Vitamin B12 deficiency occurs in 10% of the elderly (10 million in EU) and is associated with psychiatric morbidity (dementia and depression), costing EU many billions of ecru/year. Treatment is inexpensive and effective but identification of those needing therapy is hampered by the poor diagnostic accuracy of conventional tests. A European company has developed an assay to measure holotranscobalamin (helot), a sensitive marker of early B12 deficiency. The aims of the project are to demonstrate the analytical performance of this assay in 4 countries; to show its superiority to conventional methods for assessment of vitamin B12 status; and to demonstrate its value in older patients with depression or dementia. Socio-economic benefits will be: identification and early treatment of persons with B12-related morbidity; improved health economy, and promotion of European industry in laboratory diagnostics.