A 60-year-old woman presents with back pain for over 4 months, occasional irregular fever, and urinary frequency with dysuria. Laboratory tests show: urinary protein (+++), white blood cells 5-10 per high power field (HPF), positive Bence Jones protein in urine, and serum protein electrophoresis (SPEP) revealing an M protein (39.5%) between β and γ regions. Which finding would most confirm the diagnosis?