Platelets are a type of blood cell that play an important role in blood clotting. Platelets are made in the bone marrow. The bone marrow produces all types of blood cells.

A normal platelet count is between 150,000 and 450,000 platelets per microliter of blood. The bone marrow closely regulates the number of platelets that are produced and released into the blood stream. Both too low and too high levels of platelets can indicate a problem. Doctors use the term thrombocytosis to describe an increased platelet count.

Thrombocytosis has two general causes:

• Disorders of the bone marrow itself. These usually develop when the bone marrow stops regulating its production of platelets and other blood cells. It’s often as a result of a mutation or other type of injury to the marrow.
• Disorders elsewhere in the body that trigger the bone marrow to produce too many platelets. Doctors use the terms secondary or reactive thrombocytosis to describe these conditions. Usually, the bone marrow itself is normal.

Conditions that can stimulate an otherwise normal bone marrow to produce too many platelets include:

• Iron deficiency anemia
• Major surgery
• Infections
• Prior removal of the spleen

If you have an elevated platelet count, your doctor will usually begin by looking for a specific cause. He or she may order more blood tests. You may also be referred to a hematologist. This is a specialist in disorders of the blood.

If no cause is found, you may need a bone marrow biopsy. Samples from the bone marrow are examined under the microscope. In addition, special testing can be performed that provides information to guide treatment.

If all of these tests are normal and no specific cause can be found for your high platelet count, you may be diagnosed with essential thrombocytosis. (As a medical term, essential means no other cause has been identified.) This is an uncommon condition that develops for unclear reasons.