A 28-year-old male farmer presented with fever for 5 days accompanied by petechiae, transferred from a rural hospital to Zhongshan Hospital of Shanghai Medical University in November 1993. Physical examination: temperature 38°C, BP 70/50 mmHg, marked facial and periorbital congestion, conjunctival edema and congestion, cord-like petechiae in the axilla, and significant percussion tenderness in the kidney area. Epidemic hemorrhagic fever (EHF) was suspected. Which of the following is most helpful for confirming the diagnosis?