[Standardization of clinical application of mass spectrometry method for measurement of vitamin D in capillary blood of children: a multicenter study].

Objective: To establish the norms and clinical application standards of mass spectrometry method to measure vitamin D in capillary blood. Methods: Following the "Province-City-Hospital" sampling procedure, a cross-sectional sample of 1 655 healthy children under 7 years of age were recruited from 12 provinces, autonomous regions, or municipalities in China from November 2020 to December 2021. Both venous and capillary blood samples from the same individual were collected, for which serum 25(OH)D levels were measured by high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS) method. Pearson correlation analysis and linear regression analysis were used to detect the correlation and determine a correction algorithm. The agreement was analyzed using Bland-Altman plot and Kappa statistic. The sensitivity and specificity were evaluated using receiver operating characteristic (ROC) curve method. Results: Venous and capillary 25(OH)D levels of 1 655 healthy children under 7 years of age were 74.25 (59.50, 92.00) and 68.75 (54.44, 86.25) nmol/L, respectively, showed a significant difference(Z=22.14, P<0.001) as well as a highly significant correlation between venous and capillary 25(OH)D levels(r=0.95, P<0.001). Linear regression analysis was then performed to determine the correction algorithm: lg(corrected capillary 25(OH)D)=0.13+0.95×lg(capillary 25(OH)D)(R2=0.90,P<0.001). The deviation between venous and corrected capillary 25(OH)D levels was (0.50±17.50) nmol/L, a difference value that did not reach statistical significance (P>0.05). The cut-off values of capillary blood 25(OH)D values 30.00, 50.00, 75.00 nmol/L corresponding to venous blood 25(OH)D values were 26.59, 45.56, and 69.84 nmol/L, respectively. Good consistency was observed between venous and corrected capillary 25(OH)D levels in clinical diagnosis (Kappa value 0.68-0.81). Corrected capillary 25(OH)D showed a high clinically predictive value (area under curve 0.97-0.99,sensitivity 0.72-0.92,specificity 0.89-0.99). Conclusion: The standardized capillary HPLC-MS/MS method can be used to detect 25(OH)D levels in children clinically.