The utility of MASS‐FIX to detect and monitor monoclonal proteins in the clinic

The detection and quantification of monoclonal-proteins (M-proteins) are necessary for the diagnosis and evaluation of response in plasma cell dyscrasias. Immunoglobulin enrichment-coupled with matrix-assisted laser desorption ionization time-of-flight mass-spectrometry (MASS-FIX) is a simple and inexpensive method to identify M-proteins, but its clinical generalizability has not yet been elucidated. We compared MASS-FIX to protein electrophoresis (PEL), serum/urine immunofixation-electrophoresis (IFE), and quantitative serum free-light chain (FLC) for the identification of M-proteins in different clinical diagnoses. Paired serum and urine samples from 257 patients were tested. There were six patients for whom s-IFE and FLC ratio were positive and serum MASS-FIX was negative, but when serum and urine MASS-FIX results were combined, only one patient with light chain-MGUS was missed. Serum/urine-MASS-FIX detected M-proteins in 18 patients with negative serum/urine-PEL/IFE and serum-FLC, 10 of whom had multiple myeloma or AL amyloidosis, who were mistakenly thought to have complete hematologic response by serum/urine-PEL/IFE and serum-FLC. Nearly half of the AL amyloidosis patients had atypical spectra, which may prove to be a clue to the diagnosis and pathogenesis of the disease. In conclusion, MASS-FIX has a comparable sensitivity with PEL/IFE/FLC methods and can help inform the clinical diagnosis.