TKT and Thiamine deficiency: Historically,assays for blood transketolase activity as an indirect measurement ofTPP content have been used to assess thiamine deficiency.62 However, direct measurement of TPP or thiamine via highperformance liquid chromatography has been shown to be more preciseand robust.63,64 There issome controversy about reference ranges for thiamine, as somevariation between regions is to be expected based on different dietaryand environmental factors.