Q. A 28-year-old male was admitted to the hospital with a flare-up of Crohn's disease. He had a history of Crohn's disease and had been previously treated with several medications, including aminosalicylates, corticosteroids, and immunomodulators. Despite previous treatments, his symptoms had worsened, and he had developed abdominal pain, diarrhea, and weight loss. The patient underwent a thorough evaluation, including a physical examination, laboratory tests, and imaging studies. The results of the evaluation confirmed that the patient was in a flare-up of Crohn's disease, and the decision was made to initiate treatment with infliximab, a monoclonal antibody that targets tumor necrosis factor, a protein involved in inflammation. The patient received the first dose of infliximab in the hospital, and his symptoms improved significantly over the next few days. He was discharged from the hospital and scheduled for a follow-up appointment in two weeks. Today, at his follow-up appointment, the patient reports that he has developed several new symptoms, including rash, joint pain, and fever. After further evaluation, you suspect the patient has developed serum sickness. Before you treat him with corticosteroids and antihistamines, and to evaluate the severity of his reaction, which of the following tests is most indicated?