Method of measuring target substance, and reagent therefor

A measurement method for measuring a target substance in a specimen,

• the target substance being a protein having a molecular weight of 150 kDa or more and 500 kDa or less, which is incorporated into the specimen at a concentration of 0.8 ng/ml or more and 2.0 ng/ml or less,
• the measurement method comprising:
• a first mixing step of mixing the specimen and a first reagent to provide a first mixed solution;
• a second mixing step of mixing the first mixed solution and a second reagent, which includes a particle having immobilized thereon a ligand that specifically binds to the target substance, to provide a second mixed solution;
• and
• a measurement step of measuring an absorbance of the second mixed solution,
• wherein a concentration of the specimen in the second mixed solution is 4.0 vol % or more and 30.0 vol % or less,
• wherein a mass ratio of the particle to the specimen in the second mixed solution is 1:500 or more and 1:50 or less, and
• wherein the measurement method includes measuring the specimen in which an amount of change in absorbance per 1.0 ng/ml of the target substance in the measurement step is 0.0012 or more and less than 0.0100.