Differential Diagnosis of Fever with Rash

Primary Differential Diagnoses

The American College of Rheumatology suggests that fever ≥39°C for at least 7 days is the hallmark presentation of Still's disease, making this highly consistent with an 8-day fever duration 1, 2
The European League Against Rheumatism notes that the rash in Still's disease is typically salmon-pink, erythematous, transient, and coincides with fever spikes, preferentially involving the trunk 1, 2
The American Academy of Pediatrics recommends considering Kawasaki disease in children with fever persisting at least 5 days with high spiking temperatures, and a polymorphous rash typically nonspecific diffuse maculopapular eruption appearing within 5 days of fever onset 3
Bilateral bulbar conjunctival injection without exudate is a key distinguishing feature of Kawasaki disease, according to the American Heart Association 3
The Centers for Disease Control and Prevention state that the rash in measles is a maculopapular rash that begins on the face and spreads cephalocaudally to the trunk and extremities 3

The Infectious Diseases Society of America notes that rash can involve palms and soles in Rat Bite Fever, appearing 2-10 days after exposure 5
The American Academy of Dermatology states that the rash in Rocky Mountain Spotted Fever begins as small blanching pink macules evolving to maculopapular with central petechiae by day 5-6 6

The American College of Emergency Physicians recommends measuring temperature pattern to document if fever is spiking or continuous, and characterizing rash morphology to identify the underlying condition 1, 2, 3
The Society for Pediatric Research suggests assessing for conjunctival injection, oral changes, and extremity changes to identify Kawasaki disease 3

The American Society for Clinical Pathology recommends a complete blood count with differential to look for neutrophilic leukocytosis, and inflammatory markers such as CRP, ESR, and ferritin to support the diagnosis of Still's disease 1, 2
The Clinical and Laboratory Standards Institute states that IL-18 and calprotectin can be used to support the diagnosis of Still's disease if available 2

The American College of Rheumatology notes that macrophage activation syndrome (MAS) is a life-threatening complication of Still's disease, and requires monitoring for persistent fever, splenomegaly, rising ferritin, falling cell counts, and abnormal liver function 1, 2
The American Heart Association recommends urgent echocardiography to monitor for coronary artery involvement in Kawasaki disease 3