Dengue Hemorrhagic Fever and Dengue Shock Syndrome are caused by infection of RNA Flavivirus transmitted by a mosquito vector. This test differentiates between Primary and Secondary infection. Paired acute and convalescent specimens that exhibit a significant change in titer are useful to confirm the clinical diagnosis of infection. NS1 antigen is a nonstructural protein found in infected patients from 1st day of fever up to 5 days after the onset of fever.