Polycythemia.

Abstract

Plethora ('a morbid condition due to excess of red corpuscles') has been recognized since antiquity as a manifestation of disease. The use of leeches and blood letting, which was for centuries the mainstay of therapy in many clinical situations, reflected the importance then attached to having too much blood. Greater knowledge has achieved greater precision, so that measurement of packed cell volume (PCV) or hematocrit now defines polycythemia (> 0.51 in men and > 0.48 in women, when blood is sampled without venous occlusion). Clinical and laboratory investigations, mainly in the last 30 years, have enabled the subdivision of polycythemia into different subtypes of radically different etiology which will be described in this Chapter. The last decade, however, has seen the advancing edge of research in this field passing from clinical and routine laboratory testing to studies of cellular proliferation at the molecular level. Detailed understanding is, however, as always elusive, with each new finding revealing deeper layers which in turn need elucidation. The field of polycythemia is nevertheless moving slowly into the molecular era.