Role of platelet counts in the management of essential thrombocythemia: experience with anagrelide.

An elevated platelet count is the hallmark of essential thrombocythemia (ET). Therapeutic control of platelet counts in ET has proven effective for minimizing the risk of bleeding complications. By contrast, the relationship between platelet count and thrombotic complications is less well defined. The beneficial effects of lowering the platelet count are greatest when levels are reduced close to or within the normal range, thus supporting the trend within the medical community to reduce platelet treatment targets to below 400-450 × 10(9)/l. Platelet counts have further utility in ET as a marker to indicate intolerance/resistance to hydroxyurea according to recently published guidelines. In the current era, where novel risk factors for ET complications are being discussed, this article takes a back-to-basics approach, considering what platelet counts reveal in practice as a risk factor for complications and a marker of treatment efficacy, with particular focus on data for the platelet-selective agent anagrelide.