Platelets are blood particles produced in the bone marrow that play an important role in the process of forming 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 thrombocytosis 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.

Your doctor might detect thrombocytosis in a routine blood test result that shows a high platelet level. If your blood test indicates thrombocytosis, it's important to determine whether it's reactive thrombocytosis or essential thrombocythemia to know how to manage the condition.

People with thrombocytosis often don't have signs or symptoms. Signs and symptoms of reactive thrombocytosis, if they do occur, relate to the underlying condition.

People with essential thrombocythemia might have signs and symptoms related to blood clots and bleeding, including:

• Headache
• Dizziness or lightheadedness
• Chest pain
• Weakness
• Numbness or tingling of the hands and feet

When to see a doctor

Because thrombocytosis isn't likely to cause symptoms, you probably won't know you have it unless a routine blood test reveals a higher than normal number of platelets. If that occurs, your doctor will try to determine the reason.

Bone marrow — spongy tissue inside your bones — 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:

• Acute bleeding and 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 or other type of trauma

Essential thrombocythemia

The cause of this disorder is unclear but it often appears to be connected to mutations in a variety of genes. Your bone marrow produces too many of the cells that form platelets and these platelets are often abnormal. This poses a much higher risk of clotting or bleeding complications than does reactive thrombocytosis.

During an exam for a routine physical or another condition, your doctor might find that your spleen is enlarged or you have signs or symptoms of an infection or another condition. In that case, your doctor might order a complete blood count. This test can determine whether your platelet count is higher than normal.

Because a number of conditions can cause a temporary rise in your platelet count, your doctor likely will repeat the blood test to see if your platelet count remains high over time.

Your doctor might also order tests to check for:

• Abnormal levels of iron in your blood
• Markers of inflammation
• Undiagnosed cancer
• Associated 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.

• If you've had significant blood loss from a recent surgery or an injury, your elevated platelet count might resolve on its own.
• 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.
• If you've had your spleen removed (splenectomy), you might have lifelong thrombocytosis, but you're unlikely to need treatment.

Essential thrombocythemia

People with this condition who have no signs or symptoms are unlikely to need treatment as long as the condition is stable. Your doctor might recommend that you 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 doctor.

You might need to take drugs 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 platelet count greater than 1 million

Your doctor might prescribe platelet-lowering drugs primarily in the form of hydroxyurea (Droxia, Hydrea) or interferon alfa (Intron A).

Platelets can be removed from your blood by a procedure that’s similar to dialysis. A needle connected to a tube is placed in a vein and your blood is fed through a machine that filters out platelets. Then the filtered blood is returned to your body through an intravenous (IV) line. This procedure is used only in emergencies, such as if essential thrombocythemia has caused a stroke.

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 doctor who specializes in blood diseases (hematologist).

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?