Differential Diagnoses for Lower Extremity Petechial Rash

Life-Threatening Causes Requiring Immediate Action

The Centers for Disease Control and Prevention recommends initiating doxycycline 100 mg twice daily immediately if fever is present with petechial rash on lower extremities, as Rocky Mountain Spotted Fever carries a 5-10% mortality rate and delays in treatment significantly increase death risk 1
Meningococcemia presents with rapidly progressive petechial rash that can evolve to purpura fulminans with full-thickness skin necrosis, and requires immediate empiric treatment for both meningococcemia and RMSF if neither can be ruled out 2

The MMWR Recommendations and Reports suggest that Enteroviral infections are the most common cause of maculopapular rashes but typically spare palms, soles, face, and scalp 1, 5
Human Monocytic Ehrlichiosis (HME) rash occurs in only 30% of adults, appearing later in disease course (median 5 days after onset), and varies from petechial or maculopapular to diffuse erythema 3, 1
Rocky Mountain Spotted Fever (RMSF) progresses to maculopapular with central petechiae, spreading centripetally to involve entire body including palms and soles, and critical red flags include fever + headache + tick exposure (or endemic area) + thrombocytopenia/hyponatremia 3, 1

Immune Thrombocytopenic Purpura (ITP) is characterized by petechial rash associated with low platelet count, and can be distinguished from HSP by presence of thrombocytopenia on CBC 2
Post-Streptococcal Purpura can occur after group A streptococcal pharyngitis, and patients should be evaluated for recent history of sore throat or positive strep testing 2

Secondary Syphilis can present with maculopapular rash involving palms and soles, and patients should undergo RPR/VDRL and sexual history evaluation 3
Disseminated Gonococcal Infection is characterized by maculopapular or petechial rash with associated arthritis, and should be considered in sexually active patients with joint complaints 2
Infective Endocarditis can present with petechial rash on extremities, particularly in setting of fever and new heart murmur, and patients should undergo blood cultures and echocardiography 3

Drug Hypersensitivity Reactions can present as fine reticular maculopapular rashes or broad erythematous macules, and patients should be queried about antibiotics, NSAIDs, anticonvulsants, or any new medications within past 2-3 weeks 1, 5
Thrombotic Thrombocytopenic Purpura (TTP) is characterized by petechial rash with associated thrombocytopenia, hemolytic anemia, renal dysfunction, and neurologic changes, and requires urgent hematology consultation 2

The MMWR Recommendations and Reports recommend obtaining complete blood count with differential, comprehensive metabolic panel, peripheral blood smear, acute serology for R. rickettsii, E. chaffeensis, and A. phagocytophilum, and blood cultures if sepsis is suspected 2, 1

The Centers for Disease Control and Prevention advise against excluding RMSF based on absence of tick bite history, as 40% of patients do not report tick exposure, and against waiting for serologic confirmation before starting doxycycline if RMSF is suspected 4, 1