The Vitamin B3 test measures the levels of nicotinamide and nicotinic acid, the two main forms of vitamin B3, in the blood. This test is used to evaluate vitamin B3 status in patients with symptoms of deficiency, such as pellagra, characterized by dermatitis, diarrhea, and dementia. It can also help assess the nutritional status in individuals with malabsorption issues, alcohol dependence, or those on restrictive diets.