GSTM1 and acquired polycythemia vera: Altogether it has been argued that application of these criteria may result in an underdiagnosis of PV by excluding patients with actual red cell mass that is 25% above mean predicted value, but whose Hb and Hct levels are below the WHO guidelines.49, 50, 51, 52, 53, 54, 55, 56 Recently, the term masked PV (mPV) was re-introduced57 for JAK2-mutated patients with latent (initial, occult pre-polycythemic) disease manifestations who present with a BM morphology consistent with PV and display persistently raised Hb levels between 16.0 and 18.5 g/dl for men and 15.0 and 16.5 g/dl for women.