The present invention is directed to immunoassays for detecting one or more target analytes in a fluid sample wherein the detection reaction occurs on a solid support and involves an amplification system. In particular, the invention is directed to making and using a test device having at least one site for detecting the presence of at least one target analyte, wherein a conjugate comprising dextran-polystreptavidin is immobilized at the test site(s) as a capture reagent for a complex containing the target analyze.