**2. Biomarkers**

The definition of biomarkers as established by the National Institute of Health (NIH) is as follows: biomarkers are the biological, biochemical, anthropometric, physiological, etc. characteristics, which are objectively measurable, capable of identifying physiological or pathological processes, or a pharmacological response or a therapeutic intervention [7].

There are different types of biomarkers; the ideal biomarker must be specific, sensitive, predictive, rapid, economical, noninvasive, and stable in vivo and in vitro. Additionally, it must have enough preclinical and clinical relevance to modify decisions regarding the pathological process in which applies [7].

Before a biological marker is used in human health studies, its validation is fundamental; therefore, the selection and approval process requires careful consideration of specificity and sensitivity, establishing accuracy, precision, quality assurance, analytical procedure, and interpretation of measurement data, which must be compared with other variables [8].
