Chapter 108 : Immunoassay-Based Tumor Marker Measurement: Assays, Applications, and Algorithms

(Left) Schematic diagram of a competitive binding (labeled-antigen) immunoassay. In this assay, a limited amount of immobilized antibody binds both labeled and unlabeled forms of the tumor marker antigen of interest. (Right) The higher the concentration of unlabeled tumor marker antigen in the subject's serum, the lower the signal generated by bound labeled antigen.

Schematic diagram of a noncompetitive binding two-site sandwich immunometric (labeled-antibody) assay. In this assay, molar excess immobilized antibody specific for the tumor marker binds unlabeled forms of the tumor marker antigen in the patient's serum. Bound antigen is then detected with a second antitumor marker antibody that binds to a different antigenic epitope. Either the second antibody can be detected by a third labeled antibody (left) or the second antibody can be directly labeled (middle). The higher the concentration of unlabeled tumor marker antigen in the subject's serum, the higher the signal generated by bound detection antibody (right).

In immunometric assays, a hook effect can occur when the capture and detection antibodies bind to different molecules, without the formation of a “sandwich,” as shown on the left panel. This results in an erroneous decrease in the measured signal at high concentrations of analyte; the effect on the dose-response curve is shown on the right panel.

Schematic diagram showing result deviation from linearity that may be caused by interference.