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.

While a person may have certain signs and symptoms of ET, laboratory tests are needed to confirm the diagnosis. Generally, a doctor will consider other factors first to determine if a condition called “reactive thrombocythemia” is causing an increase in platelets.

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 (known as "reactive thrombocythemia"), including:

• Inflammatory disorders such as active arthritis or gastrointestinal inflammatory disease
• Iron-deficiency anemia
• An undetected cancer
• Prior removal of the spleen (splenectomy)

A patient with reactive thrombocythemia will have a return to a normal platelet count once the primary problem is successfully treated.

Medical History and Physical Examination

Evaluation of an individual with suspected ET should start with a detailed medical history and a physical examination by a hematologist-oncologist

The medical history should include information about the patient’s:

• Cardiovascular risk factors
• Past illnesses
• Injuries
• Treatments
• Medications
• A history of the formation or presence of a blood clot inside a blood vessel (thrombosis) or loss of blood from damaged blood vessels (hemorrhagic events)
• History of blood relatives—some illnesses run in families
• Current symptoms 

After the medical history, the doctor will conduct a physical examination. During the physical examination, the doctor may:

• Listen to the patient's heart and lungs
• Examine the patient's body for signs of disease
• Check different organs of the body

Blood Tests

Complete Blood Count

This test measures the number red blood cells, white blood cells and platelets in a sample of blood. It also measures the amount of substance in the blood that carries oxygen (hemoglobin) and the percent of whole blood that is made up of red blood cells (hematocrit). In patients with ET, the platelet count is higher than normal.

Peripheral Blood Smear

A pathologist examines a small amount of blood under a microscope to see if there are any unusual changes in the size, shape, or appearance of the blood cells. The test also looks for the presence of immature (blast cells). In patients with ET, the platelets may appear enlarged and/or clumped together.

Comprehensive Metabolic Panel

This test measures the levels of certain substances released into the blood by organs and tissues in the body, including electrolytes (such as sodium, potassium and chloride), fats, proteins, glucose (sugar) and enzymes. This provides important information about how well a person’s kidneys, liver and other organs are working. These tests are not used to diagnose ET but an abnormal amount of a certain substance can be a sign of disease or other health problems.

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. 

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

If you have ET, your marrow will show a significant increase in platelet-forming cells (megakaryocytes). These cells also appear abnormal in shape and size.

Molecular Testing

Molecular genetic tests are very sensitive tests that look for mutations in genes. In suspected cases of ET, doctors test for mutations of the JAK2, MPL and CALR genes.

Criteria for Diagnosis

In 2016, the World Health Organization published new criteria for diagnosing ET. The diagnosis of ET requires the presence of

• Major Criteria 1, 2 ,3 and 4 (listed below), or
• Major Criteria 1, 2, and 3 and the one minor criterion (listed below).

Four Major Criteria

1. Platelet count equal or greater than 450 x 109/L and
2. Bone marrow biopsy showing increased numbers of platelet-forming cells (megakaryocytes) with abnormal nuclei, and
3. Exclusion of other diseases defined by World Health Organization criteria, such as:
   • BCR-ABL1+ chronic myeloid leukemia 
   • Polycythemia vera 
   • Primary myelofibrosis 
   • Myelodysplastic syndromes 

   • Other myeloid neoplasms

4. Presence of JAK2, CALR, or MPL mutation, and/or

Minor Criterion

• Presence of a clonal marker (chromosome abnormality) or no evidence that the disorder is caused by reactive thrombocytosis 

Related Links

• Download or order The Leukemia & Lymphoma Society’s free booklet, Myeloproliferative Neoplasms.
• Bone Marrow Tests
• Understanding Blood Counts
• Blood Tests