SPR is an extremely sensitive method for the detection of molecular interactions by tracking the change of signal via sensor chips. SPR signal is defined as refractive index changes, which refers the response unit (RU) is approximately equivalent to a surface concentration of protein at 1pg/mm2. Plotting the SPR signal over time during the interaction between two molecules results in a sensorgram, which could be visualized in real time

General binding response includes three phases: association, equilibrium and dissociation; fitting these sensorgram data with a mathematic model allows scientists to calculate the association (Ka) and dissociation (Kd) rate constants and ultimately determine the binding affinity (KD). Different from isothermal calorimetry (ITC) that measures antibody binding affinity based on equilibrium point, SPR could obtain full kinetic parameters to evaluate all the effects of association and dissociation as well as antibody affinity.