Red blood cell transketolase activity is reduced in deficiency of thiamine (vitamin B), and may be used in the diagnosis of Wernicke encephalopathy and other B-deficiency syndromes if the diagnosis is in doubt. Apart from the baseline enzyme activity (which may be normal even in deficiency states), acceleration of enzyme activity after the addition of thiamine pyrophosphate may be diagnostic of thiamine deficiency (0-15% normal, 15-25% deficiency, >25% severe deficiency).