ELISA test. Multi-pipettes and a multi-well sample tray during an enzyme-linked immunosorbent assay (ELISA) blood test. This test measures the amount of a target substance in the blood. An antibody that binds with the target substance is mixed with a blood sample. An enzyme is also added, which binds to the antibody-target complex. The amount of enzyme that binds to the complex is in direct proportion to the amount of the target in the sample. The enzyme's substrate is added. The action of the bound enzyme on the substrate produces a colour change. The amount of the substrate that is broken down gives a measurement of the amount of the target in the sample.