S1610 MASKED POLYCYTHEMIA VERA: ANALYSIS OF A SINGLE CENTER SERIES OF 2480 RED CELL MASSES.
Authors: Soret‐Dulphy J. , Maslah N. , Dosquet C. , Vercellino L. , Belkhodja C. , Schlageter M.‐H. , Cassinat B. , Chomienne C. , Kiladjian J.‐J. , Giraudier S. 
Published: June 2019

Background:Recently, masked polycythemia vera (mPV) was defined in the WHO 2016 Myeloproliferative Neoplasms (NMP) classification as a new entity: harboring JAK2V617F mutation, combining features of both Essential Thrombocythemia (ET) (isolated thrombocytosis) and Polycythemia vera (PV). It is impossible to distinguish ET from mPV based on hemoglobin or hematocrit values and bone marrow biopsy is required to discriminate between the two.Aims:We retrospectively analyzed the red cell masses (RCM) made in our institution to determine whether this test could help differentiating these entities.Methods:We analyzed 2480 consecutive patients in whom a RCM was performed in our center for suspected polycythemia during the last 7 years. Among the JAK2V617F positive patients, the characteristics of the following groups of patients were studied: ”PV“ (JAK2V617F mutation, RCM > 125%, hemoglobin greater than 18.5/16.5 g/dL (male/female)); ”mPV“ (JAK2V617F mutation, RCM > 125% and hemoglobin 16.5–18.5/15–16.5 g/dL); ”JAK2V617F‐positive ET with hemoglobin level similar to mPV“ (JAK2V617F mutation, VG <125%, hemoglobin 16.5–18.5/15–16.5 g/dL).Results:In this cohort, 37.3% of the 2480 patients had a RCM > 125% confirming polycythemia. Patients with lower hemoglobin levels (between 15–16.5 g/dL in 56 women and 16.5–18.5 g/dL in 112 men) could be classified according to the RCM level between mPV (n = 113) when RCM was > 125% or JAK2V617F‐positive ET (n = 55) when RCM was <125% of predicted value. Mean platelet counts were significantly lower in PV patients than in mPV and ET patients, while no difference was observed between mPV and ET patients. Mean platelet counts were 460 G/L (+/‐ 222 G/L) in PV, 560 G/L (+/‐ 245 G/L) in mPV and 563 G/L (+/‐ 252 G/L) in ET (p PV vs mPV = 0.0014, p PV vs ET = 0.0075 and p mPV vs ET = NS). Mean serum Erythropoietin levels were comparable in the mPV and PV groups and lower than in the ET group (PV: 3.41 +/−2.3 mIU/mL, mPV: 3.42 +/−2.2 mIU/mL and ET: 6.22 +/−4.09 mIU/mL, p PV vs mPV = NS, p mPV vs ET = 0.0003 and p PV vs ET = 0.0002). JAK2V617F allelic burden was significantly different between the three groups. Mean mutant allelic ratios were 46% +/‐ 22%, 31% +/‐ 23% and 19.9% +/‐ 20% in PV, mPV and ET, respectively (p PV vs mPV <0.0001, p mPV vs ET = 0.0094 and p PV vs ET < 0.0001). Finally, the proportion of patients with hemodilution was significantly higher in the mPV group with plasma volume > 110% in 35% of mPV, 14% of PV, and 2% of ET patients respectively (p mPV vs. PV < 0.001, p mPV vs ET < 0.001 and p PV vs. ET = NS). This difference between PV and mPV patients could not be attributed to an increased spleen volume as the proportion of patients with splenomegaly was 19% in PV, 19% in mPV and 5.7% in ET patients (p PV vs mPV = NS, p mPV vs ET = 0.02 and p PV vs ET = 0.02).Summary/Conclusion:This study demonstrates that mPV can be distinguished from PV and ET based on RCM measurement even when bone marrow biopsy is not performed. Our findings may explain why masked PV presents some characteristics of ET (thrombocytosis) and PV (low EPO, increased RCM, relative microcytosis), but is a distinct entity (hemodilution even in the absence of splenomegaly, intermediate JAK2 mutant allelic burden between ET and PV). These results also suggest that mPV probably needs to be managed like PV, since these patients present a true erythrocytosis.