Thrombocytosis is the presence of high platelet counts in the blood. Although often symptomless (particularly when it is a secondary reaction), it can predispose to thrombosis in some patients.

Generally speaking, a normal platelet count ranges from 150,000 and 450,000 per mm³. These limits, however, are determined by the 2.5th lower and upper percentile, and a deviation does not necessary imply any form of disease. Nevertheless, counts over 750,000 (and especially over a million) are considered serious enough to warrant investigation and intervention.

Signs and symptoms

Often, high platelet levels do not lead to clinical problems; rather, they are picked up on a routine full blood count. It is vital that a full medical history is elicited, to ensure the high platelet count is not due to a secondary process. Often, it occurs in tandem with inflammatory disease, as the principal stimulants of platelet production (e.g. thrombopoietin) are elevated in these clinical states as part of the acute phase reaction.

High platelet counts can occur in patients with polycythemia vera (high red blood cell counts), and is an additional risk factor for complications.

A very small segment of patients reports the rare skin disease of erythromelalgia, a burning sensation and redness of the extremities that resolves with cooling and/or aspirin use.

Diagnosis

Laboratory tests might include: full blood count, liver enzymes, renal function and erythrocyte sedimentation rate.

If the cause for the high platelet count remains unclear, bone marrow biopsy is often undertaken, to differentiate whether the high platelet count is reactive or essential.

Causes

Increase platelet counts can be due to a number of disease processes:

• Essential
  • leukemia or myelodysplastic syndrome
  • essential thrombocytosis (a form of myeloproliferative disease)
• Reactive
  • inflammation
  • surgery
  • hyposplenism (decreased breakdown due to decreased function of the spleen)

Treatment

Often, no treatment is required or necessary. However, if platelet counts are over 750,000 or 1,000,000, and especially if there are other risk factors for thrombosis. Aspirin at low doses is thought to be protective, and extreme levels are treated with hydroxyurea (a cytoreducing agent). The new agent anagrelide (Agrylin®) has recently been introduced for the treatment of essential thrombocytosis.