Hemoglobin-normalized thiamine diphosphate (vitamin B1) in whole blood: Reference intervals and clinical implications for status assessment

Objectives Vitamin B1 status is typically assessed by measuring whole blood thiamine diphosphate (TDP). However, volumetric expression (nmol/L) may be misleading in anemia or polycythemia, as TDP is primarily carried within erythrocytes. While hemoglobin-normalized TDP (ng/g Hb) may provide a more robust biomarker—less influenced by hematological variations—reference interval (RI) data are scarce. Thus, we aimed to establish RIs for Hb-normalized TDP and evaluate the impact of its expression (ng/g Hb vs. nmol/L) on thiamine status classification. Design and methods RIs for Hb-normalized TDP were established using 130 healthy adults (18–65 years) per CLSI EP28-A3c. Exclusion criteria included vitamin B1 supplementation, denutrition, hepatic disease, and other conditions affecting thiamine metabolism. TDP was quantified via HPLC with fluorescence detection. Outliers were excluded using Tukey's and Dixon's methods. A retrospective analysis of 5429 routine samples assessed reclassification rates using Cohen's weighted kappa and the net reclassification index (NRI). Results The Hb-normalized TDP RI was 249–531 ng/g Hb (90% CI: 241–268, 519–557), and the volumetric RI was 88–181 nmol/L (90% CI: 83–92, 175–189). The units were strongly correlated ( r = 0.829, p 〈10 −33 ). No sex- or age-specific partitioning was required (z = 2.14, z* = 3.12). Among routine samples, the reclassification rate was 11.3% (weighted κ = 0.682, good agreement), with an NRI of 8.58%. Notably, 88% of reclassifications were upward, and 68.5% of samples with low TDP in nmol/L were normal in ng/g Hb. The reclassification rate rose to 25.8% in anemic patients. Conclusions We established RI for Hb-normalized TDP according to CLSI methodology. Our findings support adopting ng/g Hb for TDP assessment, particularly in anemic patients.