Vitamin B1 is mainly excreted in urine as free thiamin and TMP, but some TDP and several other metabolites are present in smaller quantities. Urinary thiamin metabolites are related to plasma thiamin concentrations, showing values >100 μg/day with intakes of more than 0.3-0.4 mg/1000kcal, and <25 μg/day under thiamin deficient conditions (1).
This assay has been commonly used in the past and was regarded as the best functional test of thiamin status (3). Erythrocytes are lysed and ETK activity is measured before and after stimulation by the addition of TDP. However, this technique has been shown to be sensitive towards various interferences such as enzyme inhibition by alcohol or acetaldehyde, certain drugs, or high background absorbance from haemoglobin (8). Results obtained may be therefore subjected to uncertain variabilities.
Erythrocyte Transketolase (ETK) Activity
ETK activity is measured in washed erythrocytes, see methods in references:
(9) Delacoux et al. Clinica Chimica Acta 1980;108:483-6. (article in French)
(10) Takeuchi et al. Clinical chemistry 1984;30:658-61.
More recently, TDP analysis in whole blood (WB) or erythrocytes has emerged as a useful biomarker of thiamin status as it reflects body stores and offers a superior evaluation of thiamin status than total thiamin. It should be noted that the direct analysis of TDP does not reflect thiamin metabolic function (1). TDP can be measured in erythrocytes, but WB is preferred due to minimal required sample handling, and results obtained for TDP in WB are highly correlated to TDP in erythrocytes. However, WB samples for thiamin analysis need to be frozen to ensure lysis of the cells. When analysing TDP, procedures should be carried out under dim light due to the light sensitivity of thiamin vitamers (1). TDP and other thiamin vitamers can be measured simultaneously using chromatographic methods, mainly HPLC-FLD, and more recently, LC-MS.
Direct measurement of TDP and other thiamin vitamers LC-FLD, see method in reference (11): Lu et al. Clinical chemistry 2008;54:901-6.
Direct measurement of TDP and other thiamin vitamers LC-MS, see method in reference (12): Meisser Redeuil et al. Journal of Chromatography A 2015.Nov 27;1422:89-98