It is important to scrutinize the blood film when establishing a diagnosis of ET. Here platelets are increased and often larger, hypogranular forms are seen as are megakaryocyte nuclei; a neutrophilia may also be present. Features of MDS, MF, iron deficiency, or hypos-plenism (may be present in ET due to splenic infarction) should be excluded. The aspirate shows increased numbers of large, hyperlobated megakaryocytes and platelet drifts may be prominent. Trephine biopsy appearances are variable in ET; the marrow is not always hypercellular but megakaryocytes are always increased (Figure 46.1). The extent to which megakary-ocyte morphology can be used to ascertain a definitive diagnosis of ET, where the megakaryocytes are enlarged and mature, or furthermore to discriminate between true ET and a prefibrotic phase of MF24 (a subgroup with a worse prognosis)25 is unclear. The atypical features include immature megakaryocytes with clustering, cloud-like nuclei, erythroid hypoplasia, and increased and dysplastic granulopoiesis (Figure 46.1). It would be very attractive to be able to further subdivide patients prognostically; however, megakaryocyte morphology is notoriously difficult to reproducibly assess, and thus far there has been no evaluation of interobserver variation. The prognostic and diagnostic significance of varied reticulin density or scoring in ET remains similarly unclear.

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