Chronic Idiopathic Thrombocytopenic Purpura

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Abstract
I believe that an important aspect of laboratory evaluation and differential diagnosis is missing from the review article by George et al. (Nov. 3 issue)1 on chronic idiopathic thrombocytopenic purpura (ITP). Abnormalities of platelet size and morphology are common in ITP. The widespread availability of particle counters in clinical laboratories now permits accurate measurement of the mean platelet volume and the extent of heterogeneity in platelet size on the basis of the platelet-distribution width. Estimates of mean platelet volume and platelet-distribution width provide useful information for evaluating patients with ITP.2 There is accelerated production of platelets in response to the peripheral destruction of platelets, and the presence of large and heterogeneous platelets results in high values for mean platelet volume and platelet-distribution width. In contrast, a low mean platelet volume is observed in myeloproliferative disorders, hypersplenism, and thrombocytopenia associated with sepsis.3 Therefore, the mean platelet volume can help distinguish thrombocytopenia induced by peripheral destruction from hypoproliferative thrombocytopenias. In the former, circulating platelets are usually large and newly released, whereas in the latter platelets are small or very small and are not homogeneously distributed. This may also explain why, with the same number of platelets, there can be a greater tendency to bleed in disorders of decreased production than in thrombocytopenia induced by peripheral destruction.