Isolated spleen agenesis: a rare cause of thrombocytosis mimicking essential thrombocythemia.

Thrombocytosis is a common feature of myeloproliferative disorders but may also result from various conditions including chronic iron deficiency, hemorrhage, chronic inflammation and splenectomy. We report two cases of secondary thrombocytosis caused by isolated and congenital asplenia, mimicking essential thrombocythemia. These two adult cases of spleen agenesis were unexpected. We conclude that in thrombocytosis without clinical evidence of splenomegaly, attentive screening of blood in search of Howell-Jolly bodies and abdominal ultrasonography should always be performed not only to detect mild spleen enlargement but also to make sure of the presence of this organ.