MPNs are characterized by marked increases in one or more cell lines within both the peripheral blood and bone marrow due to incorrect regulation of the hematopoietic stem cells. Bone marrow hypercellularity is an important aspect of this group of conditions. Other characteristics that may also be associated with MPNs include cytogenetic abnormalities/acquired mutations, extramedullary hematopoiesis, bone marrow fibrosis, and thrombotic or bleeding episodes. The MPNs are not associated with cellular dysplasia as the myelodystplastic diseases are; which is a helpful differentiation feature. Classifications of the MPNs are based upon which cell line is the most affected along with the patient's history, genetic findings, and laboratory results.
The World Health Organization (WHO) classifies these disorders as myeloproliferative neoplasms:
- Chronic myeloid leukemia (BCR/ABL1+)
- Chronic neutrophilic leukemia
- Chronic eosinophilic leukemia, ot otherwise specified (NOS)
- Essential thrombocythemia
- Primary myelofibrosis (PMF)
- PMF, prefibrotic early stage
- PMF, overt fibrotic stage
- Polycythemia vera
- Myeloproliferative neoplasm, unclassifiable (MPN-u)
In the WHO 2016 updates to the classification of myeloid neoplasms, mastocytosis (which had previously been classified as an MPN) has been removed from the list of MPNs and placed in its own disease category.