Essential Thrombocytosis Workup

Updated: Aug 22, 2019
  • Author: Asheesh Lal, MBBS, MD; Chief Editor: Emmanuel C Besa, MD  more...
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Workup

Approach Considerations

The principal diagnostic criteria for essential thrombocytosis are an elevated platelet count, bone marrow megakaryocytic proliferation, and presence of JAK2, CALR, or MPL mutation. [23]  Accordingly, the tests and procedures used in the workup for essential thrombocytosis include the following:

  • Complete blood cell count (CBC)
  • Genetic studies – For JAK2 V617F, CALR, and MPL mutations
  • Bone marrow aspirate and biopsy
  • Coagulation studies
  • Platelet aggregation studies
  • Serum chemistry

Cytogenetic study results are usually normal. Molecular studies (eg, polymerase chain reaction [PCR], Southern [genomic] blotting) may be used as sensitive means of excluding chronic myelogenous leukemia.

Computed tomography (CT) scanning or ultrasonography of the spleen may reveal splenomegaly in patients with essential thrombocytosis even when this condition is not physically detectable.

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Histologic Findings

Approximately 90% of patients with essential thrombocytosis show an increase in bone marrow cellularity. Megakaryocytic hyperplasia is present. Giant megakaryocytes are frequently observed, and clusters of megakaryocytes may be present. Significant dysplasia of the megakaryocytes is unusual. Hyperplasia of granulocyte and reticulocyte precursors is common. Bone marrow reticulin is usually increased, but collagen fibrosis is uncommon.

In essential thrombocytosis, as in other myeloproliferative disorders, bone marrow iron stain results may be negative when other studies do not support the presence of iron deficiency. For practical purposes, a ferritin level that is within the reference range or increased, along with an RBC mean corpuscular volume (MCV) that is within the reference range, is sufficient to exclude reactive thrombocytosis secondary to iron deficiency and the possibility of polycythemia vera masked by iron deficiency.

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Blood Studies

A complete blood cell count (CBC) is essential for the diagnosis of essential thrombocytosis. CBC findings are as follows:

  • The hallmark of essential thrombocytosis (primary thrombocythemia) is a sustained, unexplained elevation in the platelet count

  • Leukocytosis, erythrocytosis, and mild anemia may also be found

  • The peripheral blood may show occasional immature precursor cells (eg, myelocytes, metamyelocytes); large platelets (thrombocytes) are typically identifiable on routine peripheral blood smear

  • Mild basophilia and eosinophilia may be found

The results of the prothrombin time (PT) and activated partial thromboplastin time (aPTT) studies are usually within reference ranges. The bleeding time may or may not be prolonged.

Platelet aggregation study findings are abnormal and show impaired platelet aggregation to epinephrine, adenosine diphosphate, and collagen but not to ristocetin and arachidonic acid. Some patients may present with spontaneous platelet aggregation.

A red blood cell (RBC) mass study helps to exclude polycythemia vera. The RBC mass is elevated in polycythemia vera, but is normal in essential thrombocytosis.

Other blood work results are as follows:

  • Uric acid (UA) levels are elevated in 25% of patients at diagnosis

  • Pseudohyperkalemia may occur, and falsely elevated phosphorus (P) and acid phosphatase levels may be noted

  • Pseudohypoxemia may develop from extreme thrombocytosis

  • Vitamin B-12 levels are increased in 25% of patients

  • Elevation of C-reactive protein (CRP), fibrinogen, and interleukin 6 levels suggests secondary thrombocytosis, because those are acute-phase reactants

On in vitro studies using peripheral blood mononuclear cells, an increase in the formation of endogenous erythroid cells and/or megakaryocytic colonies with increased sensitivity to interleukin-3 indicates the presence of abnormal hematopoietic progenitor cells. This finding is a diagnostic criterion for essential thrombocytosis; however, the test is primarily a research tool.

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Genetic Studies

The principal mutations found in patients with essential thrombocytosis are in the JAK2 and CALR genes. Mutations in the MPL gene have been found in up to 4% of patients with essential thrombocytosis, but the prognostic significance of its presence is unclear.

JAK2

The JAK2 V617F mutation is present in 50% of patients with essential thrombocytosis. [24] V617F -positive patients displayed multiple features resembling polycythemia vera, with significantly higher hemoglobin levels, neutrophil counts, bone marrow erythropoiesis and granulopoiesis, more venous thromboses and a higher incidence of polycythemic transformation.

Teofili et al noted that specific JAK2 mutations activate the JAK-STAT pathway in polycythemia vera and essential thrombocytosis, with the presence of biologic markers such as endogenous erythroid colony (EECs) growth or over expression of polycythemia rubra vera 1 gene (PRV-1), RNA. [25] Because these findings are also seen in patients with wild-type JAK2, the authors hypothesized that a deregulation of the suppressor of cytokine signaling (SOCS) protein system in these cases could produce the activation of the JAK-STAT pathway.

Teofili et al evaluated 81 patients with polycythemia vera and essential thrombocytosis (53 adults and 28 children) for the methylation status of the SOCS-1, SOCS-2 and SOCS-3 CpG islands and for several myeloproliferative markers (including JAK2 and MPL mutations and clonality of hematopoiesis). The authors identified SOCS-1 or SOCS-3 hypermethylation in 23 patients, which was associated with a significant decrease of SOCS-1 or SOCS-3 RNA and protein levels, and demonstrated that the gene expression was restored by exposing cells to the demethylating agent 2-deoxyazacytidin. [25] The investigators concluded that "SOCS-1 or SOCS-3 hypermethylation can activate the JAK-STAT signaling pathway in alternative or together with JAK2 mutations," which may "represent a potential therapeutic target." [25]

Ohyashiki et al studied JAK2 mutational status with cytogenetic analysis in 54 patients with essential thrombocythemia to obtain evaluate the correlation between clinicohematologic features and genetic abnormalities. [26, 27] These investigators found that recurrent der(9;18) in essential thrombocythemia with JAK2 V617F is highly linked to myelofibrosis development.

Ohyashiki et al reported that of six patients with essential thrombocytosis who developed myelofibrosis, four had JAK2 V617F mutation. In addition, three of four patients with JAK2 V617F had add(18)(p11). However, the remaining two patients with essential thrombocytosis who developed myelofibrosis did not have JAK2 V617F or add(18)(p11), and none of the patients with essential thrombocytosis and JAK2 V617F and chromosome changes other than add(18)(p11) developed myelofibrosis. The authors concluded that "add(18)(p11), possibly due to der(9;18), may contribute a link to myelofibrosis in" JAK2 V617F -positivity in patients with essential thrombosis, whereas "those with wild-type JAK2 may use another pathway toward myelofibrosis." [26]

CALR

Mutations in the calreticulin (CALR) gene have been reported in 15-25% of patients with essential thrombocytosis. CALR mutations have been found to be exclusive of JAK-2 and MPL mutations. Initial studies have indicated that patients with CALR mutations are younger; more frequently male; and have higher platelet counts, lower hemoglobin and leukocyte counts, and a lower risk of thrombosis than those with the JAK2 mutation. There has been no difference in the rate of transformation to myelofibrosis.

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Bone Marrow Aspirate and Biopsy

A bone marrow aspirate and biopsy are useful. Use specialized needles to obtain the aspirate and biopsy material over the posterior iliac crest. Obtaining an aspirate over the sternum may also be helpful, although most physicians prefer the posterior iliac crest. Do not attempt to obtain a biopsy from the sternum.

Bone marrow findings in essential thrombocytosis are as follows:

  • Approximately 90% of patients with show an increase in bone marrow cellularity

  • Megakaryocytic hyperplasia is common

  • Giant megakaryocytes are often observed; clusters of megakaryocytes may be present; significant dysplasia of the megakaryocytes is unusual

  • Hyperplasia of granulocyte and reticulocyte precursors is common

  • Bone marrow reticulin is usually increased, [28] but collagen fibrosis is uncommon

  • Iron stores may be absent in the bone marrow; this may be due to gastrointestinal tract bleeding or menorrhagia. However, in essential thrombocytosis, as in other myeloproliferative disorders, bone marrow iron stain results may be negative even when other studies do not support the presence of iron deficiency

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