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

The feasibility of using the chromogenic dye Thorin to spectrophotometrically measure lithium concentration in human saliva was explored.  Absorbance wavelength maximum of the Li-Thorin complex was found to be 480 nm.  Absorbance at 480 nm was obtained for saliva calibration standards containing 0.00-5.29 mEq/L of lithium.  A least-squares fit produced a regression equation y = 0.128x + 1.449, R = 0.997. This was used to predict lithium concentrations in both artificially prepared lithium/saliva test solutions and in hospitalized patients treated with lithium. Results agreed well with atomic absorption spectroscopy. Using a reagent blank with an equivalent amount of saliva as the test samples eliminated protein and electrolyte absorbance interference. This study supports the continued exploration of this method as a non-invasive point-of-care testing approach for monitoring saliva lithium in bipolar disorder.