PURPOSE: To prevent the poisoning at the time of medical treatment of a depression patient, by adding a specific chelating agent to a blood serum removed protein together with thorin and acetone and measuring absorbance and then, quantitatively determing simply and accurately Li in the blood serum by using a calibration curve prepared preliminarily by a standard wave.

CONSTITUTION: At the time of determining quantitatively Li in blood by utilizing the fact that an orange color complex is formed by reacting thorin i.e. sodium salt of [o-(2-hydroxy-3,6-disulfo-1-naphthylazo) benzoarsonic acid] with Li, ethylene glycol (β-aminoethyl ether)-N, N, N', N'-tetraacetate is addes as a chelating agent for eliminating the interruption of Ca++ and also, acetone is added as a determination sensitivity improving agent together with the thorin to a blood serum which is removed protein by adding an aqueous solution of Cl3CCOOH to the blood serum. The absorbance of 482nm wavelength light generated by the existence of Li is measured and Li in the blood serum is determined quickly with high accuracy from a preliminarily obtained calibration curve.