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Due to its rare incidence, erythrocytosis frequently represents a challenge for the treating doctors. The erythropoiesis (= production of erythrocytes) is located in the bone marrow, and the hormone erythropoietin (EPO) takes control in its regulation. Therefore, measurement of EPO in serum is one of the main diagnostic steps. In erythrocytosis, congenital causes have to be distinguished from acquired ones. Furthermore, there are primary and secondary forms. Congenital causes of erythrocytoses occur very infrequently, are mainly diagnosed in young age and should be treated in specialized centers. Polycythemia vera (PV), a clonal disorder and one of the main myeloproliferative neoplasms (beside essential thrombocythemia and primary myelofibrosis), represents the most frequent primary acquired cause of erythrocytosis. Clinically, increased thrombophilia and microvascular disturbance occur. The first-line treatment in patients with PV includes administration of aspirin and phlebotomies. Secondary acquired forms of erythrocytosis mainly occur due to hypoxia triggered by nicotine abuse or chronic heart and lung diseases. Regarding other differential diagnoses, a cancer-associated EPO production, kidney diseases or exogenous supply with EPO (= EPO doping) have to be considered., Competing Interests: Disclosure The authors report no conflicts of interest in this work., (© Georg Thieme Verlag KG Stuttgart · New York.)

Due to the central role of platelets in hemostasis, the clinical relevance of quantitative changes in platelet counts (< 150 G/l or > 450 G/l) may be significant. Thrombopoesis (= production of platelets) occurs in the bone marrow, and the hormone thrombopoetin takes control on its regulation.In thrombocytosis, primary causes have to be distinguished from the far more common reactive (= secondary) reasons. The most important form of primary thrombocytosis occurs in myeloproliferative neoplasms especially in essential thrombocythemia (ET). Clinically, increased thrombophilia, microcirculatory disturbances as well as an increased hemorrhagic diathesis occur in patients with myeloproliferative neoplasms. According to the WHO diagnosis criteria 2016 standard diagnostic procedure in myeloproliferative neoplasms includes bone marrow biopsy and the detection of one of the acquired and typical MPN mutations in the JAK2, MPL or CALR gene.In contrast, patients with thrombocytopenia more often suffer from bleeding complications, however, in antiphospholipid syndrome or thrombotic microangiopathy (TMA) thrombotic events occur in spite of a low platelet count. Generally it makes sense to differentiate between pathological changes in thrombopoesis and the various causes of increased peripheral platelet turnover. Concerning differential diagnosis a careful anamnesis is very important in order to get hints like drugs associated with thrombocytopenia, signs of infection or autoimmune disorders. As an initial diagnostic approach we recommend examination of the blood smear in order to exclude pseudothrombocytopenia or disorderes like thrombotic microangiopathy, myelodysplasia or other hematological diseases., Competing Interests: Disclosure The authors report no conflicts of interest in this work., (© Georg Thieme Verlag KG Stuttgart · New York.)