GSTM1 and polycythemia: In polycythemia vera, polycythemia is defined as an increased mass of red blood cells and can be evaluated by laboratory findings (Hb or HCT >99th percentile of method-specific reference range for age, sex, and altitude of residence) [2] or measurement of red cell volume by radioisotopic methods, allowing to differentiate between true polycythemia, as in polycythemia vera, chronic obstructive pulmonary disease, and paraneoplastic syndromes, and pseudoerythrocytosis found in a dehydration state, for example.