A 27-year-old man presents to his primary care physician with complaints of a fever, headache, muscle aches, and swollen glands. The physician observes disseminated lymphadenopathy, pharyngitis, and a rash on the man's upper chest. The patient states that he had been to a party 2 weeks ago where he experimented with injecting drugs to get high. Needles were shared among the party-goers. A rapid latex test for human immunodeficiency virus (HIV) antibodies performed in the physician's office is negative. The doctor has a strong suspicion that this man has acute retroviral syndrome. Which of the following tests is most likely to support a diagnosis of HIV infection at this time?