Reagent kit, measurement kit, and measurement method

A measurement method for serum amyloid A in a biological sample, the method comprising:

• a step of preparing a mixed solution which contains a biological sample containing serum amyloid A, first particles having a label and modified with a first binding substance having a property of specifically binding to serum amyloid A, at least one nonionic surfactant having a hydrophilic-lipophilic balance value of 17 to 20, which is defined by (inorganicity value/organicity value)×10, and a molecular weight of 1000 or less, and a buffer adjusting a pH of a reaction solution to be in a range of 5.5 to 7.0 or a range of 8.5 to 9.0;
• a step of adding the mixed solution to an injection port at one end of a substrate on which a first metal film on which a third binding substance having a property of specifically binding to serum amyloid A is fixed is formed;
• a step of allowing the mixed solution to flow down onto the substrate;
• and
• a step of acquiring information of the label on the first metal film.