Differential Diagnosis
- Single most likely diagnosis
- Idiopathic thrombocytopenic purpura (ITP): The patient presents with petechiae, a history of epistaxis, and a significantly low platelet count (100 × 10^3/mcL) without other cytopenias, which is consistent with ITP. The recent viral upper respiratory infection could be a triggering factor.
- Other Likely diagnoses
- Von Willebrand disease: Although less likely given the acute presentation and the lack of a known family history, von Willebrand disease could explain the epistaxis and petechiae. However, the significantly low platelet count is not typical for von Willebrand disease alone.
- Disseminated intravascular coagulation (DIC): While DIC could explain the low platelet count and the presence of petechiae, the lack of other clinical signs such as organ dysfunction, the normal prothrombin time, and the absence of schistocytes on the peripheral smear make it less likely.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Meningococcemia: Although the patient does not have the classic signs of meningococcemia such as fever, meningismus, or a characteristic rash (which is typically more purpuric and can start as petechiae), this diagnosis is critical to consider due to its high mortality rate if not promptly treated. The recent viral illness could potentially be a predisposing factor.
- Thrombotic thrombocytopenic purpura (TTP) or Hemolytic uremic syndrome (HUS): These conditions, although rare, can present with thrombocytopenia and could be considered in the differential, especially if there are signs of microangiopathic hemolytic anemia (which is not clearly indicated here but would be a critical finding).
- Rare diagnoses
- Pseudothrombocytopenia: This is a condition where platelets clump in vitro, leading to a falsely low platelet count. It's less likely given the clinical presentation of petechiae and epistaxis, suggesting a true thrombocytopenia.
- Other rare bleeding disorders: Such as factor deficiencies or platelet function disorders, which could present with similar symptoms but are less likely given the lack of a known family history or previous bleeding episodes.