With the blood test they check for antibodies. Once you are positive, it means you have had it before within a short period of time. Doesn't necessarily means you are having it right now. IgM and IgG that they check for could go down after a while. If your antibodies always stay high, you are in constant state of inflammation due to fighting infections. You might have IgA antibodies which are in mucous membranes and kill the virus before it gets inside from previous corona virus infections. Corona viruses are common cold viruses.