A 28-year-old male from Himachal Pradesh presents with fever, severe headache, and petechial rash on the trunk and extremities for 5 days. He has a history of tick bite 10 days ago. Laboratory findings show thrombocytopenia (platelets 80,000/μL) and elevated transaminases (AST 120 U/L, ALT 95 U/L). Which investigation would be most specific for confirming Rocky Mountain spotted fever (RMSF)?