A method for measuring TAT complexes in a sample separated from a living body includes measuring TAT by performing latex immunoagglutination reaction under a condition of pH 5.8 to 6.6 using a TAT assay reagent. The TAT assay reagent includes a first antibody bound to a first latex particle, which binds to the antithrombin part of the TAT complex and recognizes the complex, and a second antibody bound to a second latex particle, which binds to the thrombin part of the TAT complex and recognizes the complex.