I am working on identifying biomarkers of the outcome of treatment in a disease. I have 20 potential biomarkers and more than 30 potential confounders. Actually, I know that some of those potential confounders affects treatment outcome. However, before determining the association between each biomarker and the therapeutic outcome, I want to know which of these confounders affects the expression level of each biomarker. I want to understand the impact of covariates on the expression of biomarkers to include them as confounders in multivariate analysis.