Peer Review Report For: Quantification of 25-hydroxyvitamin D3 in dried blood spots as compared to plasma among Indian adults [version 2; peer review: 1 approved, 2 approved with reservations]

Background Vitamin D may play an important role in later-life physical and cognitive health. Vitamin D status is standardly assessed in serum and plasma; however, collection, transport, and storage costs make large epidemiologic studies challenging. We assessed the agreement of 25-hydroxyvitamin D 3 (25(OH)D 3 ) quantification from dried blood spots (DBS) as compared to standard plasma assessment among older Indian adults. Methods A total of 58 adults over 45 years of age who resided in Pune, India were enrolled in the study from July 2020 to June 2021. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to assess 25(OH)D 3 concentrations in paired plasma and DBS samples. Results Plasma and DBS 25(OH)D 3 concentrations were highly correlated (Pearson’s correlation = 0.976). The median 25(OH)D 3 concentration of the study population assessed by plasma was 14.6 ng/mL (Q1=12.0, Q3= 18.1) while the median concentration assessed in DBS was 12.8 ng/mL (Q1=11.0, Q3= 16.6). 25(OH)D 3 concentrations measured from DBS were on average 6% (95% CI: 2-13%) lower than concentrations assessed by plasma across the observed 25(OH)D 3 distribution. Conclusions We found good agreement between 25(OH)D 3 quantification between DBS and plasma and our findings indicate that DBS can be used in epidemiologic studies of vitamin D among Indian adults.