In our small cohort of patients, we could clearly diagnose PV in 87% of cases, and none of the secondary erythrocytosis patients had elevated IGF-1R by flow cytometry; one patient with negative JAK2V617F mutation also had trilineage proliferation of bone marrow, increased RCM, and low serum EPO who was diagnosed as PV also had elevated IGF-1R values as other PV patients. Here, EPO is linked to acquired polycythemia vera.