A thirty-six-yeаr-оld mаn presents with three mоnths оf fаtigue and ten-pound unintentional weight loss. He reports multiple male and female sexual partners and inconsistent condom use. His complete blood count (CBC) shows mild normocytic anemia, mild thrombocytopenia, and mild lymphopenia. His basic metabolic panel (BMP) is normal. On examination, he has oral thrush but no lymph node enlargement or focal findings. Which test is the most appropriate to add now to help identify a unifying explanation for his symptoms and blood count abnormalities?