Blood & Lymphoreticular

Essential thrombocythemia: management

Clinical Overview and When to Suspect Essential Thrombocythemia

— Driver mutations: JAK2 V617F (~50–60%), CALR (~20–25%), MPL (~3–5%); ~10–15% "triple-negative"

— Median age at diagnosis ~60 years; second smaller peak in young women

— Incidental persistent platelets >450 × 10⁹/L on routine CBC, especially when iron studies, CRP, and ferritin are normal (excluding reactive cause)

— Unexplained arterial or venous thrombosis in a younger patient (TIA, MI, portal/splanchnic vein thrombosis, Budd-Chiari)

— Erythromelalgia — burning, erythematous, painful hands/feet relieved by aspirin

— Migraine-like headaches, visual disturbances, paresthesias, livedo reticularis

— Splenomegaly on exam (only ~20–30%) or pruritus (more typical of PV)

— Mucocutaneous bleeding when platelets are markedly elevated (>1000–1500 × 10⁹/L) due to acquired von Willebrand syndrome

— Repeat CBC in 4–8 weeks; rule out reactive causes (iron deficiency, infection, inflammation, post-splenectomy, malignancy, hemolysis, recent surgery/trauma)

— Reactive thrombocytosis rarely causes thrombosis and platelets seldom exceed 1000 × 10⁹/L

— ET is fundamentally a vascular risk management disease, not a "high platelet" disease — morbidity comes from thrombosis (more common than bleeding) and, over decades, transformation to myelofibrosis (~5–10%) or AML (<5%)

— Management decisions are entirely risk-stratified and longitudinal

Board pearl: A platelet count >450 with low ferritin in a middle-aged woman is iron-deficiency thrombocytosis until proven otherwise — replete iron and recheck before launching a JAK2 workup.

Definition: Essential thrombocythemia (ET) is a chronic BCR-ABL–negative myeloproliferative neoplasm (MPN) characterized by sustained thrombocytosis (platelets ≥450 × 10⁹/L) driven by clonal megakaryocyte proliferation

When to suspect ET (Step 3 outpatient lens):

Confirm thrombocytosis is real and persistent:

Why it matters on Step 3:

Presentation Patterns and Key History

— Step 3 stems often hide ET behind a "routine preop labs" or "annual physical" framing

— Erythromelalgia — burning red painful digits, worse with heat, dramatically improved by low-dose aspirin (a near-pathognomonic response)

— Atypical chest pain, headache, lightheadedness, transient visual blurring/scotomata, acral paresthesias, tinnitus

— These reflect platelet-mediated microvascular occlusion, not large-vessel disease

— Arterial > venous (stroke, TIA, MI, peripheral arterial)

— Venous: DVT/PE, but classically splanchnic (portal, mesenteric, hepatic — Budd-Chiari), cerebral venous sinus thrombosis

— Unprovoked splanchnic thrombosis in a young patient → screen for JAK2 V617F even with normal CBC

— Prior thrombosis (single biggest risk factor going forward)

— Cardiovascular risk factors: tobacco, HTN, DM, hyperlipidemia — modifiable and independently affect ET risk score

— Pregnancy history: prior miscarriage, IUGR, placental abruption (clues to undiagnosed MPN)

— Medications: recent corticosteroids, epinephrine, splenectomy — all cause reactive thrombocytosis

— Family history of MPN (germline predisposition exists but rare)

Key distinction: Erythromelalgia + aspirin response strongly favors a clonal MPN (ET or PV); reactive thrombocytosis does not cause erythromelalgia.

Most common presentation: Asymptomatic incidental thrombocytosis on routine CBC — roughly 50% of patients

Vasomotor / microvascular symptoms (~40%): Highly suggestive of ET when paired with thrombocytosis

Thrombotic events (~20% at diagnosis, ~10–20% during follow-up):

Bleeding (~5–10%): Mucocutaneous, GI; risk rises sharply when platelets >1000–1500 × 10⁹/L due to acquired vWF deficiency (high-MW vWF multimers adsorbed onto platelets)

Constitutional: Fatigue is the dominant quality-of-life complaint; night sweats, weight loss, bone pain are uncommon early — if prominent, suspect transformation to myelofibrosis

Key history to elicit:

Physical Exam Findings and Vascular Assessment

— Most ET patients look entirely well — the exam is often unrevealing, which is itself a clue distinguishing ET from secondary causes (where the underlying illness usually shows)

— Vital signs typically normal; check BP carefully — uncontrolled hypertension compounds thrombotic risk and changes the risk score

— Erythromelalgia: symmetric, dusky-red to violaceous, warm, tender digits or soles; may have small ischemic ulcers or dry gangrene of fingertips/toes in severe microvascular disease

— Livedo reticularis, acrocyanosis

— Petechiae or ecchymoses if bleeding phenotype predominates (high platelets, acquired vWD)

— Pruritus is uncommon (think PV if aquagenic pruritus is prominent)

— Splenomegaly in 20–30%, usually mild; massive splenomegaly should prompt reconsideration of primary myelofibrosis or post-ET myelofibrosis

— Hepatomegaly less common

— Tenderness/ascites/jaundice → consider Budd-Chiari or portal vein thrombosis

— Document carotid bruits, peripheral pulses, ankle-brachial index in older patients — baseline for monitoring

— Focused neuro exam if any TIA-like history; fundoscopy for retinal vascular changes

— Assess for prior MI sequelae; document baseline ECG (atrial fibrillation independently raises stroke risk and shifts anticoagulation decisions)

Board pearl: Massive splenomegaly + cytopenias + leukoerythroblastic smear in a "thrombocytosis" patient = primary myelofibrosis, not ET. ET spleens are modest.

General exam pearls:

Skin and extremities — the highest-yield system in ET:

Abdominal exam:

Neurovascular exam:

Cardiovascular:

CCS pearl: On a CCS case of incidental thrombocytosis, your initial location should be office; orders should include vitals, full physical exam, peripheral smear, iron studies, CRP, and a repeat CBC in 4–8 weeks before advancing to JAK2 testing — don't skip the reactive workup.

Diagnostic Workup — Initial Labs and Smear

— Platelets ≥450 × 10⁹/L sustained on at least two occasions, separated by weeks

— WBC may be mildly elevated; marked leukocytosis suggests CML or PMF

— Hgb/Hct must be normal — if elevated, evaluate for polycythemia vera (check JAK2, EPO); if low with teardrop cells, evaluate for myelofibrosis

— Large, anisocytic platelets, occasional megakaryocyte fragments

— Look for teardrops, nucleated RBCs, leukoerythroblastic picture → myelofibrosis

— Basophilia or left shift → suspect CML, send BCR-ABL

— Ferritin / iron studies — iron deficiency is the #1 mimic and is fixable

— CRP, ESR — inflammation, occult infection, IBD, rheumatologic disease

— Consider age-appropriate cancer screening (occult malignancy)

— Review meds (steroids, epinephrine, TPO mimetics) and recent splenectomy

— JAK2 V617F PCR — positive in ~55%

— If JAK2 negative → CALR exon 9 mutations (~25%)

— If both negative → MPL W515 mutations (~3%)

— BCR-ABL1 — required to exclude CML, even if WBC normal

— Triple-negative ET (~10–15%) is a diagnosis of exclusion and warrants meticulous bone marrow review

— LDH (elevation suggests higher turnover or transformation), uric acid, BMP, LFTs

— Lipid panel, HbA1c, BP — cardiovascular risk factors directly enter the IPSET-thrombosis score

— Pregnancy test in women of reproductive age (alters drug choice)

— vWF activity / ristocetin cofactor if platelets >1000 × 10⁹/L or any bleeding — screens for acquired von Willebrand syndrome before starting aspirin

Step 3 management: Before ordering JAK2, confirm thrombocytosis is persistent and reactive causes are excluded — premature molecular testing is a common Step 3 distractor.

CBC with differential:

Peripheral smear:

Exclude reactive thrombocytosis (essential first step):

Mandatory MPN molecular panel:

Baseline chemistry and risk panel:

Diagnostic Workup — Bone Marrow and WHO Criteria

— Major 1: Platelets ≥450 × 10⁹/L sustained

— Major 2: Bone marrow biopsy showing proliferation of mature, enlarged, hyperlobulated ("staghorn") megakaryocytes; no significant granulocytic/erythroid proliferation; reticulin fibrosis grade ≤1 (excludes prefibrotic PMF)

— Major 3: Does not meet WHO criteria for BCR-ABL+ CML, PV, PMF, MDS, or other myeloid neoplasm

— Major 4: Presence of JAK2, CALR, or MPL mutation

— Minor: Clonal marker present OR no evidence of reactive thrombocytosis

— Cytogenetics on marrow to exclude del(5q), t(9;22), and other myeloid clones

— Key distinction: prePMF has dense clusters of atypical megakaryocytes with hyperchromatic, bulbous nuclei and increased reticulin (grade 1); ET shows loosely scattered, mature-appearing megakaryocytes — pathologist's call but prognostically critical

— Serum erythropoietin (suppressed in PV), red cell mass if needed

— JAK2-positive with rising Hct over time may unmask masked PV

— Age >60 (1 pt), prior thrombosis (2 pts), CV risk factors (1 pt), JAK2 V617F (2 pts)

— Very low (0): none; Low (1–2): no thrombosis hx, age ≤60; Intermediate: age >60 without other; High: prior thrombosis OR age >60 + JAK2+

Board pearl: A JAK2-positive patient with platelets 800 and rising hematocrit isn't ET — repeat the workup for polycythemia vera. Treatment (especially phlebotomy goal Hct <45%) changes immediately.

WHO 2022 diagnostic criteria for ET — all 4 major OR first 3 major + minor:

Bone marrow biopsy is required for diagnosis — even when JAK2 is positive, biopsy distinguishes true ET from prefibrotic primary myelofibrosis (prePMF), which has worse prognosis and a different treatment trajectory

Distinguishing from PV (when Hgb is borderline):

Risk stratification once diagnosis is secured (IPSET-thrombosis, revised):

Risk Stratification and First-Line Management Logic

— Very low risk: age ≤60, no thrombosis history, JAK2 negative

— Low risk: age ≤60, no thrombosis, JAK2 positive

— Intermediate risk: age >60, no thrombosis, JAK2 negative

— High risk: age >60 AND JAK2 positive, OR any prior thrombosis at any age

— Aggressive modifiable CV risk factor control: statin per ASCVD risk, BP <130/80, HbA1c targets, tobacco cessation counseling at every visit

— Avoid estrogen-containing contraceptives in high-risk patients

— Update vaccinations (influenza, pneumococcal, COVID, shingles per age)

— Counsel on signs of thrombosis and bleeding

— Hold aspirin if platelets >1000–1500 × 10⁹/L until vWF activity confirms no acquired vWD (aspirin can precipitate bleeding)

— BID dosing (81 mg twice daily) reduces residual microvascular symptoms in some patients with incomplete platelet COX-1 inhibition

Step 3 management: Age >60 + JAK2 V617F automatically equals high risk even without prior thrombosis — start hydroxyurea plus aspirin.

Treatment is entirely risk-driven — platelet count alone does not dictate therapy. Memorize the four IPSET-thrombosis tiers:

Observation ± low-dose aspirin only if microvascular symptoms or CV risk factors

Many CALR-mutated young patients fall here and do not need aspirin routinely (CALR mutations carry lower thrombotic risk)

Low-dose aspirin 81 mg daily (consider BID if microvascular symptoms persist)

No cytoreduction unless extreme thrombocytosis or bleeding

Low-dose aspirin

Cytoreduction is reasonable but not mandatory; individualize

Cytoreduction is required plus aspirin (or anticoagulation if prior venous thrombosis)

Target platelets <400 × 10⁹/L

Universal measures for every ET patient (Step 3 outpatient core):

Aspirin caveats:

Pharmacotherapy — Cytoreductive First-Line Regimens

— Starting dose 500 mg PO BID (or 15 mg/kg/day); titrate every 2–4 weeks to platelets <400 × 10⁹/L while keeping ANC >1.5 and Hgb >10

— Monitor CBC every 2 weeks during titration, then every 3 months

— Adverse effects: macrocytosis (expected, not a reason to stop), cytopenias, oral/leg ulcers (often dose-limiting), nail hyperpigmentation, nonmelanoma skin cancers (counsel sun protection, annual derm exam), rare pulmonary toxicity

— Theoretical leukemogenicity is small and not a reason to withhold in older patients; concern is higher in young patients with long expected exposure

— Preferred first-line in young patients (<40–60), women planning pregnancy, and patients intolerant of hydroxyurea

— Non-leukemogenic, can induce molecular responses (especially in CALR-mutated ET)

— Adverse effects: flu-like symptoms, depression (screen and avoid in active mood disorders), autoimmune phenomena, transaminitis, thyroid dysfunction

— Selective megakaryocyte inhibitor; reserved for hydroxyurea failure/intolerance

— Avoid in cardiac disease — causes palpitations, tachycardia, fluid retention, headache, and may accelerate progression to myelofibrosis

— Does not reduce arterial thrombosis as effectively as hydroxyurea (PT-1 trial)

— Prior arterial thrombosis: aspirin + cytoreduction

— Prior venous thrombosis: systemic anticoagulation (warfarin or DOAC) + cytoreduction; add aspirin only if concurrent arterial indication

— Splanchnic vein thrombosis: indefinite anticoagulation

Board pearl: A 35-year-old woman with high-risk ET who wants future pregnancy → choose pegylated interferon, not hydroxyurea (teratogenic, category D).

Hydroxyurea (first-line for most high-risk adults):

Pegylated interferon alfa-2a (ropeginterferon alfa-2b is the related agent approved for PV; off-label use in ET is common):

Anagrelide (second-line):

Antiplatelet/anticoagulation overlay:

Ruxolitinib: Not standard first-line in ET; reserved for post-ET myelofibrosis with symptoms/splenomegaly

Managing Extreme Thrombocytosis, Acute Events, and Drug Switching

— Check vWF ristocetin cofactor activity; if <30%, hold aspirin to prevent hemorrhage

— Start or intensify cytoreduction (hydroxyurea) to bring platelets down; vWF activity typically normalizes as platelets fall, after which aspirin can be safely added

— Treat the acute event per usual standards (tPA/thrombectomy, PCI, dual antiplatelet therapy as indicated)

— Initiate cytoreduction urgently if not already on it

— Target platelets <400 × 10⁹/L; some experts target <250 after stroke

— Therapeutic anticoagulation; DOACs are acceptable in ET, though warfarin is preferred for splanchnic thrombosis in many centers

— Duration: indefinite for unprovoked or splanchnic events given ongoing prothrombotic state

— Add cytoreduction

— Reserved for life-threatening thrombosis or hemorrhage with extreme thrombocytosis as a temporizing bridge while cytoreduction takes effect

— Not used for asymptomatic high counts

— Platelets >600 after 3 months at ≥2 g/day

— Platelets >400 with WBC <2.5 or Hgb <10

— Unacceptable toxicity (leg ulcers, mucocutaneous, fever, pneumonitis)

— Switch to pegylated interferon (younger) or anagrelide (older without cardiac disease); ruxolitinib trials ongoing

— Continue cytoreduction through surgery; target platelets <400 preop

— Hold aspirin 5–7 days before high-bleed-risk surgery, resume 24–48 h postop

— Pharmacologic VTE prophylaxis is mandatory — ET patients are high VTE risk even after platelets normalize

CCS pearl: In a CCS case of new stroke + platelets 1.2 million, your orders are: stroke pathway + neuro consult + hematology consult + start hydroxyurea + check vWF activity before adding aspirin.

Extreme thrombocytosis (>1000–1500 × 10⁹/L):

Acute arterial thrombosis (stroke, MI):

Acute venous thrombosis:

Plateletpheresis:

Hydroxyurea resistance/intolerance (ELN criteria — any one):

Perioperative management:

Special Populations — Elderly and Renal/Hepatic Impairment

— Automatically meet age criterion for high-risk ET; JAK2 positivity further amplifies risk

— Cytoreduction is virtually always indicated — hydroxyurea is preferred due to oral route, decades of safety data, and low cost

— Lower starting dose (500 mg daily) and slower titration; check CBC every 2 weeks initially

— Watch for polypharmacy: hydroxyurea + allopurinol + aspirin + antihypertensives — review at every visit

— Falls assessment matters: aspirin + anticoagulation in a frail faller may need re-evaluation per shared decision-making

— Hydroxyurea is renally cleared — dose-reduce

— Increased risk of cytopenias and mucocutaneous toxicity

— Anagrelide: no specific renal adjustment but use caution

— Interferons: generally safe in CKD but monitor for fatigue, depression, thyroid changes

— DOACs for anticoagulation: apixaban preferred in CKD; avoid dabigatran if CrCl <30

— Hydroxyurea: caution; monitor LFTs but no formal dose adjustment

— Interferon: avoid in decompensated cirrhosis (can precipitate hepatic failure, depression)

— Anagrelide: contraindicated in severe hepatic impairment (Child-Pugh C)

— Anagrelide is contraindicated/avoided in CHF, arrhythmias, significant CAD due to positive inotropy, tachycardia, and fluid retention

— Older patients with QT prolongation: anagrelide can prolong QT — get baseline ECG

— Daily dosing reliability matters; consider pillboxes, caregiver involvement, and simpler regimens

Key distinction: In an elderly patient with ET and CAD requiring cytoreduction, choose hydroxyurea over anagrelide — anagrelide's cardiotoxicity makes it the wrong answer on the boards in this scenario.

Elderly (>60–75 years):

Renal impairment:

CrCl 30–59: reduce dose ~50%

CrCl <30 or dialysis: start at 25% dose, monitor closely

Hepatic impairment:

Cardiovascular comorbidity:

Cognitive and adherence considerations:

Special Populations — Pregnancy, Young Patients, and Surgery

— Pregnancy itself is hypercoagulable; ET adds clonal prothrombotic state

— Complications: first-trimester miscarriage (~25–40%), IUGR, placental abruption, preeclampsia, preterm birth, maternal venous thrombosis

— Higher risk pregnancies: prior thrombosis, prior pregnancy loss, JAK2 positive, platelets >1500

— Low-dose aspirin 81 mg daily throughout pregnancy unless bleeding contraindication

— Prophylactic LMWH (enoxaparin 40 mg daily) postpartum for 6 weeks in all ET patients; antepartum LMWH if prior thrombosis or other high-risk features

— Cytoreduction if needed: pegylated interferon alfa is the agent of choice (does not cross placenta significantly, non-teratogenic)

— Avoid hydroxyurea (teratogenic) and anagrelide (crosses placenta) — stop hydroxyurea ≥3 months before conception in both partners

— Monitor: monthly CBC, serial growth ultrasounds, uterine artery Dopplers in high-risk patients

— Multidisciplinary MFM + hematology co-management

— Most are low/very-low risk → aspirin ± observation

— If cytoreduction needed: interferon preferred to minimize cumulative lifetime exposure to hydroxyurea and theoretical leukemogenicity concerns

— Avoid combined estrogen-containing OCPs (additive thrombotic risk)

— Progestin-only methods, IUDs (copper or levonorgestrel) are preferred

Step 3 management: A 30-year-old JAK2-positive woman with ET planning pregnancy → switch from hydroxyurea to pegylated interferon ≥3 months pre-conception, continue low-dose aspirin, plan postpartum LMWH for 6 weeks.

Pregnancy in ET — high-risk obstetric scenario:

Management of pregnancy:

Young patients (<40) without pregnancy plans:

Contraception:

Pediatric ET: Rare; managed at specialized centers; aspirin for symptomatic disease, interferon for cytoreduction

Complications and Adverse Outcomes

— Arterial: ischemic stroke/TIA, MI, peripheral arterial occlusion, retinal artery occlusion, digital ischemia

— Venous: DVT, PE, splanchnic vein thrombosis (portal, mesenteric, hepatic — Budd-Chiari), cerebral venous sinus thrombosis

— Lifetime thrombosis risk ~20% even on therapy; risk halved by appropriate cytoreduction + aspirin

— Microvascular: erythromelalgia, transient visual loss, headaches, paresthesias

— GI and mucocutaneous bleeding, often when platelets >1000–1500 due to acquired von Willebrand syndrome

— Increased risk with aspirin + extreme thrombocytosis — always screen vWF before antiplatelet therapy in extreme counts

— Post-ET myelofibrosis: ~5–10% at 15 years; presents with worsening splenomegaly, cytopenias (anemia, then thrombocytopenia), constitutional symptoms, leukoerythroblastic smear, rising LDH

— AML/MDS transformation: ~1–5% at 15 years; risk modestly increased by prolonged hydroxyurea + alkylators, age, and certain mutations (TP53, ASXL1)

— Triple-negative and high-molecular-risk mutations (SRSF2, U2AF1, IDH1/2) predict worse outcomes

— Hydroxyurea: leg ulcers (often around malleoli, painful, slow-healing — typically requires drug discontinuation), oral ulcers, skin cancers, macrocytic anemia, alopecia

— Anagrelide: palpitations, HF exacerbation, headache, diarrhea, accelerated myelofibrosis

— Interferon: depression (with suicide risk), autoimmune thyroiditis, flu-like symptoms, neutropenia

Board pearl: A patient on hydroxyurea presents with a painful non-healing malleolar ulcer — the answer is discontinue hydroxyurea and switch cytoreductive agent; topical wound care alone won't heal it.

Thrombotic complications (the dominant morbidity):

Hemorrhagic complications:

Disease transformation:

Treatment-related complications:

Pregnancy losses and obstetric complications (as above)

Quality of life: chronic fatigue is the most common patient-reported symptom and often underrecognized; use MPN-SAF TSS to track

When to Escalate — Consults, Inpatient Triage, and ICU

— Any new diagnosis of ET (or strong suspicion) to confirm with bone marrow biopsy

— Treatment failure or intolerance to first-line cytoreduction

— Pregnancy planning in ET

— Suspected disease transformation (myelofibrosis or acute leukemia)

— Pre-allogeneic transplant evaluation in young patients with high-risk features

— Any acute thrombotic event (stroke, MI, PE, splanchnic thrombosis, limb ischemia) — admit, treat per standard protocols, urgent heme consult, start cytoreduction

— Major hemorrhage with extreme thrombocytosis — admit, hold antiplatelet/anticoagulant, vWF studies, plateletpheresis if life-threatening, hydroxyurea load

— New constitutional symptoms + cytopenias + splenomegaly → admit if symptomatic, evaluate for myelofibrotic transformation

— Febrile neutropenia on cytoreduction — admit, broad-spectrum antibiotics

— Massive PE with hemodynamic instability — thrombolysis, possible thrombectomy

— Acute Budd-Chiari with hepatic failure — hepatology + transplant evaluation, TIPS consideration

— Catastrophic stroke with herniation risk — neuro ICU

— Plateletpheresis often requires step-down or ICU monitoring depending on hemodynamics

— Maternal-fetal medicine for pregnancy

— Vascular surgery for limb ischemia or chronic venous insufficiency

— Dermatology for hydroxyurea-related skin lesions and skin cancer surveillance

— Cardiology before initiating anagrelide

— Genetic counseling rarely, for familial MPN clusters

— Hospital discharge after thrombotic event must include: cytoreduction prescription, anticoagulation plan with duration, hematology follow-up within 1–2 weeks, CBC in 1 week, and clear written instructions for the patient's PCP

CCS pearl: "Schedule outpatient hematology follow-up in 1 week" is almost always a correct discharge order in an ET CCS case — don't forget it.

Hematology referral — required for:

Emergency department / inpatient triage:

ICU-level care:

Multidisciplinary consults to consider:

Transitions of care (Step 3 favorite):

Key Differentials — Other Myeloproliferative and Myeloid Causes

— Elevated Hct (>49% men, >48% women) and/or red cell mass

— Suppressed EPO, JAK2 V617F in >95% (or JAK2 exon 12)

— Splenomegaly more common; aquagenic pruritus characteristic

— Masked PV: patients with iron deficiency may have normal Hct masking underlying PV — repeat after iron repletion or check red cell mass

— Treatment: phlebotomy to Hct <45%, aspirin, hydroxyurea/interferon if high-risk

— Anemia, teardrop RBCs, leukoerythroblastic smear, massive splenomegaly, constitutional symptoms

— Elevated LDH, marrow fibrosis (grade ≥2 in overt PMF; ≤1 in prePMF — distinguishing prePMF from ET requires expert hematopathology)

— Worse prognosis than ET; ruxolitinib for symptomatic disease; allogeneic transplant for high-risk young patients

— Key distinction: prePMF has higher thrombosis risk, more frequent progression, and worse survival than true ET — biopsy is essential

— Leukocytosis with full myeloid spectrum, basophilia, eosinophilia, splenomegaly

— BCR-ABL1 / Philadelphia chromosome positive — must be excluded in every thrombocytosis workup

— Rarely presents with isolated thrombocytosis but can mimic ET

— Treatment: tyrosine kinase inhibitors (imatinib, dasatinib, nilotinib)

— Especially MDS/MPN with ring sideroblasts and thrombocytosis (MDS/MPN-RS-T) — features of both MDS (dysplasia, ring sideroblasts) and ET-like thrombocytosis; often SF3B1 + JAK2 mutated

— Treatment combines MDS approach (lenalidomide, luspatercept) with thrombosis prevention

Board pearl: Thrombocytosis + basophilia + splenomegaly = check BCR-ABL before treating as ET. CML can hide as "ET."

Differentiating ET from same-category clonal myeloid disorders is the #1 diagnostic task:

Polycythemia vera (PV):

Primary myelofibrosis (PMF) — including prefibrotic PMF (prePMF):

Chronic myeloid leukemia (CML):

Myelodysplastic syndromes with thrombocytosis:

Key Differentials — Reactive and Other-Category Causes

— Iron deficiency anemia: the single most common cause of elevated platelets in clinical practice; check ferritin, treat with iron, recheck CBC — platelets often normalize within weeks

— Infection/inflammation: acute bacterial infections, abscesses, IBD, rheumatoid arthritis, vasculitis, tuberculosis — elevated CRP/ESR are clues

— Malignancy: paraneoplastic thrombocytosis, particularly in lung, GI, gynecologic, lymphoma; in older patients with new thrombocytosis and weight loss/anemia, age-appropriate cancer evaluation is warranted

— Post-splenectomy / functional asplenia: persistent thrombocytosis is expected; Howell-Jolly bodies on smear are the giveaway

— Hemolysis or acute blood loss: rebound thrombocytosis during recovery

— Post-surgical / post-trauma: transient rebound, peaks at 1–2 weeks

— Medications: epinephrine, corticosteroids, vincristine, retinoic acid, TPO receptor agonists, recovery from chemotherapy

— Alcohol withdrawal — rebound thrombocytosis after marrow recovery

— Platelets usually <1000 × 10⁹/L (though can exceed it)

— Normal-sized, normal-shaped platelets on smear (vs. anisocytic giant platelets in ET)

— Elevated CRP/ESR, ferritin, fibrinogen (acute phase reactants)

— Resolves with treatment of underlying cause

— No erythromelalgia, no thrombotic predisposition — reactive thrombocytosis rarely causes thrombosis

— Rare; germline MPL or THPO mutations; familial pattern; usually benign course

— Consider in young patients with persistent thrombocytosis, family history, and negative MPN molecular workup

— Microcytic RBC fragments, cryoglobulins, or bacterial particles miscounted as platelets — confirm with manual smear review

Key distinction: Reactive thrombocytosis is transient, normal-platelet-morphology, elevated CRP, and not thrombogenic — distinguish before any cytoreduction.

Reactive (secondary) thrombocytosis — must exclude before diagnosing ET:

Distinguishing features of reactive thrombocytosis:

Hereditary thrombocytosis:

Spurious thrombocytosis:

Secondary Prevention and Long-Term Plan

— Low-dose aspirin 81 mg daily for low/intermediate/high-risk JAK2-positive patients

— Anticoagulation indefinitely after unprovoked or splanchnic venous thrombosis

— Reassess bleeding/thrombosis balance annually; check vWF if platelets rise

— Target platelets <400 × 10⁹/L in high-risk; lower (<250) if recent thrombosis

— Monitor for tachyphylaxis, intolerance, transformation

— Document indications and reassess yearly

— Blood pressure <130/80 per ACC/AHA

— Statin based on ASCVD risk score; consider lower threshold given inherent prothrombotic state

— HbA1c to individualized target (typically <7%)

— Tobacco cessation counseling at every visit — pharmacotherapy (varenicline, bupropion, NRT) covered by most plans

— Weight management, Mediterranean-style diet, regular aerobic exercise

— Annual influenza

— Pneumococcal per CDC schedule

— Recombinant zoster (Shingrix) for age ≥50

— COVID-19 updated boosters

— Hepatitis B if not immune

— Age-appropriate colonoscopy, mammography, cervical screening, lung CT (if eligible)

— Annual full-body skin exam by dermatology for hydroxyurea-treated patients (nonmelanoma skin cancer risk)

— Repeat marrow if clinical change suggests transformation (worsening cytopenias, growing spleen, new symptoms, rising LDH)

— Verify cytoreductive dose, antiplatelet/anticoagulant, statin, antihypertensive

— Counsel on bleeding precautions and thrombosis warning signs

Step 3 management: Don't neglect statin and BP targets — modifiable CV risk factors are an independent component of the IPSET-thrombosis score and must be optimized at every visit.

Pillars of long-term ET management (Step 3 ambulatory voice):

Antiplatelet/anticoagulation maintenance:

Cytoreduction maintenance:

Aggressive CV risk factor management (often overlooked, board favorite):

Vaccinations:

Cancer surveillance:

Bone marrow re-evaluation:

Discharge medication reconciliation after any hospitalization:

Follow-Up, Monitoring Parameters, and Counseling

— Newly diagnosed or initiating therapy: CBC every 2 weeks until stable, then every 4–8 weeks

— Stable on hydroxyurea: CBC every 3 months, CMP/LDH/uric acid every 6 months

— Stable on interferon: CBC monthly initially, then every 3 months; TSH every 6 months, depression screening (PHQ-9) at each visit

— Anagrelide: CBC + ECG monitoring, especially with dose changes; cardiac symptoms at each visit

— Hematology visit at least every 6 months in low-risk; every 3 months in high-risk

— Platelet count trend (target <400, or <250 post-event)

— Hemoglobin (watch for macrocytic anemia from hydroxyurea, transformation anemia)

— WBC and differential (cytopenia, blast monitoring)

— LDH, uric acid (cell turnover, transformation)

— MPN-SAF Total Symptom Score (TSS) at visits — captures fatigue, pruritus, early satiety, bone pain, concentration; rising score may signal transformation

— Spleen size by exam, imaging if equivocal

— BP, lipids, A1c, smoking status — every visit

— Thrombosis warning signs: sudden unilateral weakness/numbness, vision loss, chest pain, dyspnea, calf swelling, severe abdominal pain → ED immediately

— Bleeding warning signs: melena, hematuria, prolonged epistaxis, easy bruising

— Hydroxyurea: contraception in both sexes (3 months washout before conception), sun protection, annual skin checks, oral hygiene for ulcer surveillance

— Interferon: mood monitoring, thyroid symptoms, expect flu-like symptoms with first injections (premedicate acetaminophen)

— Aspirin: take with food, avoid concurrent NSAIDs when possible

— Regular moderate aerobic exercise reduces thrombosis risk

— Hydration, compression stockings during long travel

— Avoid prolonged immobility; perioperative VTE prophylaxis non-negotiable

Board pearl: Rising MPN-SAF score + new anemia + growing spleen on follow-up = order repeat bone marrow biopsy — concern for post-ET myelofibrosis.

Follow-up cadence:

What to monitor and document:

Patient counseling priorities:

Rehabilitation and lifestyle:

Ethical, Legal, and Patient Safety Considerations

— Discuss theoretical leukemogenic risk of hydroxyurea, especially in younger patients, and offer interferon as alternative — document shared decision-making

— Interferon: disclose risk of severe depression and suicide; obtain mental health history; co-manage with psychiatry if mood disorder present

— Anagrelide: discuss cardiac risks, progression to myelofibrosis concerns

— Both partners on hydroxyurea must use effective contraception during therapy and for 3+ months before conception

— Document teratogenicity counseling at every visit for women of reproductive age

— Pregnancy in ET is a high-risk obstetric scenario requiring formal multidisciplinary consent and management plan

— After acute thrombosis hospitalization: medication reconciliation, written discharge instructions, scheduled hematology follow-up within 1–2 weeks, CBC in 1 week, and explicit communication with the PCP are the chain links most likely to break

— Verify the patient can afford and access cytoreductive therapy before discharge — hydroxyurea is generic and cheap; interferon and anagrelide are not — engage social work/financial counseling

— Always provide a written "warning sign" handout in the patient's preferred language and at a 6th-grade reading level

— Do not reflexively start aspirin in a patient with platelets >1000 — check vWF activity first to prevent iatrogenic hemorrhage (a Joint Commission–style sentinel event if missed)

— JAK2/CALR/MPL mutations are somatic, not germline — counsel patients that ET is not directly heritable to relieve family anxiety, while acknowledging rare familial MPN clusters

— Counsel after TIA/stroke per state DMV reporting laws; some states require physician reporting of patients with recent loss of consciousness or visual events

— Document fatigue and cognitive symptoms objectively for FMLA/disability paperwork when warranted

Step 3 management: The single highest-yield safety move in a discharged ET patient is scheduled hematology follow-up within 1–2 weeks with a CBC — this question shows up repeatedly.

Informed consent for cytoreductive therapy:

Reproductive counseling and consent:

Transitions of care — Step 3's favorite safety target:

Patient safety in extreme thrombocytosis:

Genetic counseling and disclosure:

Driving and occupational safety:

Disability and employment:

High-Yield Associations and Rapid-Fire Clinical Facts

— JAK2 V617F: higher thrombosis risk, higher Hct/WBC, more PV-like features

— CALR (type 1 > type 2): higher platelet counts but lower thrombosis risk, better survival in ET, more common in younger patients

— MPL W515: rare, intermediate phenotype

— Triple-negative: indolent ET phenotype, but rule out hereditary thrombocytosis and prePMF carefully

Board pearl: "Asymptomatic 45-year-old, platelets 720, JAK2 negative, CALR positive, no thrombosis history" → low-dose aspirin only (or observation if no microvascular symptoms) — do not start hydroxyurea.

Driver mutations and clinical phenotype:

Erythromelalgia: Burning red painful digits, dramatically relieved by a single low-dose aspirin — near-pathognomonic for ET/PV microvascular disease

Acquired von Willebrand syndrome: Platelets >1000–1500 → high-MW vWF multimers consumed → bleeding; check vWF before aspirin in extreme thrombocytosis

Splanchnic vein thrombosis: Unprovoked portal/hepatic/mesenteric vein thrombosis in a young patient with normal CBC → screen JAK2 V617F (occult MPN found in ~30% of "idiopathic" splanchnic VTE)

IPSET-thrombosis high-risk = age >60 + JAK2-positive, OR any prior thrombosis — automatic cytoreduction

Hydroxyurea ankle ulcer: Painful malleolar non-healing ulcer in an ET patient on long-term hydroxyurea → discontinue and switch to interferon or anagrelide

Pregnancy in ET: Aspirin throughout, postpartum LMWH × 6 weeks, interferon if cytoreduction needed, avoid hydroxyurea and anagrelide

PT-1 trial: Hydroxyurea + aspirin beat anagrelide + aspirin in high-risk ET for arterial thrombosis prevention — hydroxyurea remains first-line in older patients

WHO criterion to remember: Bone marrow biopsy showing mature, hyperlobulated, staghorn megakaryocytes with reticulin grade ≤1 = ET (not PMF)

Reactive vs clonal smear: Reactive = small normal platelets; clonal = giant anisocytic platelets, occasional megakaryocyte fragments

Transformation rates at 15 years: post-ET myelofibrosis ~5–10%, AML ~1–5%

Vaccinations not to forget: influenza, pneumococcal, Shingrix ≥50, COVID boosters

Board Question Stem Patterns

"A 58-year-old man on routine preoperative labs has platelets 685. Smear shows large platelets. Ferritin and CRP normal."

— Next step: persistent thrombocytosis confirmation + JAK2 testing + bone marrow biopsy

— Distractor: jumping to hydroxyurea before risk stratifying

"Burning red painful soles relieved by aspirin, platelets 612, JAK2 V617F positive."

— Diagnosis: ET; if age ≤60 and no prior thrombosis, low-dose aspirin alone; treat erythromelalgia symptomatically with aspirin BID if needed

"32-year-old JAK2-positive ET on hydroxyurea wants pregnancy."

— Switch to pegylated interferon ≥3 months before conception; continue aspirin; plan postpartum LMWH × 6 weeks

"Platelets 1.6 million, nosebleeds, prolonged bleeding time."

— Check vWF ristocetin cofactor activity; hold aspirin if vWF activity low; start hydroxyurea

"68-year-old with new left hemiparesis, platelets 920, JAK2 positive."

— Acute stroke management + start hydroxyurea + low-dose aspirin (or anticoagulate if cardioembolic); hematology follow-up at discharge

"Platelets 580, microcytic anemia, low ferritin." → Iron deficiency, not ET

"Platelets 700, splenomegaly, basophilia, leukocytosis." → CML, send BCR-ABL

"Platelets 600, anemia, teardrops, massive splenomegaly." → Primary myelofibrosis

"Painful non-healing malleolar ulcer in ET patient on hydroxyurea × 4 years." → Discontinue hydroxyurea, switch to interferon

"32-year-old man with new portal vein thrombosis, normal CBC." → Send JAK2 V617F (occult MPN); anticoagulate indefinitely

"Patient discharged after ET-related MI; what's the most important follow-up order?" → Hematology appointment within 1–2 weeks with CBC

Step 3 management: When the stem gives you "platelets, JAK2, age, prior clot," calculate IPSET-thrombosis mentally — the answer almost always falls out of the risk tier.

Stem 1 — The incidental thrombocytosis:

Stem 2 — Erythromelalgia:

Stem 3 — Young woman planning pregnancy:

Stem 4 — Extreme thrombocytosis with mucosal bleeding:

Stem 5 — Stroke + thrombocytosis:

Stem 6 — The mimics:

Stem 7 — Hydroxyurea complication:

Stem 8 — Splanchnic thrombosis:

Stem 9 — Discharge planning:

One-Line Recap

Essential thrombocythemia is a clonal JAK2/CALR/MPL-driven myeloproliferative neoplasm whose management is dictated entirely by thrombotic risk stratification (age, prior thrombosis, JAK2 status, CV risk factors) — universal aspirin in JAK2-positive disease, cytoreduction (hydroxyurea, or interferon in young/pregnant patients) for high-risk disease, aggressive CV risk factor control, and vigilant surveillance for transformation.

Board pearl: When in doubt on Step 3, the right next step in ET is almost always risk-stratify first, treat modifiable cardiovascular risk factors, and schedule structured hematology follow-up — not reflexive cytoreduction.

Diagnosis: Confirm persistent platelets ≥450, exclude reactive causes (iron, inflammation, malignancy), exclude BCR-ABL, send JAK2/CALR/MPL, perform bone marrow biopsy to distinguish ET from prefibrotic primary myelofibrosis — WHO criteria require all four major criteria or three major plus one minor.

Risk-based treatment (IPSET-thrombosis): Very low/low risk → aspirin only (or observation if CALR-positive young); intermediate → aspirin ± cytoreduction; high risk (age >60 + JAK2+, or any prior thrombosis) → hydroxyurea + aspirin/anticoagulation with target platelets <400 × 10⁹/L; switch to interferon for hydroxyurea failure, young patients, or pregnancy; avoid anagrelide in cardiac disease.

Safety must-knows: Check vWF activity before starting aspirin when platelets >1000–1500 (acquired vWD risk); avoid hydroxyurea in pregnancy and discontinue ≥3 months pre-conception; postpartum LMWH × 6 weeks for all ET pregnancies; annual dermatology surveillance for hydroxyurea-treated patients; aggressive control of BP, lipids, glucose, and tobacco — these are independently part of the risk score.

Follow-up and transitions: CBC every 2 weeks during titration then every 3 months when stable; hematology follow-up within 1–2 weeks after any thrombotic hospitalization; track MPN-SAF symptom score, LDH, and spleen size to detect post-ET myelofibrosis or AML transformation early.