A 40-year-old male farmer presents with fever, headache, and diarrhea for 3 days, visiting on November 10. Physical exam: temperature 39°C, BP 50/30 mmHg, apathy, facial flushing, conjunctival congestion and edema, extensive fine petechiae in both axillae, no hepatosplenomegaly. Laboratory tests: WBC 32×10⁹/L, Hb 160 g/L, platelets 34×10⁹/L, urinary protein (+++), stool microscopy shows WBC 5-8/HP. Which lab test is most appropriate for diagnosis?