A method of calibrating an assay for detecting the presence or amount of one or more analytes in a biological sample, the method comprising: (a) providing: (i) one or more magnetic particles capable of being positioned at a localized area of an assay vessel; (ii) a first SERS-active nanoparticle which has associated therewith at least one specific binding member having affinity for one or more analytes of interest in the biological sample and capable of producing a first detectable signal, wherein the first detectable signal is dependent on an amount of the one or more analytes in the biological sample; and (iii) a second SERS-active nanoparticle which is attached to said one or more magnetic particles and which has associated therewith at least one specific binding member having affinity for one or more analytes of interest in the biological sample and capable of producing a second detectable signal, wherein the second detectable signal is dependent on an amount and a position of the one or more magnetic particles and is substantially independent of the amount of the one or more analytes in the biological sample; and (b) comparing the first detectable signal to the second detectable signal to calibrate the assay for detecting the presence or amount of one or more analytes in the biological sample. (a) providing: (i) one or more magnetic particles capable of being positioned at a localized area of an assay vessel; (ii) a first SERS-active nanoparticle which has associated therewith at least one specific binding member having affinity for one or more analytes of interest in the biological sample and capable of producing a first detectable signal, wherein the first detectable signal is dependent on an amount of the one or more analytes in the biological sample; and (iii) a second SERS-active nanoparticle which is attached to said one or more magnetic particles and which has associated therewith at least one specific binding member having affinity for one or more analytes of interest in the biological sample and capable of producing a second detectable signal, wherein the second detectable signal is dependent on an amount and a position of the one or more magnetic particles and is substantially independent of the amount of the one or more analytes in the biological sample; and (i) one or more magnetic particles capable of being positioned at a localized area of an assay vessel; (ii) a first SERS-active nanoparticle which has associated therewith at least one specific binding member having affinity for one or more analytes of interest in the biological sample and capable of producing a first detectable signal, wherein the first detectable signal is dependent on an amount of the one or more analytes in the biological sample; and (iii) a second SERS-active nanoparticle which is attached to said one or more magnetic particles and which has associated therewith at least one specific binding member having affinity for one or more analytes of interest in the biological sample and capable of producing a second detectable signal, wherein the second detectable signal is dependent on an amount and a position of the one or more magnetic particles and is substantially independent of the amount of the one or more analytes in the biological sample; and (b) comparing the first detectable signal to the second detectable signal to calibrate the assay for detecting the presence or amount of one or more analytes in the biological sample.