Differential Diagnosis for Petechial Rash

The following is a differential diagnosis for a petechial rash, categorized for clarity and emphasis on critical conditions.

• Single Most Likely Diagnosis
  • Idiopathic Thrombocytopenic Purpura (ITP): This condition is characterized by low platelet count due to immune-mediated destruction, leading to petechiae, purpura, and sometimes more severe bleeding. It's a common cause of petechial rash, especially in children and young adults.
• Other Likely Diagnoses
  • Viral Infections: Certain viral infections, such as influenza, mononucleosis, and enterovirus, can cause petechial rashes. The mechanism may involve direct viral effects, immune complex deposition, or secondary thrombocytopenia.
  • Allergic Reactions: Allergic reactions to drugs or other substances can lead to petechial rashes, often as part of a more generalized allergic response.
  • Trauma: Physical trauma, including minor injuries, can cause petechiae, especially in areas of significant impact or pressure.
• Do Not Miss Diagnoses
  • Meningococcemia: This is a life-threatening infection caused by Neisseria meningitidis, characterized by a petechial or purpuric rash, fever, and potentially severe systemic symptoms. Early recognition and treatment are crucial.
  • Disseminated Intravascular Coagulation (DIC): DIC is a condition characterized by both widespread clotting and bleeding in the vascular system, which can lead to petechiae among other symptoms. It's often associated with severe underlying conditions like sepsis, trauma, or cancer.
  • Thrombotic Thrombocytopenic Purpura (TTP): TTP is a rare disorder of the blood-coagulation system, causing extensive microscopic thrombi to form in the smallest blood vessels throughout the body. It presents with thrombocytopenia, microangiopathic hemolytic anemia, renal failure, neurological symptoms, and fever, in addition to petechiae.
• Rare Diagnoses
  • Henoch-Schönlein Purpura (HSP): Although more commonly associated with palpable purpura, HSP can sometimes present with petechiae. It's a form of vasculitis that affects blood vessels and is often seen in children.
  • Leukemia: Certain types of leukemia, particularly acute leukemias, can cause petechial rashes due to bone marrow failure leading to thrombocytopenia or due to leukemic infiltration of the skin.
  • Vasculitis: Various forms of vasculitis, such as microscopic polyangiitis or granulomatosis with polyangiitis, can present with petechial rashes among other systemic symptoms.

Each of these conditions has distinct features and associated symptoms that can help guide the diagnosis. A thorough history, physical examination, and appropriate laboratory tests are essential for determining the underlying cause of a petechial rash.