The Western blot test, also called immunoblotting, is a test for a specific protein within a protein mixture. The Western blot test is performed after gel-electrophoresis or an enzyme-linked immunosorbent assay (ELISA) test, and it uses antibodies to identify specific proteins.

Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) is a common test used to separate proteins for use in the Western blot. The proteins are separated by weight and electrical properties as they move through a gel matrix.

The ELISA test uses enzymes or antibodies attached to a solid surface to create the test surface. A sample is then added to the test surface. Antibodies or enzymes reacting or attaching to proteins indicates a positive result.

The Western blot test is performed after the gel-electrophoresis. The separated proteins are transferred (or blotted) onto nitrocellulose or nylon membranes and identified by specific antibodies that are tagged by a secondary protein.

The Western blot test is used to confirm positive results from either gel-electrophoresis or ELISA tests. The Western blot test can identify proteins more specifically and can rule out false positives.

The Western blot test is generally used to confirm positive test results for human immunodeficiency virus (HIV) and Lyme disease.