Cellulose derivative fine particle, dispersion liquid thereof, dispersion body thereof and diagnostic reagent

A diagnostic reagent comprising a cellulose derivative fine particle, and a buffer solution, wherein the cellulose derivative fine particle comprises cellulose derivative with a part of hydroxyl groups of cellulose being substituted by a substituent, the cellulose derivative fine particle is water-insoluble, the average particle diameter of the cellulose derivative fine particle obtained by observation using electron microscope is from 9 to 1,000 nm, a coefficient of variation (CV) value is 30% or less, a ratio between major axis and minor axis of the cellulose derivative fine particle is 2.0 or less, the substitutent comprises any one or more members selected from the group consisting of a carboxyl group, an amino group, a quaternary ammonium group and a hydroxyalkyl group, the substitution degree of said substitution is 0.5 or less, and biomaterial is loaded on the cellulose derivative fine particle through chemical bonding or physical adsorption.