What is polycythaemia vera (PV)?

Red blood cells are made in our bone marrow – the spongy material inside some of our bones. They are the cells which carry oxygen around the body.

In PV, blood stem cells in your bone marrow start to make too many red blood cells. This makes your blood thicker than normal.

It’s common for people with PV to have higher than normal levels of white blood cells and platelets as well, which also make the blood thicker.

When your blood is too thick, it’s harder for it to flow freely around your body.

Thick blood leads to a higher risk of developing blood clots, or thrombosis.

Clots can cause a range of different problems, depending on which blood vessels are affected:

• In the brain, clots can cause stroke or mini stroke (a TIA or transient ischaemic attack).
• In the eyes, clots can cause blurred vision or loss of sight.
• In the heart, they can cause angina (chest pain) or a heart attack.
• In the stomach area (abdomen), they can cause damage to organs such as your liver or gut.
• In the legs, they can cause deep vein thrombosis (DVT).
• In the lungs, they can cause chest pain and difficulty breathing. A clot in the lungs is called a pulmonary embolism. It can be related to DVT.

All of these things can cause serious problems and some are life-threatening. So it’s important to be aware of the symptoms of thrombosis. It’s also important to understand that once you’re diagnosed with PV, you can start taking medicine that will bring down your risk of blood clots.

If you have PV, you may have a higher risk of unusual bleeding. This is more likely for people who have a high platelet count as well as too many red blood cells.

Keeping your blood cell counts in the normal range will lower your risk of bleeding. Your hospital team will monitor your blood test results and adjust your treatment if needed.

PV is a type of blood cancer. It’s one of a group of blood cancers known as myeloproliferative neoplasms or MPNs.

The name cancer describes diseases that involve cells growing out of control. PV is a blood cancer because it involves red blood cells multiplying more quickly than normal. But it isn't like other more common cancers such as breast cancer or lung cancer, because the main risk with PV is from blood clots. Treatment aims to control that risk so you can live your life as normally as possible.

Sometimes, PV can progress to another type of blood cancer. It may develop into another type of MPN called myelofibrosis, or a type of leukaemia called acute myeloid leukaemia (AML). It’s rare for either of these things to happen.

If you want to know more about the possibility of PV transforming to another blood cancer, read our information about the risk of progression.

Some people have features of both polycythaemia vera (PV) and another MPN called essential thrombocythaemia (ET). PV and ET have a lot in common and occasionally, they can overlap.

In ET, blood stem cells in the bone marrow produce too many platelets – the cells which stick together to stop you bleeding. This makes the blood clot too easily, causing thrombosis. This can also happen in PV – some people produce too many platelets as well as red blood cells.

Treatments for PV and ET have the same aim – to manage the risk of thrombosis. Some treatments are used for both conditions.

If you are told you have features of both PV and ET, ask your hospital team to explain what this means and how it might affect your treatment and monitoring. Your doctor will choose the best treatment for your condition and your individual circumstances, whether you have PV, ET or a combination of the two.

Polycythaemia rubra vera (PRV) and primary polycythaemia are other names for PV, which can be confusing.

It helps to know that they are the same condition, and this is the right information to read if you are told you have polycythaemia rubra vera (PRV) or primary polycythaemia.

We know that PV is caused by specific genetic changes that happen during someone’s lifetime.

We don’t yet know why this happens, but we know that PV is more common in people over 50 years old, although younger adults can get it. We also know it’s slightly more common in men than women.

PV starts with a genetic change (a mutation) that causes over-production of red blood cells.

Genes are like a set of instructions that tell your cells how to behave, including how to divide to make new cells. Sometimes there’s a copying mistake as the cell divides and that causes a mutation.

We don’t know why genetic mutations happen, but we know over 95% of people have a gene mutation known as JAK2 V617F. Most other people have a mutation called JAK2 exon 12.

A few people have no JAK2 mutation and will need more tests to confirm whether PV is the correct diagnosis. See our information about tests for PV.

In some families the gene that controls red blood cell production is faulty. If you have this faulty gene, you are more likely to develop PV, but this is very rare. Speak to your hospital team if you are worried this might be the case for you.