Az essentialis thrombocythaemia Philadelphia-negatív krónikus myeloproliferatív neoplázia, amelyet emelkedett thrombocytaszám, megakaryocyta hyperplasia, fokozott thrombohaemorrhagiás és lassan progrediáló transzformációs hajlam jellemez. Ritka betegség, incidenciája 0,5–1,5/100 000 lakos/év, előfordulása női predominanciát és bimodalitást mutat: fiatal nőkben és az 50–70 évesekben gyakoribb. Az essentialis thrombocythaemia diagnózisa a 450·109/l feletti thrombocytaszámon, a normális erythroid sejttömegen és vasraktáron, valamint egyéb thrombocytosist okozó tényezők hiányán alapul, egyéb myeloproliferativ neoplasiák (a myelofibrosis, krónikus myeloid leukémia) kizárandóak. A Janus-2 pszeudokináz gén V617F-mutációja az essentialis thrombocythaemiás esetek közel 50%-ában fordul elő, a fennmaradókban a calreticulin gén (CALR) exon 9, néhányukban pedig thrombopoetin receptor gén (MPL) mutációja mutatható ki, de 10–15%-ban ezek egyike sincs jelen, ezeket tripla-negatív eseteknek nevezzük. A betegség tünetei a gyengeség, fejfájás, thrombohaemorrhagiás események mellett az erythromelalgia lehetnek. A splenomegalia általában mérsékelt fokú. A betegség prognózisa általában igen jó, a fő szövődményt a vascularis események jelentik. Az ET kezelésében elsődleges a thrombocyta aggregáció gátlás, de a 60 évnél idősebb és/vagy korábban már thromboembolián átesett betegek esetén cytoreduktív kezelés is javasolt. Az irodalmi adatok szerint a thrombosis kockázat nem függ össze a thrombocytosis mértékével.
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