Agreement between measurement of 25-hydroxyvitamin D3 in dried blood spot samples and serum in a Chinese population in the Netherlands.

Blood for determining 25-hydroxyvitamin D3 [25(OH)D3] is usually obtained through venipuncture although, as an alternative for serum, dried blood spot (DBS) can be considered. The aim of this proof-of-concept study was to investigate levels of agreement between measurements of 25(OH)D3 obtained with DBS compared with serum. 301 Chinese participants were included who completed 25(OH)D3 measurement from DBS and from simultaneously collected blood samples obtained by venipuncture. Measurements of both DBS and serum 25(OH)D3 were performed using liquid chromatography followed by tandem mass spectrometry. Agreement between the two methods was assessed with Passing and Bablok regression analysis and Bland-Altman plot. Measurements showed a good correlation (Pearson's correlation coefficient r = 0.929, P < 0.001) between the two methods. After recalculating for a 13% difference, a regression equation of DBS 25(OH)D3 = -1.91 + 1.00 serum 25(OH)D3 was found in Passing and Bablok regression analysis. Bland-Altman analysis showed a fixed bias of 1.7 nmol/L; upper and lower limit of agreement was 24.1 nmol/L and -20.7 nmol/L, respectively. Sensitivity of recalculated DBS for 25(OH)D3 concentrations <30 and <50 nmol/L was 87.8% and 91.1%, respectively, and specificity was 89.2% and 83.1%, respectively. In conclusion, a good agreement was found between the measurement of 25(OH)D3 obtained with DBS compared with serum. DBS may possibly be used in a future screening program, but it is less suitable for individualized vitamin D status assessment.