Differential Diagnosis for NS1 Antigen Positive in First 5 Days of Illness

Single Most Likely Diagnosis

• Dengue Fever: The presence of NS1 antigen in the first 5 days of illness is highly suggestive of dengue fever. NS1 antigen is a protein produced by the dengue virus and can be detected in the blood early in the course of the disease.

Other Likely Diagnoses

• Zika Virus Infection: Although less common, Zika virus infection can also present with positive NS1 antigen in the early stages. However, the clinical presentation and epidemiological context are crucial for differentiating it from dengue.
• Other Flavivirus Infections: Infections with other flaviviruses, such as yellow fever or West Nile virus, could potentially yield positive NS1 antigen results, though this is less common and depends on the specific diagnostic tests used.

Do Not Miss Diagnoses

• Chikungunya: While not typically diagnosed through NS1 antigen, chikungunya can present similarly to dengue and Zika, with severe joint pain and fever. Missing this diagnosis could lead to inappropriate management and increased morbidity.
• Severe Bacterial Infections: Conditions like sepsis or severe bacterial infections (e.g., leptospirosis, rickettsial diseases) can mimic the early stages of viral hemorrhagic fevers and must be considered to avoid missing potentially fatal bacterial infections.

Rare Diagnoses

• Other Viral Hemorrhagic Fevers: Although rare in many parts of the world, other viral hemorrhagic fevers (e.g., Ebola, Lassa fever) could potentially present with similar symptoms and must be considered in patients with a relevant travel or exposure history.
• Cross-Reactive Infections: Infections that cross-react with dengue or Zika diagnostic tests, such as Mayaro virus or O'nyong-nyong virus, are rare but could be considered in specific epidemiological contexts.