Vitamin B1 (Thiamine) is an essential water-soluble vitamin that plays a crucial role in energy metabolism and nervous system function. This test measures thiamine levels in whole blood to evaluate nutritional status and diagnose deficiency. Thiamine serves as a coenzyme for critical metabolic reactions including carbohydrate metabolism, energy production, and the synthesis of neurotransmitters like acetylcholine and GABA. Deficiency can cause severe neurological and cardiovascular complications including Wernicke encephalopathy, Korsakoff syndrome, beriberi, and heart failure. Thiamine deficiency is most common in alcoholics, patients with malabsorption syndromes, those on long-term parenteral nutrition, and individuals with increased metabolic demands such as pregnancy or hyperthyroidism. This test is particularly valuable for evaluating unexplained neurological symptoms, monitoring nutritional therapy, and assessing thiamine status in high-risk populations.