Investigations
1st investigations to order
FBC with differential
Test
An FBC should be ordered as part of the initial evaluation.
Patients with essential thrombocythaemia (ET) will have a persistent and unexplained elevated platelet count.
Thrombocytosis is the hallmark of ET. Platelet count can range from 450 × 10⁹/L to >1000 × 10⁹/L (450,000 to >1 million/microlitre).
The degree of thrombocytosis cannot be used to predict the likelihood of ET; the platelet count may be elevated to the same range in people with reactive thrombocytosis.
If thrombocytosis is reported in the initial study, it should be confirmed by repeat testing and examination of the peripheral blood smear.[36]
White blood cell (WBC) count is usually normal, but can be mildly elevated in ET. Patients with reactive thrombocytosis caused by inflammatory or infectious conditions may have an elevated WBC count.
Result
platelet count ≥450 × 10⁹/L (≥450,000/microlitre); elevated WBC count
peripheral blood smear
Test
A peripheral blood smear should be ordered as part of the initial evaluation.
In patients with essential thrombocythaemia (ET), peripheral blood smear will show thrombocytosis with varying degrees of platelet anisocytosis (ranging from normal size platelets with normal granulation, to large platelets that are hypogranular). Immature precursor cells (e.g., myelocytes, metamyelocytes) may also be seen.
Red blood cells (RBCs) on peripheral blood smear are usually normochromic and normocytic in patients with ET. Hypochromic and microcytic RBCs may indicate iron deficiency (a cause of reactive thrombocytosis).
Patients who have undergone splenectomy or have reduced splenic function (hyposplenism) may have a secondary thrombocytosis. Peripheral blood smear in these patients will show nuclear fragments (Howell-Jolly bodies) in RBCs, along with target cells and misshapen RBCs.
Result
thrombocytosis with varying degrees of platelet anisocytosis; immature precursor cells (e.g., myelocytes, metamyelocytes) may be present; normochromic and normocytic RBCs
serum iron studies
Test
Required to exclude iron deficiency and other secondary causes of thrombocythaemia.
A low serum ferritin (e.g., <27 picomol/L [<12 nanograms/mL]; varies between guidelines) is diagnostic of iron deficiency, with a specificity approaching 100%.
Result
normal
Investigations to consider
CRP
Test
Chronic inflammation needs to be excluded, as this is a cause of secondary thrombocytosis.
Serum tests for inflammatory markers (e.g., the acute phase reactants: CRP, erythrocyte sedimentation rate, and fibrinogen) should be done after thrombocytosis is confirmed on FBC and peripheral blood smear.
Serum CRP is usually normal in essential thrombocythaemia, and increased in most cases of reactive thrombocytosis.
Result
normal
erythrocyte sedimentation rate (ESR)
Test
Chronic inflammation needs to be excluded, as this is a cause of secondary thrombocytosis.
Serum tests for inflammatory markers (e.g., the acute phase reactants: CRP, ESR, and fibrinogen) should be done after thrombocytosis is confirmed on FBC and peripheral blood smear.
ESR is usually normal in essential thrombocythaemia, and increased in most cases of reactive thrombocytosis.
Result
normal
fibrinogen
Test
Chronic inflammation needs to be excluded, as this is a cause of secondary thrombocytosis.
Serum tests for inflammatory markers (e.g., the acute phase reactants: CRP, erythrocyte sedimentation rate, and fibrinogen) should be done after thrombocytosis is confirmed on FBC and peripheral blood smear.
Fibrinogen is usually normal in essential thrombocythaemia, and increased in most cases of reactive thrombocytosis.
Result
normal
bone marrow biopsy and histopathology
Test
Bone marrow biopsy is indicated if there is evidence of thrombocytosis on FBC and peripheral blood smear without an identifiable cause.
A major increase in reticulin fibres would suggest prefibrotic/early primary myelofibrosis (prePMF). It is critical to make this distinction on bone marrow biopsy as these two disorders have significantly different prognoses.
Result
proliferation of the megakaryocyte lineage with increased numbers of enlarged, mature megakaryocytes with hyperlobulated nuclei; no significant increase or left shift in neutrophil granulopoiesis or erythropoiesis, and no major increase in reticulin fibres; minor (grade 1) increase in reticulin fibres may be seen, but is rare
genetic mutation testing (JAK2 V617F, CALR, and MPL)
Test
All patients with suspected essential thrombocythaemia (ET) should undergo molecular testing (on peripheral blood or alternatively bone marrow) for the Janus kinase 2 (JAK2) V617F initially. If negative, testing for calreticulin (CALR), and myeloproliferative leukaemia virus oncogene (MPL) mutations should follow.[38] Alternatively, a next-generation sequencing panel comprising all three myeloproliferative neoplasm (MPN) driver mutations can be used.
Presence of a JAK2 V617F, CALR, or MPL mutation indicates an MPN, but these driver mutations are not specific for ET.
JAK2 V617F, CALR, and MPL mutations are present in approximately 50% to 60%, 25% to 30%, and 3% to 11% of patients with ET, respectively.[19] Approximately 10% to 15% of patients with ET are negative for all three driver mutations (triple‐negative); therefore, absence of these mutations does not exclude the diagnosis.[19] JAK2 V617F, CALR, and MPL mutations may occur in other myeloproliferative neoplasms (polycythaemia vera, primary myelofibrosis) and myeloid malignancies.
Driver mutation expression is often considered to be mutually exclusive in ET; however, there are reports of patients with coexisting JAK2 V617F and CALR, or JAK2 V617F and MPL, mutations.[20][21]
Treatment for ET may be individualised based on mutation status.
The JAK2 V617F mutation is associated with higher risk of thrombosis (particularly among homozygotes [>50% mutant allele burden] and those who are younger [aged <60 years]) and a lower risk of post-ET myelofibrosis.[27][28][29][30][31][39] Patients with JAK2 V617F mutation display higher leukocyte count and haemoglobin levels than those without the mutation.[29][32]
CALR-mutation is associated with younger age, male sex, lower haemoglobin level, higher platelet count, lower leukocyte count, and lower risk of thrombosis, compared with JAK2 V617F-mutated ET.[39][40]
MPL-mutation is inconsistently associated with older age, female sex, lower haemoglobin level, higher platelet count, and possible inferior myelofibrosis-free survival, compared with MPL wild-type ET.[39]
Result
may be positive for JAK2 V617F, CALR, or MPL mutations
cytogenetic and molecular testing: BCR::ABL1
Test
Fluorescence in situ hybridisation (FISH) or polymerase chain reaction (PCR) should be carried out to detect BCR::ABL1 (Philadelphia chromosome).
Absence of BCR::ABL1 helps rule out chronic myeloid leukaemia (CML), a myeloproliferative neoplasm that can present initially with isolated thrombocytosis.
It is important to rule out CML because prognosis and management of this condition is very different from that of essential thrombocythaemia.[41]
Result
negative for BCR::ABL1 (Philadelphia chromosome)
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