Abstract: A method for detecting the presence or concentration of an analyte in a sample, wherein: a) the sample is contacted with an immobilized first binding reagent which is capable of binding the analyte if present in the sample; b) the sample is contacted with a second binding reagent which comprises a fusion protein having a reporter domain and a binding domain, and which is capable of binding the analyte if present in the sample, the first and second binding reagents being capable of binding the analyte simultaneously if present in the sample, such that said second binding reagent becomes immobilized through the analyte bound to the first binding reagent; and detecting whether the second binding reagent has become immobilized to thereby detect the presence or concentration of said analyte.