Dengue fever in first week is BEST diagnosed using which of the following investigation?

During the first week of dengue, performing IgM ELISA has a sensitivity of 90% and is used for diagnosing it. Other lab findings includes leukopenia and a mild drop in platelets. Dengue fever is transmitted by mosquito bite. Patients presents with fever, chills, severe retro-orbital pain, severe muscle and joint pain, nausea, and vomiting. During early stage erythema of face and torso occur followed by a centrifugal maculopapular rash. This rash can become petechial, and mild hemorrhagic signs such as epistaxis, gingival bleeding, microscopic blood in stool or urine may occur. The illness usually lasts for 5-7 days. Ref: Levin M.J., Weinberg A. (2012). Chapter 40. Infections: Viral & Rickettsial. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e