Myeloproliferative neoplasms: Clinical

Myeloproliferative neoplasms are a group of neoplastic disorders characterized by proliferation of the bone marrow cells from the myeloid lineage. That includes red blood cells, platelets, megakaryocytes, as well as granulocytes, which include neutrophils, basophils, mast cells, and eosinophils.

Each disorder may potentially cause proliferation of all the myeloid cells, however, myeloproliferative neoplasms can cause proliferation of all of the myeloid cells, but they’re classified based on the dominant cell line involved.

So there’s polycythemia vera, for red blood cells, essential thrombocythemia for megakaryocytes and platelets, chronic myeloid leukemia, or CML for granulocytes, and the odd one out, primary myelofibrosis, which doesn’t really have a dominant cell line, but instead is characterized by bone marrow fibrosis.

Now, these are diagnoses of exclusion. So for example, when you see an increase in platelets, or thrombocytosis, you have to think through all of the other causes before considering essential thrombocythemia.

Alright, first up is polycythemia; which is when the hemoglobin level more than 16.5 grams per deciliter in men or more than 16.0 grams per deciliter in women, or a hematocrit more than 49% in men, or more than 48% in women.

Women have a lower cutoff because they normally lose some blood through the menses.

Now, polycythemia is classified into primary polycythemia, called polycythemia vera, and secondary polycythemia, which is further subclassified into relative polycythemia, appropriate absolute polycythemia, and inappropriate absolute polycythemia.

To differentiate the causes of polycythemia, we’ll be looking at four laboratory parameters: plasma volume, RBC mass, oxygen saturation, and erythropoietin levels.

So, starting with secondary polycythemia - relative polycythemia happens when something like dehydration, diuretic use or a burn injury causes a decrease in plasma volume, resulting in an apparent increase of RBCs, but the RBCs are only increased relative to the plasma. Therefore, we don’t expect any change in the RBC mass, oxygen saturation, and erythropoietin levels.