Methods are provided for forming a complex comprising an analyte (or an analyte analogue) from a blood-derived sample and labeling substances, and separating the complex from excess labeling substances and coexisting substances from the blood-derived sample, in a rapid, simple, convenient, and highly precise isotachophoresis (ITP) process by adding 2-(N-morpholino)ethane sulfonate (MES) salt and/or glutamate salt to the ITP sample. Methods are also provided for measuring the analyte in blood-derived samples with high precision and high sensitivity, based on the amount of the complex separated or the amount of uncomplexed labeling substance-containing molecules.