Lithium is the standard first line of treatment for bipolar disorder. Because of its potential toxicity, levels of the drug have to be constantly monitored. To this end, an accurate and rapid assay is thus required. Moreover, lithium measurement allows differential diagnosis in patients with hallucinations, dementia and amnesia. The lithium concentration in blood is usually evaluated by flame photometry, atomic absorption spectroscopy or ion selective electrode. The aim of this study was to evaluate the analytical performance of a new colorimetric assay for lithium. Within-day precision of a pool with a target value of 1.25 mmol/l was 2.18% (CV) and day-to-day precision was assessed using serum aliquots containing two concentrations of lithium, and CVs were 5.47 and 1.6% at 0.6 and 1.15 mmol/l, respectively. The lower detection limit (LLD) was <0.08 mmol/l. The assay seemed to be linear up to 5 mmol/l. Results of a comparison method using Deming regression were: y=1.161x-0.0075 (r=0.9879) for regression with the Flame Photometer and y=0.9729 x+0.0133 (r=0.9787) for regression with atomic absorption spectroscopy. Moreover, in this study we did not find any interference with drugs or other analytes tested. In conclusion, this assay may be a method of choice for measuring lithium blood concentrations, especially in emergency situations.