DEAR MAYO CLINIC: I am 33 years old and recently had an annual physical and routine blood work. I'm active with two children and feel fantastic, but my platelet count came back as 651. My health care professional said it could indicate a blood cancer and referred me to a hematologist. What can cause my platelets to be so high? I'm nervous, as my only prior medical problem was iron deficiency anemia.

ANSWER: Being told you might have cancer can be scary, but, thankfully, most people with an elevated platelet count do not have cancer. Understanding what platelets are can help you understand the situation.

Your bone marrow makes white blood cells, red blood cells and platelets. White blood cells help fight infection, red blood cells help carry oxygen to parts of the body, and platelets help keep you from bleeding too much. When a blood vessel is injured, like after a cut, platelets are activated and stick together to plug the hole and stop the bleeding.

A normal platelet count typically ranges from 150–450 cells per nanoliter of blood. Like you, most patients with a high platelet count do not have any symptoms, and the high count is found unexpectedly on routine blood work.

The referral to a hematologist can help you discern the cause of your high platelet numbers.

There are two possible causes:

1. Essential thrombocytosis
   Essential thrombocytosis is due to an underlying problem where the bone marrow starts making too many platelets on its own. Some physicians also call this primary thrombocytosis.
2. Secondary thrombocytosis
   Secondary thrombocytosis, or reactive thrombocytosis, is the more common cause for patients to have high platelet numbers. This typically occurs when something outside the bone marrow stimulates the marrow to make more platelets. The bone marrow is just increasing platelet production because something else is telling it to do so.

Primary thrombocytosis also is technically considered cancer. Patients diagnosed with this are often found to have a mutation in the bone marrow that causes it to make more platelets. The most common mutations involve the JAK2, CALR and MPL genes. Sometimes, no mutation is found.

This condition can be diagnosed at any age, but most patients are diagnosed when they're older than 50. Essential thrombocytosis can include vague symptoms like headaches, lightheadedness, chest pain and visual changes.

Unlike other cancers, like lung or colon cancer, essential thrombocytosis does not have a staging system. No tumor or "pieces" of cancer can break off and spread to other areas in the body. Treatment is not what patients fear the most. It does not include intense chemotherapy with potentially severe side effects. Many patients will not require any treatment.

The biggest challenge with this condition is that it also can increase the risk of blood clots. Patients typically are treated with aspirin and sometimes other medications to decrease the platelet count. This treatment can improve symptoms and decrease the risk of blood clots. If the platelet count is extremely high, where the platelet count is greater than 1,000 per nanoliter of blood, a higher risk of bleeding is possible.

A small percentage of patients with primary thrombocytosis may develop scar tissue in their blood marrow called myelofibrosis. They also may progress to a preleukemia condition called myelodysplastic syndrome. A portion of these patients may then develop acute leukemia, a type of aggressive blood cancer needing immediate treatment. Thankfully, though, most patients will have a normal life span, and the thrombocytosis will become a chronic medical problem that is managed over time.

Based on your past anemia diagnosis, you most likely would have reactive or secondary thrombocytosis, which is where other conditions cause a high platelet count.

Inflammatory conditions like autoimmune diseases, cancer or trauma, as well as certain infections and iron deficiency, are common causes of a high platelet count. Treatment is directed at the stimulus, and the platelet count returns to normal once resolved.

Another reason for secondary thrombocytosis is when the spleen has been removed. When blood travels throughout the body, the spleen normally holds platelets and other cells inside of it for processing. When the spleen is removed, the platelets are no longer contained in the spleen, so more of them travel in the bloodstream.

Patients diagnosed with secondary thrombocytosis are not at increased the risk of myelofibrosis, myelodysplastic syndrome or acute leukemia.

While secondary thrombocytosis by itself is not thought to increase the risk of blood clots, the underlying inflammatory or infectious cause may increase this risk. This condition is benign. If the underlying cause improves or resolves, the platelet count should decrease back to normal.

It is not possible to diagnose the cause of the high platelets based on the platelet count alone, so I recommend following up with a specialist if your platelet count is high to discern the cause and the right treatment plan for you. — Jawad Khan and Dr. Marwan Shaikh, Hematology/Oncology, Mayo Clinic, Jacksonville, Florida

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