Involved free light chain: an early independent predictor of response and progression in multiple myeloma.

Serum and urine protein electrophoresis (sPEP/uPEP) are the standard methods for monitoring of multiple myeloma (MM). However, a method of detection with shorter half-life, such as serum-free light chain (FLC), could detect the response or progression earlier. In total, 450 MM patients were assessed in first, second, and third line. Response and progression were classified according to International myeloma working group guidelines. The overall median time to partial response or better was detectable significantly earlier with involved free light chain (iFLC) 1.94 months (IQR: 1.61-2.23) compared to sPEP 5.39 months (IQR: 3.88-7.00). In first line, iFLC detected progression earlier compared to sPEP, particularly in patients with progression more than 18 months after best response. In conclusion, a response observed by iFLC occurs at least a median of 3 months before response is detected by sPEP/uPEP.