Differential Diagnosis for 45F with High Fevers, Rigors, Myalgia, and Negative Rat Test

• Single Most Likely Diagnosis
  • Influenza: Given the symptoms of high fevers, rigors, and myalgia, influenza is a highly plausible diagnosis, especially during flu season. The negative rat test (assuming this refers to a test for rat-borne illnesses) does not rule out influenza.
• Other Likely Diagnoses
  • Viral infections (e.g., COVID-19, mononucleosis): These can present with similar symptoms and are common in the general population.
  • Bacterial infections (e.g., pneumonia, sepsis): While the negative rat test might lean away from certain zoonotic infections, other bacterial infections could still be responsible for these symptoms.
  • Autoimmune disorders (e.g., rheumatoid arthritis, lupus): These conditions can cause systemic symptoms including fever and myalgia, though they might not typically present with rigors.
• Do Not Miss Diagnoses
  • Sepsis: Although it might not be the first consideration, sepsis is a life-threatening condition that requires immediate attention. The presence of high fevers and rigors could indicate a severe infection.
  • Meningitis or encephalitis: These are serious and potentially life-threatening conditions that could present with fever and systemic symptoms. Early diagnosis is crucial.
  • Leptospirosis: Despite the negative rat test, if the patient has been exposed to contaminated water, this diagnosis should not be entirely ruled out without further testing.
• Rare Diagnoses
  • Lymphoma: Certain types of lymphoma can present with systemic symptoms like fever and myalgia, though this would be less common.
  • Hematologic disorders (e.g., hemophagocytic lymphohistiocytosis): These are rare but can present with high fevers, myalgia, and other systemic symptoms.
  • Exotic infections (e.g., hantavirus, Lassa fever): Depending on the patient's travel history and exposure, these could be considered, though they are rare in most populations.