What does a positive result in a sandwich assay indicate?

In a sandwich assay, a positive result is indicated by the appearance of a specific line on the test. This type of assay typically involves two antibodies: a capture antibody that is attached to a surface (like a test strip) and a detection antibody that is linked to a signal, such as a colored reporter. When the target analyte binds to the capture antibody, the detection antibody can subsequently bind to the analyte, creating a "sandwich" complex.

The formation of this complex leads to the development of a visible line in the test area, signaling a positive result. This is crucial for confirming the presence of the target substance being tested for, as it signifies that the assay has successfully detected the analyte of interest.

The appearance of a control line is also a critical component of the test, indicating that the assay has functioned correctly. However, the specific line that indicates a positive result is directly related to the target being tested, which is why the presence of this specific line is the defining characteristic of a positive result in a sandwich assay.