The invention relates to detection the presence of a target molecule in a sample, wherein the sample is contacted with a substrate, the substrate subsequently being washed in a wash step. In particular, the invention relates to a method of detecting the presence of a target molecule in a sample, the method comprising: (a) contacting the sample (37) with a substrate having immobilized thereon probe molecules that specifically binds to the target molecule; (b) washing the substrate (38) in a wash step by a wash fluid in order to remove or dilute unbound target molecules; (c) detect the presence of resultant binding complexes (39) on the substrate to determine whether the target molecule is present in the sample. The wash fluid being substantially refractive index matched to the substrate.