A 32-year-old man presents to an Australian emergency department with fever, headache and myalgia 10 days after returning from Papua New Guinea. He took doxycycline intermittently for malaria prevention. He is alert, normotensive and tolerating oral fluids; there is no jaundice, respiratory distress or neurological deficit. An initial thick and thin blood film is negative, but the platelet count is 92 × 10^9/L (150–400 × 10^9/L). Which one of the following is the most appropriate next step?