Tears and Saliva as Biological Matrices for Vitamin D and Glucose Assessment: A Pilot Study

Background: Several studies have established correlations between low serum levels of vitamin D and various pathologies, such as diabetes mellitus and its complications. However, few studies analyze its levels in matrices other than blood plasma, such as tears and saliva. In this study, we aimed to demonstrate the feasibility of using tears and saliva as alternative biological matrices for laboratory assessment of vitamin D and glucose concentration in individuals with type II diabetes mellitus and healthy individuals, using the electrochemiluminescence method. Methods: This study included volunteers with type II diabetes and healthy controls, excluding those with certain comorbidities or a BMI ≥ 40. Blood, tear, and saliva samples were taken after 3 h of fasting for biochemical analysis of fasting glucose and vitamin D. Statistical analysis was conducted using GraphPad Prism® 8.0—with Pearson and other tests to evaluate correlations—at a significance level of 5% and test power > 95%. Results: A negative correlation between serum vitamin D values and those found in saliva (p = 0.041) was found, as well as a positive correlation between serum glucose values and those found in tears (p = 0.0254). Conclusions: Tears and saliva samples can be used as proxies for venous blood samples in specific situations, such as studying blood glucose levels and vitamin D levels. However, expanding the sample size is essential to confirm the correlation and develop an accurate equation for estimating serum levels of these markers using these alternative matrices.