Differential Diagnosis

• Single most likely diagnosis
  • Meningococcemia: This condition is characterized by the presence of fever, altered mental status, headache, and purpura, which are all consistent with the patient's presentation. The laboratory findings of thrombocytopenia (low platelet count), elevated INR (indicating coagulopathy), and schistocytes on smear (indicating microangiopathic hemolytic anemia) also support this diagnosis. Meningococcemia is a severe infection caused by Neisseria meningitidis that can lead to sepsis and disseminated intravascular coagulation (DIC), explaining the patient's hypotension and tachycardia.
• Other Likely diagnoses
  • Severe sepsis or septic shock due to other bacterial infections: The patient's presentation with fever, hypotension, tachycardia, and altered mental status could be indicative of severe sepsis or septic shock from other bacterial sources. However, the presence of purpura and specific laboratory findings points more towards meningococcemia.
  • Thrombotic thrombocytopenic purpura (TTP): TTP is characterized by thrombocytopenia, microangiopathic hemolytic anemia, renal failure, neurological symptoms, and fever. While some of these features are present, the acute onset of fever, headache, and purpura, along with the severity of the patient's condition, makes meningococcemia more likely.
• Do Not Miss diagnoses
  • Rocky Mountain spotted fever (RMSF): Although less common, RMSF can present with fever, headache, and a petechial or purpuric rash. It is a tick-borne illness caused by Rickettsia rickettsii and can be severe if not treated promptly. Missing this diagnosis could be fatal.
  • Hemorrhagic fever with renal syndrome (HFRS) or other viral hemorrhagic fevers: These conditions can present with fever, renal failure, and hemorrhage, including purpura. They are less common but can be deadly if not recognized and treated appropriately.
• Rare diagnoses
  • Atypical hemolytic uremic syndrome (aHUS): This is a rare condition characterized by microangiopathic hemolytic anemia, thrombocytopenia, and renal failure, but it typically does not present with the acute onset of fever and purpura seen in this patient.
  • Purpura fulminans: This is a rare, severe disorder of the blood vessels that can cause purpura, but it is more commonly associated with severe deficiency of protein C or protein S, and the presentation might not fully align with the patient's symptoms and lab findings.