Supplementary Material for: Saliva for Assessing Vitamin A Status in Extremely Preterm Infants: A Diagnostic Study

Background: Salivary measurement of hormones and vitamins is gaining prominence as a minimally invasive procedure with the negligible potential for harm. We aimed to assess the utility of saliva for assessing vitamin A status in extremely preterm infants. Methods: Paired saliva and blood samples were collected at 4 weeks of age from infants born <28 weeks of gestation using a proprietary polymer swab. Plasma retinol was measured using high-performance liquid chromatography, and salivary retinol was measured using enzyme-linked immunosorbent assay. Results: Thirty infants were recruited with a median (IQR) gestation and birth weight of 26.2 weeks (24.8–27.2) and 865 g (718–1,002), respectively. An adequate volume of saliva (>50 µL) was obtained in 68%. There was no significant correlation (Spearman’s correlation coefficient = 0.16, p = 0.3) between individual plasma and salivary retinol levels. Bland-Altman analysis showed wide limits of agreement (–113 to +119%) between individual plasma and salivary retinol levels. Conclusion: Individual vitamin A status cannot be determined reliably from saliva in extremely preterm infants using current collection materials and analysis techniques.