A precise laboratory measurement assessing the concentration of Thiamine (Vitamin B1) directly within the blood serum or plasma. Thiamine biologically functions as an absolutely indispensable enzymatic cofactor driving both energy and carbohydrate cellular metabolism. Clinically, maintaining an optimal thiamine concentration is biologically vital to ensure the proper, uninterrupted physiological functioning of the entire nervous system, myocardial tissues (the heart), and skeletal muscle networks.