High platelet count (thrombocytosis): causes and what happens next

Platelets are small cell fragments in the blood that help it to clot. A high platelet count is called thrombocytosis. Most people have no symptoms and the result is found during a full blood count taken for another reason, which is why it often causes alarm when it appears in a results portal without explanation.

The South Tees pathway used by North Yorkshire GPs says that a high platelet count is most often reactive, in the same way that CRP or ESR may be raised in response to illness. That means the platelet count is acting as a marker that something else is going on, rather than being a disease in itself. The NNUH haematology document uses 450 x10^9/L as the threshold for a high count in its investigation advice.

A single raised result does not need to be treated as an emergency. It does need a reason, which is found by thinking about recent illness, bleeding or surgery, and by repeating the blood count.

In reactive thrombocytosis the bone marrow is normal and is simply making more platelets in response to a stimulus. The NNUH document lists the following causes, and the South Tees pathway gives a very similar list.

• Infection, acute or chronic.

• Inflammatory conditions such as inflammatory bowel disease, arthritis and connective tissue disorders.

• Acute blood loss, or chronic blood loss and iron deficiency (the pathway notes chronic blood loss may show iron deficiency anaemia as well).

• Trauma or surgery.

• Malignancy.

• Rebound after chemotherapy.

• Splenectomy or a spleen that is not working (hyposplenism), because platelets are cleared less from the blood.

Less often, the marrow itself makes too many platelets. The NNUH document lists clonal bone marrow disorders called myeloproliferative disorders: essential thrombocythaemia (isolated raised platelets), polycythaemia vera (raised platelets together with raised red cells), myelofibrosis (raised platelets and anaemia due to scarring of the marrow) and chronic myeloid leukaemia. It says myelodysplastic syndrome is rarely a cause, since platelets are usually low in that condition.

The South Tees pathway notes that most people with these disorders will have a JAK2, CALR or MPL gene mutation, which is why a JAK2 test is part of the work-up when no reactive cause is found. These are conditions that a haematologist manages, often with monitoring, aspirin or, in some people, medicines that lower the platelet count, according to the NNUH document.

For persistent thrombocytosis the NNUH document lists a history (for example splenectomy, blood loss, inflammatory disorder, chronic infection, thrombosis), an examination for an enlarged spleen or signs of inflammation, a repeat full blood count, ferritin or iron studies to look for iron deficiency, CRP or ESR to look for inflammation, an autoantibody screen if an inflammatory disease is suspected and a JAK2 mutation test if there is no secondary cause. CALR and MPL are checked if JAK2 is negative.

The South Tees pathway adds a blood film, kidney and liver function tests, calcium and a chest X-ray as baseline investigations, with other tests depending on what the history and examination suggest. The NNUH document also notes that kidney blood tests often show a falsely high potassium in primary thrombocytosis, which is a laboratory artefact and does not affect the patient.

Many people read that a raised platelet count can be linked to cancer and become frightened. It helps to see what the evidence says. The South Tees pathway states that thrombocytosis can be a marker for cancer, including lung, endometrial, gastric, oesophageal or colorectal.

The University of Exeter research, reported by the NIHR Applied Research Collaboration South West, found that around 2% of people over 40 have a raised platelet count. Among those with thrombocytosis, more than 11% of men and 6% of women went on to be diagnosed with cancer within a year, rising to 18% of men and 10% of women if a second raised count was recorded within six months. Put the other way round, most people with a raised count did not have cancer, and the study was among people who had been sent for a blood test by their GP.

The practical result is that a persistent, unexplained raised count is taken seriously and usually triggers a review of symptoms and sensible further tests, not that every raised count means cancer.

The NNUH document advises GPs to consider haematology advice if the patient is positive for JAK2, CALR or MPL, if thrombocytosis of 600 x10^9/L or more persists over 6 to 8 weeks with no obvious reactive cause, if it of 450 x10^9/L or more is persistent with other full blood count abnormalities (excluding iron deficiency anaemia), if it is associated with thrombotic events, or if it is associated with an enlarged spleen.

The South Tees pathway gives a similar criterion: persistently raised platelets above 450 x10^9/L with no obvious underlying cause and normal inflammatory markers, or an enlarged spleen or a blood film suggesting a primary bone marrow disorder. If the raised count is part of an unexplained inflammatory picture it suggests referral to general medicine instead.

If a reactive cause is found and treated, the platelet count normally settles on its own. The NNUH document says that if no reactive cause has been identified, it may be prudent to recheck the blood count every 3 to 6 months to make sure a marrow disorder is not developing.

The NNUH document advises people to return for a blood count if they develop symptoms of a clot, such as a transient ischaemic attack, or bleeding symptoms, or discomfort in the upper left of the tummy, which can suggest an enlarged spleen. Sudden weakness of the face or arm, difficulty speaking or a sudden severe headache should be treated as a 999 emergency in the usual way.

Lifestyle matters too. The same document advises reviewing lifestyle risk factors for arterial and venous clots, such as smoking, blood pressure and cholesterol, as part of care.