A 60-year-old woman presents with irregular fever, low back pain, and facial edema for 5 months. Laboratory tests show: urinary protein (+++), WBC 5-10/HPF, positive Bence Jones protein in urine. Bone marrow aspiration reveals 20% myeloma cells (large cells with prominent nucleoli, including binucleated and trinucleated plasma cells). Diagnosis: multiple myeloma (MM). Which of the following tests is NOT relevant to MM staging?