The word biomarker is short for biological markers. Biomarkers are measurable indicators of a biological process. Biomarkers can usually be measured through external testing such as tests of blood, urine or other body fluids such as cerebrospinal fluid (CSF) or can be measured by scans or other physical tests. Often the term biomarker is used to describe indicators which are observable during disease, or a factor which may predict disease onset. Biomarkers may also been used during clinical trials to determine whether treatment being tested is working.