Vitamin B3 (niacin) is a water-soluble vitamin that exists in two forms: nicotinic acid and nicotinamide. This test measures both forms to evaluate niacin nutritional status and diagnose deficiency. Niacin is essential for the production of coenzymes NAD and NADP, which are involved in hundreds of enzymatic reactions including energy metabolism, DNA repair, and cellular signaling. Severe niacin deficiency causes pellagra, characterized by the 'four Ds': dermatitis, diarrhea, dementia, and death. Deficiency can result from inadequate dietary intake, malabsorption syndromes, or conditions that interfere with tryptophan metabolism (the amino acid precursor to niacin). This test is particularly valuable for evaluating patients with unexplained skin rashes, gastrointestinal symptoms, or neurological problems, as well as monitoring nutritional therapy in patients with malabsorption disorders.