Polycythaemia vera (PV) is a type of blood cancer that causes the body to produce too many red blood cells.
Red blood cells are made in the bone marrow, the soft tissue inside some bones. Their job is to carry oxygen around the body. In PV, the bone marrow produces too many of these cells, which makes the blood thicker than normal. Thicker blood does not flow as easily, increasing the risk of blood clots (thrombosis). Some people with PV also have too many platelets. Platelets help stop bleeding by forming clots, but having too many can increase the risk of both clotting and unusual bleeding (haemorrhage).
PV begins when a change (mutation) occurs in a gene that controls how blood cells grow and divide. Genes act like instructions for the body’s cells. Sometimes, when cells copy themselves, mistakes happen and a mutation develops. We do not know exactly why these mutations occur. However, more than 95% of people with PV have a mutation called JAK2 V617F, and most of the remaining people have another mutation known as JAK2 exon 12. A small number of people do not have a JAK2 mutation and may need further tests to confirm a diagnosis of PV.
Some people have features of both PV and another similar blood condition called essential thrombocythaemia (ET). In ET, the bone marrow produces too many platelets. Because platelets help the blood to clot, having too many can increase the risk of thrombosis. This can also happen in PV, especially in people who have raised platelet counts as well as high red blood cell levels.
The main goal of treatment for both PV and ET is to reduce the risk of blood clots, and some treatments are used for both conditions.
