You cannot use the same reasoning for the thesis and the antithesis. If certain signs and symptoms are indicative of abnormality it does not follow that the absence of those signs and symptoms are indicative of a state of normalcy. The signs and symptoms of an abnormal state can be set down, memorized, and determined by diagnostic evaluation. The signs and symptoms of a normal state are not known. The problem in the medical realm then very clearly is attempting to ascertain a normal state using the same criteria with which an abnormal state is ascertained.
X is present, it is known to be abnormal, therefore lack of X indicates a state of normalcy.
You cannot use the same criteria for normal and abnormal. Being normal is a continually changing adapting activity. Its signs or symptoms, whatever they might be, are not the opposite of the signs and symptoms of abnormality. Hegelian logic has its place and is valuable in certain philosophical discussion. In other situations it cannot be applied.