Particle, affinity particle, test reagent, and detection method

A method of detecting a substance to be measured contained in a specimen, the method comprising:

• a first step of adding a specimen containing a substance to be measured and a test reagent containing an affinity particle and a dispersion medium for dispersing the affinity particle into a container having a first ligand immobilized on a lower side in a gravity direction;
• a second step of applying a magnetic field so that the affinity particle binds to the first ligand via the substance to be measured;
• and
• a third step of applying a magnetic field so that the affinity particle free from binding to the first ligand via the substance to be measured is separated away from the first ligand,
• wherein the affinity particle includes a particle comprising a magnetic particle containing a magnetic material,
• wherein the magnetic particle has a resin on a surface,
• wherein the particle has a volume average particle diameter of 0.4 μm or more and 1.5 μm or less,
• wherein the particle has a density of 5.1 g/cm3 or more and 10.0 g/cm3 or less, and
• wherein the resin has a functional group capable of binding a ligand, and
• further wherein the first ligand and the second ligand are each bound to the substance to be measured.