Which are the best parameters to obtain proteins and malondialdehyde from a tear sample?

Predictive, preventive and personalized medicine (PPPM) is a current concept in healthcare based on the analysis of biomarkers through non-invasive methods. Biomarkers for inflammation and oxidative stress are especially used for screening. Quantification of tear total protein content is important to identify potential, specific biomarkers, such as malondialdehyde concerning oxidative stress. The Schirmer strip test is an accessible and simple method for tear analysis. However, it is limited by the low concentration of biomarkers in the human tear. In this preliminary study, different procedures were compared for the extraction of tear proteins and malondialdehyde. Schirmer strips were used to obtain tears from healthy subjects. Ionic strength and surfactant agents were assessed, as well as different centrifugation parameters. Finally, several volumes of n-butanol on the process of malondialdehyde extraction were evaluated. The results showed that ionic strength strongly influences the extraction process, although most studies have suggested that surfactant agents are the most relevant factor; the most efficient results were obtained using a 2 M solution of NaCl in phosphate buffered saline. Regarding centrifugation, leaving the Schirmer strip tip left hanging outside the tube cap and using 1000 rpm was the best option, which is a lower centrifugation speed than the usually reported on literature. Moreover, 250 μL was the optimal n-butanol volume for malondialdehyde extraction. The importance of this study relies on the increasing relevance of the biomarkers in the field of PPPM and the need of a standardized method to extract biomarkers from the tears, to optimise its use.