Blood tests and sometimes a bone marrow test are used to diagnose essential thrombocythemia (ET). Your doctor may suspect ET as a result of a routine blood test showing a higher-than-normal platelet count. Or, a doctor may order blood tests for a patient who has a blood clot, unexpected bleeding or a mildly enlarged spleen, and note a markedly elevated platelet count.
After your doctor takes your blood, he or she sends it to a lab for a complete blood count (CBC), which measures the number of red cells, white cells and platelets in your blood. A ET diagnosis is considered if:
- Your platelet count is above 600,000 per microliter (µL) of blood and remains high over a period of observation. (In comparison, normal platelet values range from about 175,000 to 350,000 platelets/µL.) Occasionally, ET is diagnosed in patients with platelet counts that are high normal (between 350,000 and 600,000 platelets/µL of blood).
- There's no other evident cause for your elevated platelet count.
Your doctor can't confirm an ET diagnosis by laboratory tests alone. Generally, he or she considers other conditions first to determine if any of them are the cause of platelet elevation.
You'll need to undergo further examination and testing so your doctor can rule out other disorders. Several conditions can cause an increase in platelets, including:
- Inflammatory disorders such as active arthritis or gastrointestinal inflammatory disease
- Iron-deficiency anemia
- An undetected (occult) cancer
- A past splenectomy (removal of the spleen)
In most cases, your doctor bases your ET diagnosis on:
- A high platelet count that persists over time
- A JAK2 gene mutation in your blood cells
- The absence of evidence for other clonal blood diseases associated with high platelet levels
- A bone marrow examination
Bone Marrow Tests
Although a bone marrow examination isn't strictly necessary to make a diagnosis, doctors often use it to help confirm a ET diagnosis. If you have ET, your marrow has a significant increase in platelet-forming cells (megakaryocytes) and masses of platelets.
Bone marrow testing involves two steps usually performed at the same time in a doctor's office or a hospital:
- A bone marrow aspiration to remove a liquid marrow sample
- A bone marrow biopsy to remove a small amount of bone filled with marrow