Platelets are parts of the blood that help form blood clots. Thrombocytosis (throm-boe-sie-TOE-sis) is a disorder in which your body produces too many platelets.

It's called reactive thrombocytosis or secondary thrombocytosis when the cause is an underlying condition, such as an infection.

Less commonly, when the high platelet count has no apparent underlying condition as a cause, the disorder is called primary thrombocythemia or essential thrombocythemia. This is a blood and bone marrow disease.

A high platelet level may be detected in a routine blood test known as a complete blood count. It's important to determine whether it's reactive thrombocytosis or essential thrombocythemia to choose the best treatment options.

People with high platelet levels often don't have signs or symptoms. When symptoms occur, they're often related to blood clots. Examples include:

• Headache.
• Confusion or changes in speech.
• Chest pain.
• Shortness of breath and nausea.
• Weakness.
• Burning pain in the hands or feet.

Less commonly, very high platelet levels may cause bleeding. This can cause:

• Nosebleeds.
• Bruising.
• Bleeding from your mouth or gums.
• Bloody stool.

Bone marrow is a spongy tissue inside your bones. It contains stem cells that can become red blood cells, white blood cells or platelets. Platelets stick together, helping blood to form a clot that stops bleeding when you damage a blood vessel, such as when you cut yourself. Thrombocytosis occurs when your body produces too many platelets.

Reactive thrombocytosis

This is the more common type of thrombocytosis. It's caused by an underlying medical problem, such as:

• Blood loss.
• Cancer.
• Infections.
• Iron deficiency.
• Removal of your spleen.
• Hemolytic anemia — a type of anemia in which your body destroys red blood cells faster than it produces them, often due to certain blood diseases or autoimmune disorders.
• Inflammatory disorders, such as rheumatoid arthritis, sarcoidosis or inflammatory bowel disease.
• Surgery and other types of traumas.

Essential thrombocythemia

The cause of this disorder is unclear. It often appears to be connected to changes in certain genes. The bone marrow produces too many of the cells that form platelets, and these platelets often don't work properly. This poses a much higher risk of clotting or bleeding complications than does reactive thrombocytosis.

Essential thrombocythemia can lead to a variety of potentially life-threatening complications, such as:

• Strokes. If a blood clot occurs in the arteries that supply the brain, it may cause a stroke. A mini stroke, also called a transient ischemic attack, is a temporary interruption of blood flow to part of the brain.
• Heart attacks. Less commonly, essential thrombocythemia can cause clots in the arteries that supply blood to your heart.
• Cancer. Rarely, essential thrombocythemia may lead to a type of leukemia that progresses quickly.

Pregnancy complications

Most women who have essential thrombocythemia have normal, healthy pregnancies. But uncontrolled thrombocythemia can lead to miscarriage and other complications. Your risk of pregnancy complications may be reduced with regular checkups and medication, so be sure to have your doctor regularly monitor your condition.

A blood test called a complete blood count (CBC) can show if your platelet count is too high. You may also need blood tests to check for:

• High or low iron levels.
• Markers of inflammation.
• Undiagnosed cancer.
• Gene mutations.

You might also need a procedure that uses a needle to remove a small sample of your bone marrow for testing.

Reactive thrombocytosis

Treatment for this condition depends on the cause.

• Blood loss. If you've had significant blood loss from a recent surgery or an injury, your elevated platelet count might resolve on its own.
• Infection or inflammation. If you have a chronic infection or an inflammatory disease, your platelet count likely will remain high until the condition is under control. In most cases, your platelet count will return to normal after the cause is resolved.
• Spleen removed. If you've had your spleen removed, you might have lifelong thrombocytosis, but you're unlikely to need treatment.

Essential thrombocythemia

People with this condition who have no signs or symptoms usually don't need treatment. You might need to take daily, low-dose aspirin to help thin your blood if you're at risk of blood clots. Don't take aspirin without checking with your health care team.

You might need to take prescription medicine or have procedures to lower your platelet counts if you:

• Have a history of blood clots and bleeding.
• Have risk factors for heart disease.
• Are older than 60.
• Have a very high platelet count.

Your doctor might prescribe platelet-lowering drugs such as hydroxyurea (Droxia, Hydrea), anagrelide (Agrylin) or interferon alfa (Intron A).

In emergencies, platelets can be filtered from your blood with a machine. This procedure is called plateletpheresis. The effects are only temporary.

It's likely that a routine blood test showing a high platelet count will be your first indication that you have thrombocytosis.

Besides taking your medical history, examining you physically and running tests, your doctor might ask about factors that could affect your platelets, such as a recent surgery, a blood transfusion or an infection. You might be referred to a hematologist, which is a doctor who specializes in blood diseases.

Here's some information to help you get ready for your appointment.

What you can do

Be aware of pre-appointment restrictions. When you make the appointment, ask if there's anything you need to do in advance, such as restrict your diet.

Make a list of:

• Your symptoms and when they began.
• Your medical history, including recent infections, surgical procedures, bleeding and anemia.
• All medications, vitamins and other supplements you take, including doses.
• Questions to ask your doctor.

Take a family member or friend along, if possible, to help you remember the information you're given.

For thrombocytosis, questions to ask include:

• What tests do I need?
• Is my condition likely temporary or chronic?
• What treatment do you recommend?
• What follow-up care will I need?
• Do I need to restrict my activity?
• I have other health conditions. How can I best manage them together?
• Should I see a specialist?
• Do you have brochures or other printed material that I can have? What websites do you recommend?

Don't hesitate to ask other questions.

What to expect from your doctor

Your doctor is likely to ask you questions, such as:

• Have your signs and symptoms worsened over time?
• Do you drink alcohol? Do you smoke?
• Have you had your spleen removed?
• Do you have a history of bleeding or a lack of iron?
• Do you have a family history of high platelet counts?