Fig. 1.

Illustration of single step competitive immunoassay. a In single step competitive immunoassay, capture antibodies are anchored to a solid phase in the testing tube. A certain concentration of labeled analyte (pre-prepared labeled antigen) is added to the testing tube together with the blood sample containing the studied hormone. b Both the studied analyte (the hormone) and the labeled analyte compete for the binding sites of the capture antibodies. After discarding the supernatant with both unbound endogenous and labeled analytes, the remaining signal of the bound labeled analyte is measured. The higher the concentration of the hormone (illustrated in green), the less labeled analyte will be bound, and thus, the less signal will be measured. In other words, the weaker the signal, the more of the endogenous hormone is present in the sample and vice versa (i.e. the signal strength is inversely proportional to the hormone concentration)