Enumeration of Circulating Platelet-Derived Microparticles Clearly Separates PV and ET From Secondary Polycythemia or Thrombocytosis

The diagnosis of Polycythemia Vera (PV) and Essential Thrombocytemia (ET) relies on a set of criteria gathered in the 2008 WHO classification; however, it may occasionnaly be difficult, especially in the absence of molecular markers such as Jak2^V617→F or Mpl mutations. Having previously reported a significant increase in circulating platelets microparticles (PMP) in patients bearing Myeloproliferative neoplasms versus healthy donors (V.Tintillier-Colin, ASH Annual Meeting 2009, Abstr. 1906), our purpose has been to assess the usefulness of blood PMP count as an additional criteria in dubious cases of ET and PV.

Patients and methods: We performed PMP counts in 100 newly diagnosed and still untreated patients (55 ET and 45 PV in accordance to WHO criteria), 40 secondary or reactive states (erythrocytosis and thrombocytosis, 20 cases each) and 75 controls matched for age and sex. The PMP were characterized by their size and co-expression of Annexin V and CD41; their concentration was measured on plasma extracts using the FC500 flow-cytometer (Beckman-Coulter™). Pre-analytical and testing procedures complied with the recommendations of the ISTH Standardization Sub-committee with each test performed in duplicate.

The 3 groups were homogeneous regarding age and sex-ratio. Mean platelet counts were 703.10⁹/l in the TE group, 652.10⁹/l in the cohort with secondary thrombocytosis and 247.10⁹/l in the control group. Mean Hb level was 18.0g/dl in the PV group, 18.1g/dl in the secondary erythrocytosis group and 15g/dl in controls. The median value of PMP concentration is much higher (p < 0.05, Krusall-Wallis test) in the TE group (5900/μl) than in the reactive thombocytosis group (1283/μl), this latter being no different from the control group (996/μl). In the ET group, PMP count is similarly increased whether patients beared Jak2^V617→F mutation or not. Using the ROC statistical method, we defined a threshold value of 3400 PMP/μl able to discriminate ET from reactive thrombocytosis with 93% specificity and 67% sensitivity.

Similarly, the median value of PMP count was neatly higher in the PV group (3258/μl) than in secondary erythocytosis (671/μl) and healthy controls (996/μl) (p < 10⁻⁵, Krusall-Wallis test). The ROC method fixed a 1300/μl threshold separating PV from secondary erythrocytosis with 89% sensibility and 63% specificity.

We report an evaluation of the blood PMP count in a quite large series of newly diagnosed untreated ET and PV patients compared to patients with secondary thrombocytosis or erythrocytosis and healthy volunteers; this study emphasizes the interest of the circulating PMP count in order to differentiate MPN neoplasms from normal subjects as well as patients with reactive conditions. This test, quick, reproducible and cheap, especially shows its value as an additional diagnostic criteria in ET or PV when clonal markers are lacking.

No relevant conflicts of interest to declare.