Thrombocytosis

Thrombocytosis is the presence of high platelet counts in the blood, and can be either reactive or primary (also termed essential and caused by a myeloproliferative disease). Although often symptomless (particularly when it is a secondary reaction), it can predispose to thrombosis in some patients.

Generally, 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.

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Signs and symptoms

High platelet levels do not necessarily signal any clinical problems, and are picked up on a routine full blood count. However, it is important that a full medical history be elicited to ensure that the increased platelet count is not due to a secondary process. Often, it occurs in tandem with an 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 report symptoms 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 (primary)
  • Essential thrombocytosis (a form of myeloproliferative disease)
  • Other myeloproliferative disorders such as chronic myelogenous leukemia, polycythemia vera, myelofibrosis
• Reactive (secondary)
  • Inflammation
  • Surgery (which leads to an inflammatory state)
  • Hyposplenism (decreased breakdown due to decreased function of the spleen)
  • Hemorrhage and/or iron deficiency

Treatment

Often, no treatment is required or necessary for reactive thrombocytosis.

However, in primary thrombocytosis, 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. However, recent studies show that anegrilide is not significantly more effective than traditionally used hydroxyurea (Harrison et al 2005).

References

• Harrison CN, Campbell PJ, Buck G, Wheatley K, East CL, Bareford D, Wilkins BS, van der Walt JD, Reilly JT, Grigg AP, Revell P, Woodcock BE, Green AR; United Kingdom Medical Research Council Primary Thrombocythemia 1 Study. Hydroxyurea compared with anagrelide in high-risk essential thrombocythemia. N Engl J Med 2005;353:33-45. PMID 16000354.