The upper limit of the platelet count in normal adults differs among various sources and laboratories, but is generally accepted to be in the range of 350,000 to 450,000/microL (350 to 450 x 109 /L). As an example, in one study in 10,000 subjects, the 99th percentile for the platelet count was 409,000 and 381,000/microL, respectively, in women and men .
In this review, thrombocytosis will be arbitrarily defined as a platelet count greater than 450,000/microL, a value that has also been chosen by World Health Organization investigators as one of the criteria required for the diagnosis of essential thrombocythemia (table 1).
In any patient with newly recognized thrombocytosis, two questions should be addressed in a timely fashion:
- Is the thrombocytosis a reactive phenomenon or a marker for the presence of a clonal (neoplastic, autonomous) hematologic disorder?
- What is the immediate risk to the patient from the increased platelet count and how should it be managed?
This topic review will discuss the approach to the patient with thrombocytosis, with a view to answering these two clinically important questions. The diagnosis, clinical manifestations, treatment, and prognosis of essential thrombocythemia (essential thrombocytosis) are discussed separately. (See "Diagnosis and clinical manifestations of essential thrombocythemia" and "Prognosis and treatment of essential thrombocythemia".)