Patent ID: 7226777

Claim:
A turbidimetric immunoassay for quantifying a concentration of an analyte in a sample solution by mixing the sample solution containing an antigen as the analyte with a reagent containing an antibody that specifically binds to said analyte and detecting signals representing turbidity caused by an antigen-antibody complex produced by said specific binding reaction, said turbidimetric immunoassay comprising the steps of: (1) mixing a plurality of solutions with different known concentrations of analyte and said reagent, respectively, to give a plurality of mixed solutions with different known concentrations of analyte; (2) measuring the turbidity levels of said plurality of mixed solutions over time; (3) determining for each said mixed solutions, the maximum value of said measured turbidity levels (S peak ), the elapsed time at which S peak is observed (T peak ), and the turbidity level at certain elapsed time T after the mixing, (S(T)); (4) creating a first calibration curve indicating a first correlation between said known concentrations of analyte and the turbidity levels S(T) detected in said step (3), and a second calibration curve indicating a second correlation between said maximum values of said measured turbidity levels (S peak ) detected in said step (3), said elapsed time at which S peak is observed (T peak ), and said known concentrations of analyte, wherein said second calibration curve has the maximum value of said S peak (Sm peak ); (5) after step (4), mixing said sample solution and said reagent to give a mixed sample solution; (6) measuring the turbidity levels of said mixed sample solution over time; (7) detecting for said mixed sample solution, the maximum value of the turbidity levels measured in said step (6) (Ss peak ), the elapsed time at which Ss peak is observed (Ts peak ), and the turbidity level Ss(T) of said mixed sample solution at the elapsed time T after the mixing; (8) comparing said Ts peak with the T peak at which Sm peak is observed in said second calibration curve (Tm peak ); (9) when said Ts peak is greater than said Tm peak , converting said Ss(T) into the concentration of said analyte using said first calibration curve; and (10) when said Ts peaks is equal to or smaller than said Tm peak , determining the concentration of analyte to be equal to or greater than the concentration of analyte corresponding to Sm peak in said second calibration curve.