A method for determining the presence or absence of an analyte in a liquid sample, comprising: (a) providing a test device for determining the presence or absence of an analyte in a liquid sample, comprising: (i) a sample receiving member; (ii) a carrier in fluid communication with the sample receiving member; (iii) a labeled reagent which is mobile in the carrier in the presence of the liquid sample, the labeled reagent comprising a ligand that binds to the analyte and a coated nanoparticle consisting of a gold core and a shell having the ligand attached thereto, wherein the shell comprises glass, a ceramic material such as perovskite and ZrO 2, or a polymer such as polyethylene glycol, polymethylmethacrylate and polystyrene, and increases the reflectance of the nanoparticle, and wherein the coated nanoparticle does not include a Raman-active molecule; and (iv) a binding reagent effective to capture the analyte, when present, immobilized in a defined detection zone of the carrier; (b) contacting the liquid sample with the sample receiving member of the test device; (c) allowing the liquid sample applied to the sample receiving member to mobilize said labeled reagent such that the liquid sample and labeled reagent move along the length of the carrier to pass into the detection zone; (d) detecting the presence of the labeled reagent in the detection zone by measuring reflectance, wherein detection of the labeled reagent in the detection zone is indicative of the presence of analyte in the liquid sample, and failure to detect the presence of the labeled reagent in the detection zone is indicative of the absence of the analyte in the liquid sample. (a) providing a test device for determining the presence or absence of an analyte in a liquid sample, comprising: (i) a sample receiving member; (ii) a carrier in fluid communication with the sample receiving member; (iii) a labeled reagent which is mobile in the carrier in the presence of the liquid sample, the labeled reagent comprising a ligand that binds to the analyte and a coated nanoparticle consisting of a gold core and a shell having the ligand attached thereto, wherein the shell comprises glass, a ceramic material such as perovskite and ZrO 2, or a polymer such as polyethylene glycol, polymethylmethacrylate and polystyrene, and increases the reflectance of the nanoparticle, and wherein the coated nanoparticle does not include a Raman-active molecule; and (iv) a binding reagent effective to capture the analyte, when present, immobilized in a defined detection zone of the carrier; (i) a sample receiving member; (ii) a carrier in fluid communication with the sample receiving member; (iii) a labeled reagent which is mobile in the carrier in the presence of the liquid sample, the labeled reagent comprising a ligand that binds to the analyte and a coated nanoparticle consisting of a gold core and a shell having the ligand attached thereto, wherein the shell comprises glass, a ceramic material such as perovskite and ZrO 2, or a polymer such as polyethylene glycol, polymethylmethacrylate and polystyrene, and increases the reflectance of the nanoparticle, and wherein the coated nanoparticle does not include a Raman-active molecule; and (iv) a binding reagent effective to capture the analyte, when present, immobilized in a defined detection zone of the carrier; (b) contacting the liquid sample with the sample receiving member of the test device; (c) allowing the liquid sample applied to the sample receiving member to mobilize said labeled reagent such that the liquid sample and labeled reagent move along the length of the carrier to pass into the detection zone; (d) detecting the presence of the labeled reagent in the detection zone by measuring reflectance, wherein detection of the labeled reagent in the detection zone is indicative of the presence of analyte in the liquid sample, and failure to detect the presence of the labeled reagent in the detection zone is indicative of the absence of the analyte in the liquid sample.