What is erythrocytosis? Erythrocytosis is a condition in which the red blood cell count exceeds the upper limit of the normal range and/or the hemoglobin concentration exceeds the upper limit of the normal range on routine blood tests. The main thing to consider when finding an increase in red blood cells is to identify whether it is secondary or primary erythrocytosis. 1, depends on whether the patient has obvious dehydration at the time of blood collection, if there is obvious dehydration, blood concentration, red blood cell count will relatively increase. 2, to see if there is secondary erythrocytosis, generally speaking common in chronic severe lung diseases resulting in long-term chronic hypoxic diseases, such as chronic obstructive pulmonary disease, due to long-term hypoxia, resulting in compensatory erythrocytosis; and longer time living in high altitude areas, due to the plateau hypoxia will cause secondary erythrocytosis. There are also heart disease, renal vascular disease, severe snoring, etc. can lead to secondary erythropoiesis due to hypoxia, while adrenal tumors, kidney tumors, other tumors, etc. all lead to secondary erythropoiesis due to excessive secretion of erythropoietin. 3, to see if it is a true erythropoietic sign, which is due to the abnormal bone marrow hematopoietic stem cell gene, especially the presence of JAK-2V617F gene mutation, the bone marrow unregulated overproduction of red blood cells, resulting in a sustained and significant increase in peripheral blood erythrocytes, hemoglobin concentration is often greater than 185g/L in men and 165g/L in women, mostly with increased white blood cells and platelets. This is the time to check the bone marrow and the JAK-2V617F gene mutation. This is a disease of a malignant nature and requires aggressive treatment. Erythrocytosis is not uncommon in clinical practice and needs to be carefully identified and treated aggressively for the cause of the erythrocytosis, as well as actively preventing thrombosis.