Essential Thrombocythemia

• JAK2 V617F mutation (present in ~50-60% of ET cases): This gain-of-function point mutation in the JAK2 kinase domain (exon 14) results in constitutive activation of JAK-STAT signaling. The V617F substitution allows JAK2 to signal independently of erythropoietin and thrombopoietin receptor (TPOR) binding, leading to enhanced phosphorylation of STAT proteins and downstream proliferation signals. Megakaryocytic progenitors become hypersensitive to thrombopoietin (TPO), resulting in autonomous growth. JAK2 V617F positivity is associated with higher risk of arterial thrombosis and transformation to secondary myelofibrosis compared to JAK2-negative disease.

• CALR mutation (present in ~25% of ET cases): Mutations in calreticulin (CALR), a chaperone protein critical for protein folding and calcium homeostasis, occur as frameshift mutations in exon 9 that create a novel C-terminus. These altered CALR proteins cannot bind to the endoplasmic reticulum (ER) signal peptide, instead localizing to the cytoplasm where they bind and activate TPOR (MPL) in a ligand-independent manner. This activation of the thrombopoietin signaling pathway drives megakaryocyte expansion. CALR mutations are often associated with a lower risk of arterial thrombosis but higher risk of venous thromboembolism compared to JAK2-mutated ET.

• MPL mutation (present in ~5% of ET cases): Mutations in MPL (the thrombopoietin receptor gene) cause constitutive activation of the TPO-signaling cascade without requiring TPO binding. The most common mutation is MPL W515L/K, which disrupts the autoinhibitory domain of the receptor, allowing spontaneous JAK2 association and activation. These mutations directly sensitize megakaryocytic progenitors to low levels of circulating TPO, driving proliferation.

• Triple-negative ET and additional molecular events: Approximately 10-15% of ET patients lack JAK2, CALR, and MPL mutations ("triple-negative"). These cases may harbor mutations in other genes including ASXL1, EZH2, TET2, DNMT3A, and TP53. ASXL1 and TP53 mutations in ET are associated with poor prognosis and increased risk of transformation. Triple-negative ET has a more indolent course on average but requires molecular profiling to exclude other myeloid neoplasms.

• Consequences of clonal megakaryocyte expansion: The proliferation of mutant megakaryocytes leads to markedly increased platelet production via increased ploidy of megakaryocytes and increased megakaryocyte numbers. Bone marrow shows hypercellularity with clustering of enlarged, morphologically abnormal megakaryocytes. Circulating platelets are often functionally abnormal with defective aggregation responses and abnormal calcium signaling, paradoxically predisposing to both bleeding (from platelet dysfunction) and thrombosis (from excessive numbers and platelet activation). Additionally, megakaryocytes and platelets produce elevated levels of inflammatory cytokines (IL-6, IL-8, TNF-α) and prothrombotic molecules (tissue factor, phosphatidylserine), promoting endothelial injury and thrombosis.

• JAK2 V617F mutation: Present in approximately 50-60% of ET patients, this is the most common driver mutation. The presence of JAK2 V617F strongly supports the diagnosis of ET and is considered a major diagnostic criterion per WHO 2016/2022 classification. Homozygous JAK2 V617F is rare in ET but, when present, suggests increased disease burden.

• CALR mutations: Present in 25% of ET cases, CALR mutations are typically heterozygous frameshift mutations in exon 9. Type 1 (52-bp deletion, L367fs*46) and Type 2 (5-bp insertion, K385fs*47) are the most common variants. CALR mutations are mutually exclusive with JAK2 and MPL mutations, and their presence strongly supports ET diagnosis.

• MPL mutations: Found in 5% of ET cases, MPL mutations (particularly W515L and W515K) represent the third major driver mutation. Like CALR mutations, MPL mutations are mutually exclusive with JAK2 and CALR mutations and support ET diagnosis.

• Absence of driver mutations (triple-negative ET): In 10-15% of ET cases, none of the three major mutations are detected. These patients should be evaluated for other myeloid neoplasms and other myeloid mutations. The diagnosis of ET in triple-negative patients requires careful morphologic review and exclusion of other entities. Additional molecular testing for ASXL1, EZH2, TET2, and other genes may be warranted.

• Age and constitutional factors: ET typically presents in older adults (median age 60-65 years), though younger patients can be affected. Female predominance is noted in epidemiologic studies but is not a direct risk factor.

• No environmental or lifestyle risk factors: Unlike some malignancies, ET has no established association with tobacco, radiation, chemical exposures, or other modifiable risk factors that directly cause disease.

• Asymptomatic presentation (40-50% of patients): Many ET patients are identified only when a platelet count >450,000/μL is found during routine laboratory evaluation or workup for another condition. These patients may remain asymptomatic for years despite marked thrombocytosis, emphasizing that absolute platelet count does not correlate with symptom burden or thrombotic risk.

• Thrombotic complications: These are the most frequent clinical manifestation and major driver of morbidity. Patients may present with arterial thrombosis (myocardial infarction, ischemic stroke, peripheral arterial thrombosis) or venous thromboembolism (deep vein thrombosis, pulmonary embolism, splanchnic vein thrombosis). Thrombosis in ET is often unusual in location (mesenteric, hepatic, or portal vein thrombosis) and tends to occur in younger patients than expected for primary cardiovascular disease, a red flag that should prompt evaluation for ET. The hyperplatelet state, combined with platelet dysfunction and endothelial damage from circulating cytokines, creates a prothrombotic milieu.

• Bleeding manifestations: Paradoxically, some ET patients experience bleeding despite thrombocytosis, a phenomenon termed "paradoxical bleeding" resulting from platelet dysfunction with defective aggregation, impaired thrombin generation on platelet surfaces, and platelet sequestration. Manifestations include mucosal bleeding (epistaxis, gingival bleeding, menorrhagia), gastrointestinal bleeding (often from underlying vascular lesions or angiodysplasia), and petechiae/ecchymoses. Severe bleeding is uncommon in ET but can occur, especially if anticoagulation is needed for thrombotic events.

• Microvascular symptoms: These are characteristic of ET and reflect platelet microthrombi in small vessels. Erythromelalgia is a distinctive manifestation present in 5-10% of patients, characterized by episodic burning pain, erythema, and warmth in the extremities (particularly feet), often triggered by stress or alcohol. The pain may last minutes to hours and responds dramatically to aspirin, making aspirin response a diagnostic clue. Transient ischemic attacks (TIAs) and amaurosis fugax (transient monocular blindness) represent microvascular events in the cerebral circulation. Acral paresthesias and headache are common complaints reflecting microvascular insufficiency.

• Constitutional symptoms: Splenomegaly occurs in 20-40% of ET patients and results from extramedullary hematopoiesis and sequestration of abnormal platelets. Hepatomegaly is less common. Patients may experience fatigue, though severe systemic symptoms are uncommon at diagnosis (unlike in PMF or blast-phase disease).

• Physical examination findings: Splenomegaly is the most common abnormal finding on examination. Petechiae or ecchymoses may be visible if bleeding complications have occurred. Examination of lower extremities may reveal signs of prior thrombosis (edema, skin changes). In patients with erythromelalgia, acute examination during an episode shows striking erythema and edema of affected areas.

• Disease variants and special presentations: Some ET patients present in the setting of pregnancy, which accelerates platelet production and increases thrombotic risk. Others present with ET transformation to myelofibrosis or acute myeloid leukemia, marked by worsening constitutional symptoms, progressive splenomegaly, and worsening cytopenias. Patients with triple-negative ET or those with unfavorable mutations (ASXL1, TP53) may have more aggressive disease courses.

• Complete blood count (CBC) and platelet count: The threshold for diagnosis is a sustained platelet count of ≥450,000/μL on at least one occasion and confirmed on repeat testing. The platelet count is typically markedly elevated, often >600,000/μL at presentation. Hemoglobin and white blood cell count are normal or near-normal (by definition, if hemoglobin is elevated, PV must be considered; if immature forms are abundant, CML or AML must be excluded). Red cell mass is not elevated, distinguishing ET from PV. The peripheral blood smear shows numerous platelets, often with giant forms, and may show abnormal platelet morphology including hypogranularity or large forms.

• Bone marrow biopsy and aspirate: A bone marrow biopsy is essential for diagnosis and must show proliferation mainly of the megakaryocytic lineage with increased numbers of enlarged, morphologically abnormal megakaryocytes (with hyperlobulated nuclei and abundant cytoplasm) with clustering or sheets. The cellularity is increased, particularly in the megakaryocytic compartment. Importantly, there is no significant increase in granulopoiesis or erythropoiesis, no increase in blasts, and <1% fibrosis (distinguishing ET from PMF). Reticulin staining quantifies fibrosis using a 4-point scale (0-3); ET requires ≤grade 1 reticulin fibrosis. The marrow background should not show the characteristic features of other MPNs or myeloid neoplasms.

• JAK2, CALR, and MPL mutation testing: Molecular testing for mutations in these three genes is essential for diagnosis and is obtained from peripheral blood or bone marrow. JAK2 V617F is present in 50-60% of ET patients; testing requires allele burden quantification (percentage of mutant alleles), with higher burden (>50%) suggesting JAK2 homozygosity. CALR mutations are found in 25% of patients, detected via sequencing or specialized PCR assays. MPL mutations are present in 5% of patients. Presence of any one of these mutations fulfills a major diagnostic criterion. The vast majority of ET patients are positive for at least one of these mutations; triple-negative cases require careful morphologic review and consideration of alternative diagnoses.

• WHO diagnostic criteria (2016/2022):
• Major Criterion 1: Sustained platelet count ≥450,000/μL
• Major Criterion 2: Bone marrow biopsy showing proliferation mainly of megakaryocytic lineage with increased, enlarged, morphologically abnormal megakaryocytes with hyperlobulated nuclei; no significant granulopoiesis or erythropoiesis increase; <1% fibrosis
• Major Criterion 3: Presence of JAK2 V617F, CALR, or MPL mutation
• Major Criterion 4: No evidence of BCR-ABL1 fusion gene (ruling out CML)
• Major Criterion 5: Does not meet WHO criteria for PV, PMF, or other myeloid neoplasm

ET is diagnosed if platelet count ≥450,000/μL plus bone marrow findings (Criterion 2) plus either Criterion 3 (mutation present) OR all of Criterion 4 and 5 (mutation-negative but BCR-ABL1 negative and other disorders excluded).

• Exclusion criteria: ET diagnosis requires exclusion of secondary causes of thrombocytosis (reactive thrombocytosis). Iron deficiency, chronic inflammation, malignancy, hemorrhage, hypersplenism, and medications must be ruled out through clinical history and laboratory testing. BCR-ABL1 fusion gene must be excluded to rule out CML. PV is excluded by normal hemoglobin/hematocrit (and normal red cell mass if uncertain). PMF is excluded by absence of significant marrow fibrosis. Other myeloid neoplasms (AML, MDS) are excluded by morphology and cytogenetics.

• Cytogenetics and additional molecular testing: Conventional cytogenetics is typically normal in ET, though some clonal abnormalities (13q deletion, 20q deletion) may occur. These do not preclude ET diagnosis. In suspected triple-negative ET, additional molecular sequencing for ASXL1, EZH2, TET2, DNMT3A, TP53, and other genes may be considered to identify unfavorable mutations and guide prognosis. However, these additional mutations are not required for ET diagnosis.

• LDH and uric acid: Elevated LDH and uric acid reflect increased cell turnover and should be measured at baseline. Marked elevation might suggest myelofibrotic transformation.

• Coagulation studies: In asymptomatic ET, coagulation studies (PT, aPTT, fibrinogen) are often normal. If bleeding is present, further evaluation for acquired von Willebrand syndrome or platelet function defects may be warranted (platelet aggregation studies).

• Differential diagnosis considerations:
• Reactive (secondary) thrombocytosis: Iron deficiency, hemorrhage, hemolysis, inflammation, infection, malignancy, medications. Distinguishing feature: underlying cause is identified; bone marrow shows normal megakaryocytes without clustering; molecular mutations are absent.
• Chronic myeloid leukemia (CML): BCR-ABL1 fusion gene present; immature granulocytes in blood; left-shifted myeloid maturation.
• Polycythemia vera (PV): Elevated hemoglobin/hematocrit; JAK2 V617F often present but red cell mass is increased; bone marrow shows panmyelosis.
• Primary myelofibrosis (PMF): Grade 2-3 marrow fibrosis; often more symptomatic with constitutional symptoms; leukoerythroblastic picture on blood smear.

Disease-related complications

• Arterial and venous thrombosis: the dominant cause of morbidity. Clonal, hyperreactive platelets plus JAK2-mutant leukocyte–endothelial interaction create a prothrombotic surface. Signals: acute stroke/TIA, MI in a patient younger than expected, or unprovoked DVT/PE. Emergency — treat as standard acute arterial or venous thrombosis while working up the MPN.
• Splanchnic vein thrombosis (Budd-Chiari, portal/mesenteric): classically the presenting event in JAK2-mutated MPN, sometimes before the platelet count is impressive (portal hypertension and hypersplenism can blunt it). Signals: new ascites with tender hepatomegaly, or abdominal pain out of proportion to exam. Emergency.
• Acquired von Willebrand syndrome: at extreme thrombocytosis (generally platelets well above one million/μL), high-molecular-weight vWF multimers are adsorbed onto the platelet surface and proteolyzed. Signal: mucocutaneous bleeding with a low vWF ristocetin cofactor activity relative to antigen. Antiplatelet therapy must be withheld until it is corrected.
• Digital ischemia/gangrene: progression of untreated erythromelalgia to acral necrosis from platelet microthrombi.
• Progression to post-ET myelofibrosis or blast phase (AML): signaled by new anemia, growing splenomegaly, constitutional symptoms, and a leukoerythroblastic smear with teardrop cells; blast phase is a hematologic emergency.
• Pregnancy loss and placental insufficiency: placental microvascular thrombosis; recurrent first-trimester loss and fetal growth restriction.

Treatment-related complications

• Hydroxyurea: myelosuppression, macrocytosis, painful oral and lower-leg ulcers, and non-melanoma skin cancer (counsel on photoprotection and skin surveillance per NCCN MPN guidance).
• Anagrelide: phosphodiesterase-3 inhibition produces headache, palpitations, fluid retention and high-output cardiac failure; in the PT-1 trial anagrelide plus aspirin was associated with more arterial thrombosis, bleeding, and myelofibrotic transformation than hydroxyurea plus aspirin.
• Interferon alfa (including pegylated forms): flu-like symptoms, depression, and autoimmune thyroiditis.
• Aspirin: GI hemorrhage, amplified when acquired von Willebrand syndrome is unrecognized.
• Alkylators/radiophosphorus: leukemogenic; avoided in modern practice.

• Burning red hands and feet relieved by low-dose aspirin is erythromelalgia — the single most examined ET buzzword. A dramatic aspirin response is itself a diagnostic clue, and low-dose aspirin is the answer for microvascular symptoms in essentially all ET patients.
• Bleeding with a platelet count over a million is not a distractor — it is acquired von Willebrand syndrome. Best next step: vWF ristocetin cofactor activity/antigen before starting or continuing aspirin. Cytoreduction (hydroxyurea) lowers the platelet count and restores the multimers.
• Thrombocytosis plus basophilia, left-shifted granulocytes, or splenomegaly = check BCR-ABL1 first. CML can present with isolated thrombocytosis; excluding the Philadelphia chromosome is mandatory before calling it ET.
• Rule out reactive thrombocytosis before invoking a neoplasm: iron deficiency, infection/inflammation, post-splenectomy, and occult malignancy. Iron studies and inflammatory markers are cheap and high-yield; reactive marrows lack megakaryocyte clustering and driver mutations.
• JAK2 V617F is the association examiners test — shared with PV and PMF, and the reason an unexplained splanchnic or hepatic vein thrombosis should trigger JAK2 testing even when the CBC looks unremarkable.
• Risk stratification, not the platelet number, drives therapy. Age over 60, prior thrombosis, JAK2 mutation, and cardiovascular risk factors define risk (IPSET-thrombosis, used in NCCN MPN guidance). A platelet count of 1.2 million in a 35-year-old with no history does not by itself mandate cytoreduction.
• Pregnancy: interferon alfa is the cytoreductive agent of choice; hydroxyurea and anagrelide are avoided — hydroxyurea is teratogenic.
• Common distractor: choosing plateletpheresis routinely. It is reserved for acute, life-threatening thrombosis or hemorrhage with extreme thrombocytosis, as a bridge to hydroxyurea — not for an asymptomatic high count.