symptoms, oral aciclovir, valaciclovir, or famciclovir are indicated rather than topical agents 1, 6
• Treatment window: The natural healing process starts within the first 24 hours, so if she is already at day 3, antiviral benefit may be reduced but can still shorten duration and severity 1

If Impetigo Cannot Be Excluded

• Consider empiric coverage only if: Lesions worsen, develop thick honey-colored crusts, or if bacterial superinfection is suspected 1
• Topical mupirocin or oral antibiotics: Would target Staphylococcus aureus and Streptococcus pyogenes, the typical impetigo pathogens 1

Common Pitfalls to Avoid

• Don't dismiss systemic symptoms: The presence of fever and tachycardia makes impetigo less likely and should prompt consideration of primary HSV infection 1, 2
• Don't wait too long for antivirals: If HSV is strongly suspected clinically, consider starting treatment empirically rather than waiting days for confirmatory testing, as therapeutic benefit decreases with delay 1
• Don't assume "cold sores" are always recurrent HSV: This could be primary HSV-1 gingivostomatitis, which is more severe than recurrent herpes labialis and warrants more aggressive management 2, 3
• Don't overlook immunocompromised states: While your patient appears immunocompetent, severe or atypical presentations should prompt HIV testing or evaluation for other immunodeficiencies 2, 7