Assay

The present invention provides an assay kit for detecting an analyte of interest in a sample. The kit comprises a) a reporter species; b) a labelled species having first and second binding regions, wherein the first binding region is capable of binding to the analyte of interest and the second binding region is capable of binding to the reporter species; c) an immobilised species capable of binding to the first binding region of the labelled species; and d) immobilised capture reagent capable of binding to the reporter species. The arrangement is such that the sample is contacted with the labelled species, is then contacted with the immobilised species and is subsequently contacted with the immobilised capture reagent, the reporter species being added prior to exposure of the sample to the immobilised capture reagent. If no analyte is present in the sample, the labelled species becomes bound to the immobilised species and is therefore unable to bind to the immobilised capture reagent. If analyte is present in the sample, the analyte binds to the labelled species such that the labelled species is unable to bind to the immobilised species but can bind to the immobilised capture reagent via the reporter species, the presence of the analyte thus being determined by the presence of labelled species bound to the immobilised capture reagent via the reporter species.