Primary Thrombocythemia: When a High Platelet Count Comes From the Marrow

What a high platelet count means

Platelets are small blood cells that help blood clot. The normal range is about 150,000 to 450,000 per microliter. A count above 450,000 is called thrombocytosis. It has two broad causes:

Common causes of reactive thrombocytosis

• iron deficiency anemia, often from heavy periods or gut bleeding
• infections, especially pneumonia and other bacterial infections
• inflammation: rheumatoid arthritis, inflammatory bowel disease, vasculitis
• recent surgery, trauma or burns
• cancer, particularly lung, gut, ovarian and breast cancer, and lymphoma
• after spleen removal or with a non-working spleen
• acute bleeding or recovery from it
• exercise and stress (short-lived)

How a high platelet count is investigated

1. Repeat the blood count and look at a blood smear.
2. Review the history: recent infection, surgery, bleeding, weight loss, symptoms of inflammation or cancer.
3. Look for reactive causes: ferritin and iron studies, CRP or ESR, tests guided by symptoms.
4. If the count stays high without a cause: test for JAK2, CALR and MPL mutations and, if needed, BCR-ABL to rule out chronic myeloid leukemia.
5. Bone marrow biopsy confirms the diagnosis and separates ET from other myeloproliferative neoplasms such as polycythemia vera and early myelofibrosis.

Diagnostic criteria

The World Health Organization criteria for ET require:

• a sustained platelet count of 450,000 or higher
• a bone marrow showing increased large, mature megakaryocytes
• not meeting criteria for other myeloid cancers
• a JAK2, CALR or MPL mutation, or another marker of a clonal disorder, or no evidence of reactive thrombocytosis

Symptoms of primary thrombocythemia

Many people have none. Others have headaches, dizziness, visual disturbances, tingling, burning red hands or feet (erythromelalgia), fatigue, and clots such as stroke or deep vein thrombosis. Very high counts can paradoxically cause bleeding.

Treatment overview

Treatment of primary thrombocythemia is guided by clot risk:

• low-dose aspirin for many people, especially with a JAK2 mutation or circulation symptoms
• platelet-lowering medicines (hydroxyurea, interferon or anagrelide) for those over 60 or with a previous clot
• controlling heart risk factors: smoking, blood pressure, cholesterol and diabetes

Reactive thrombocytosis needs no platelet-lowering treatment; the cause is treated instead. For full details of ET symptoms, risk categories and outlook, see essential thrombocythemia.

Outlook

Primary thrombocythemia is usually slow-growing, and most people have a near-normal life expectancy. A small number progress to myelofibrosis or, rarely, acute leukemia over many years, so regular monitoring is needed.

Frequently asked questions

Is primary thrombocythemia the same as essential thrombocythemia?

Yes, in most usage. Both terms describe a high platelet count caused by a bone marrow disorder rather than another illness. "Essential thrombocythemia" is the official World Health Organization name.

What is the difference between primary and secondary thrombocytosis?

Primary thrombocytosis comes from a gene change in marrow stem cells and persists. Secondary, or reactive, thrombocytosis is the marrow's response to iron deficiency, infection, inflammation, surgery or cancer, and it resolves when the cause is treated.

What platelet count is considered high?

A count above 450,000 per microliter is considered high. Most mildly high counts are reactive. Counts that stay high for months without a cause, or are very high, need further investigation by a hematologist.

How is primary thrombocythemia diagnosed?

Doctors repeat the blood count, rule out reactive causes with iron and inflammation tests, test for JAK2, CALR and MPL mutations, and often perform a bone marrow biopsy to confirm the diagnosis.

Is a high platelet count dangerous?

Reactive high counts rarely cause clots. Primary thrombocythemia increases the risk of clots and, at very high counts, bleeding. The risk depends on age, past clots, gene mutations and heart risk factors.