SPEP

Serum protein electrophoresis (SPEP) is a laboratory test that separates the proteins in blood serum by their electric charge and size, producing a pattern of bands that reflect distinct protein fractions. The test is typically performed on serum using cellulose acetate or agarose at a fixed alkaline pH, and results are analyzed both visually and by densitometry to estimate the concentration of each fraction. The usual fractions are albumin, and the globulins: alpha-1, alpha-2, beta, and gamma.

SPEP is commonly used to screen for and characterize monoclonal gammopathies. A normal pattern shows relatively smooth, distinct bands in each fraction. Abnormal patterns may reveal a monoclonal spike, or M-spike, usually narrow and prominent in the gamma region but sometimes in the beta region, indicating a clonal immunoglobulin protein. A broad-based increase in the gamma region suggests a polyclonal gammopathy, often due to inflammation or liver disease. Abnormal patterns may be accompanied by changes in the albumin or globulin ratios.

Follow-up testing typically includes serum immunofixation electrophoresis (IFE) to identify the specific immunoglobulin class and light chain involved, as well as quantitative immunoglobulins and sometimes the serum free light chain assay. Urine protein electrophoresis (UPEP) can be used to detect light chains not evident in serum.

Limitations include reduced sensitivity for very small M-proteins and potential masking by polyclonal background or technical factors. SPEP is frequently used to monitor treatment response in conditions such as multiple myeloma.