A 35-year-odl man presents with 1 week of fever, sore throat, malaise, and anorexia. He reports travel to New York City for vacation 4 weeks ago. He has no significant PMH and no drug allergies. He is taking no meds. He denies IVDU. He does report unprotected sexual contractions while in New York City. On examination, he is alert but uncomfortable. Temp 38.5C, PO2 96, RR 16, BP 115/70. HEENT: oropharyngeal erythema and mild thrush. Chest/heart: unremarkable. Abdomen benign. He has cervical, axillary, and lingual adenopathy. There is a fine macular rash on his trunk and extremities. WBC count is 7,600. Heterophilic antibody tests are negative. Your differential diagnosis includes acute retroviral syndrome. The best approach for making a specific diagnosis of acute HIV infection is?