Electrophoresis: A laboratory technique that uses an electric current to separate substances, such as proteins or nucleic acids. The size and electrical charge (either positive or negative) of a substance determines how far it moves with the current. Electrophoresis may be used to help diagnosis certain diseases. This test separates substances, especially proteins, and analyzes molecular structure. Electrophoresis may be used to provide a confirmation of a diagnosis of multiple myeloma (9732/3). Electrophoresis is used to detect M proteins in the urine or serum (M-spike).