Differential Diagnosis for Fever with Rash without Thrombocytopenia

Single Most Likely Diagnosis

• Viral Exanthem: This is the most common cause of fever with rash in children and adults, often due to viruses like enteroviruses, adenoviruses, or parvovirus B19. The rash can vary in appearance, and the absence of thrombocytopenia helps to distinguish it from other conditions.

Other Likely Diagnoses

• Bacterial Infections: Such as streptococcal pharyngitis or scarlet fever, which can present with fever and rash. The rash in scarlet fever is typically sandpapery and follows a sore throat.
• Kawasaki Disease: An acute febrile illness of childhood characterized by fever, rash, conjunctivitis, and lymphadenopathy. Although thrombocytosis (not thrombocytopenia) is a hallmark in the later stages, early in the disease, platelet counts can be normal.
• Measles: Characterized by high fever, cough, runny nose, and a rash that starts on the face and spreads downward. The absence of thrombocytopenia does not rule out measles.

Do Not Miss Diagnoses

• Meningococcemia: A severe infection caused by Neisseria meningitidis, which can present with fever, rash (often petechial or purpuric), and shock. Although thrombocytopenia can occur, it's not a universal finding, and the condition's severity necessitates prompt recognition.
• Rocky Mountain Spotted Fever (RMSF): Caused by Rickettsia rickettsii, transmitted by ticks. It presents with fever, headache, and a characteristic rash that may not appear until later in the illness. Early treatment is crucial, and thrombocytopenia is not always present initially.
• Toxic Shock Syndrome: Caused by bacterial toxins, typically from Staphylococcus aureus or Streptococcus pyogenes. It presents with high fever, rash, hypotension, and involvement of three or more organ systems. Thrombocytopenia may not be an initial finding.

Rare Diagnoses

• Erythema Multiforme: An immune-mediated condition characterized by target lesions, often precipitated by infections or medications. It can present with fever and rash but is less common than other diagnoses listed here.
• Secondary Syphilis: Can present with a rash on the palms and soles, fever, and lymphadenopathy. The rash is highly variable and can be mistaken for other conditions.
• Leptospirosis: A zoonotic infection that can cause fever, rash, and a wide range of systemic symptoms. It's more common in individuals with exposure to contaminated water or soil.