A method of detecting soluble lithium in a liquid biological sample comprising combining: (a) an amount of the biological sample; (b) 2,3,7,8,12,13,17,18-octobromo-5,10,15,20-tetrakis(4-sulphonatophenyl) porphyrin; (c) alkali; (d) detergent; (e) chelating agent; and (f) optionally a suitable solvent; to produce a test sample; detecting absorbance change of the test sample relative to a lithium deficient standard sample at a wavelength of between about 475 nm and about 485 nm, between about 515 nm and about 525 nm or simultaneously between about 475 nm and about 485 nm and between about 515 nm and about 525 nm, wherein absorbance change indicates presence of soluble lithium in the biological sample. Also disclosed is a method of quantitatively measuring soluble lithium ions in a biological sample and a reagent for use in such methods.