Spectrophotometric Measurement of Lithium in Human Saliva Using the Chromogenic Reagent Thorin

This study explored the feasibility of using the chromogenic dye Thorin to spectrophotometrically measure the lithium concentration in human saliva. The absorbance wavelength maximum of the Li-Thorin complex was determined to be 480 nm. Lithium concentrations were measured spectrophotometrically at 480 nm in human pooled saliva with lithium added to produce calibration standards of 0.00-5.29 mEq/L of lithium, which corresponds to a blood lithium range of 0.00-2.60 mEq/L, assuming a saliva/blood ratio of 2/1. A least-squares fit of the absorbance vs lithium concentration calibration data produced a regression equation y = 0.128x + 1.449 with correlation coefficient = 0.997. This regression equation was then used to predict lithium concentrations from absorbance data in prepared lithium/saliva test solutions and in hospitalized patients being treated with lithium. The results generally agreed well with those determined by atomic absorption spectroscopy. By measuring absorbance of test saliva vs reagent blank containing the same amount of saliva, interfering effects of saliva protein and electrolytes in the test samples were avoided. This study supports the continued exploration of this method as a non-invasive point-of-care testing approach for monitoring saliva lithium during lithium treatment.