The M spike (or monoclonal spike) refers to a sharp peak seen in the results of a specific blood test called serum protein electrophoresis (SPEP). This test measures the different types of proteins in the blood, and the M spike indicates the presence of a high level of abnormal monoclonal protein (also called M protein), produced by malignant plasma cells in multiple myeloma.

The M spike is a key marker used by doctors to diagnose multiple myeloma and monitor the disease's progression or response to treatment. A higher M spike suggests an increased amount of M protein, which typically correlates with the burden of disease, while a decreasing M spike can indicate successful treatment.