Clues Pointing TO This Diagnosis

Break in the skin (shin laceration from the garden gate) as a portal of entry — even minor skin breaks allow strep/staph to enter and cause cellulitis
Onset a few days after the injury — the typical incubation interval before erythema develops
Spreading erythema (redness climbing up the shin) — the cardinal expanding sign of cellulitis
Warmth over the affected area — reflects the underlying dermal infection/inflammation
Swelling and tenderness of the leg — classic accompanying signs of cellulitis
Unilateral single-limb involvement — bilateral cellulitis is rare, so a single involved leg supports true cellulitis over stasis dermatitis
Ascending red streak with tender inguinal lymph nodes — lymphangitis/regional lymphadenopathy is consistent with a streptococcal skin infection
Diabetes with poor glycemic control — impaired healing and immune function raise infection risk and worsen outcomes
Non-pitting appearance rather than the fluid pattern of edema — helps separate infection from stasis changes
Elevated white blood cell count — supports an infectious/inflammatory process and helps gauge severity
Response to an oral antistreptococcal agent (cephalexin) with a stable marked border — improvement on first-generation cephalosporin supports the diagnosis

Why It's NOT the Similar Diseases

NOT venous stasis dermatitis — stasis dermatitis is typically bilateral and chronic; Gladys has acute, unilateral, spreading erythema after a skin break
NOT deep vein thrombosis — DVT causes unilateral swelling and pain but not a spreading warm erythematous plaque with a portal of entry and ascending lymphangitic streak
NOT necrotizing fasciitis — there are no bullae, skin necrosis, crepitus, pain out of proportion, or systemic toxicity in Gladys
NOT contact/atopic dermatitis — these are pruritic and eczematous rather than a tender, warm, expanding infectious plaque
NOT gout — gout causes acute monoarticular joint inflammation (classically the great toe), not a spreading cellulitic plaque along the shin

Differential Diagnosis List

Venous stasis dermatitis — ruled out by unilateral acute spread with a clear skin-break portal of entry
Deep vein thrombosis — ruled out by warmth, spreading erythema, and ascending lymphangitis rather than isolated limb swelling
Necrotizing fasciitis — ruled out by absence of bullae, necrosis, crepitus, or pain out of proportion
Contact/atopic dermatitis — ruled out by tender expanding warm plaque rather than pruritic eczematous rash
Gout — ruled out by shin plaque distribution rather than acute joint inflammation

Key Distinguishing Test

Cellulitis is a clinical diagnosis based on the characteristic spreading erythema, warmth, swelling, and tenderness; routine blood cultures, aspirates, and biopsies are not recommended in typical cases.
A white blood cell count can help gauge severity, and point-of-care ultrasound helps distinguish an underlying abscess when purulence is suspected.
When the presentation is ambiguous, a clinical prediction tool (the ALT-70 score) helps separate true lower-extremity cellulitis from mimics.

Board Note

Nonpurulent cellulitis is usually streptococcal (and MSSA); treat mild disease with an oral antistreptococcal agent such as cephalexin, dicloxacillin, penicillin, or amoxicillin—clindamycin if penicillin-allergic
Standard duration is 5 days, extended if not improving; elevate the limb and treat predisposing factors such as edema and tinea pedis
Cover MRSA (e.g., TMP-SMX, doxycycline, or vancomycin depending on severity) for purulent infection, penetrating trauma, known MRSA colonization, injection drug use, or severe/systemic illness
Assess severity with SIRS criteria; patients with 2 or more criteria, hypotension, immunocompromise, or rapid progression warrant IV therapy and admission
Suspect necrotizing infection (surgical emergency) with bullae, skin necrosis, crepitus, sensory loss, or pain out of proportion—normal vital signs are not reassuring
Recurrence is common (up to ~29%, and up to 47% within 3 years); reduce risk by treating tinea pedis, moisturizing, controlling diabetes, and using compression stockings for chronic edema, with antibiotic prophylaxis for frequent recurrences
The following figure contrasts true cellulitis with common mimics such as DVT and stasis dermatitis, reinforcing the diagnostic reasoning tested below:
Figure 1. Clinical Presentation of Conditions That Mimic Cellulitis and True Cellulitis
Cellulitis: A Review. JAMA. July 18, 2016.
Content used under license from the JAMA Network®
© American Medical Association

“Gladys’s Story”