A method of determining the presence of an analyte present in a liquid sample, the method comprising: a) contacting the liquid sample with a predetermined amount of an analogue of the analyte and a first binding moiety, wherein: the analogue comprises an amino acid sequence which has been recombinantly expressed or synthesized in vitro to include a binding region composed of amino acids that can be bound by a second binding moiety, such that the analogue incorporates only one of said binding region, wherein the binding region is not present in the analyte or is present in the analyte but not accessible to the second binding moiety; and the first binding moiety is capable of binding each of the analyte and the analogue independently, such that a mixture is formed comprising an analogue-first binding moiety complex, or, when analyte is present in the sample, either (i) an analogue-first binding moiety complex and an analyte-first binding moiety complex, or (ii) an analyte-first binding moiety complex and no analogue-first binding moiety complex; b) contacting the mixture with the second binding moiety, wherein the second binding moiety is an antibody capable of binding the analogue-first binding moiety complex but not the analyte-first binding moiety complex, and is immobilised or immobilisable on a solid phase; c) determining the level of a signal indicative of the presence of the analogue-first binding moiety complex bound to the second binding moiety; wherein if the level of the signal determined in step c) is lower than the level of a maximum signal determined when no analyte is present, then analyte is present in the sample. a) contacting the liquid sample with a predetermined amount of an analogue of the analyte and a first binding moiety, wherein: the analogue comprises an amino acid sequence which has been recombinantly expressed or synthesized in vitro to include a binding region composed of amino acids that can be bound by a second binding moiety, such that the analogue incorporates only one of said binding region, wherein the binding region is not present in the analyte or is present in the analyte but not accessible to the second binding moiety; and the first binding moiety is capable of binding each of the analyte and the analogue independently, such that a mixture is formed comprising an analogue-first binding moiety complex, or, when analyte is present in the sample, either (i) an analogue-first binding moiety complex and an analyte-first binding moiety complex, or (ii) an analyte-first binding moiety complex and no analogue-first binding moiety complex; the analogue comprises an amino acid sequence which has been recombinantly expressed or synthesized in vitro to include a binding region composed of amino acids that can be bound by a second binding moiety, such that the analogue incorporates only one of said binding region, wherein the binding region is not present in the analyte or is present in the analyte but not accessible to the second binding moiety; and the first binding moiety is capable of binding each of the analyte and the analogue independently, such that a mixture is formed comprising an analogue-first binding moiety complex, or, when analyte is present in the sample, either (i) an analogue-first binding moiety complex and an analyte-first binding moiety complex, or (ii) an analyte-first binding moiety complex and no analogue-first binding moiety complex; b) contacting the mixture with the second binding moiety, wherein the second binding moiety is an antibody capable of binding the analogue-first binding moiety complex but not the analyte-first binding moiety complex, and is immobilised or immobilisable on a solid phase; c) determining the level of a signal indicative of the presence of the analogue-first binding moiety complex bound to the second binding moiety;