Polycythemia is defined as an increase in the volume of circulating RBCs per kilogram of body weight or, equivalently, an increase in the RBC mass. Clinically, this is expressed as an absolute increase in the number of RBCs, usually but not always accompanied by corresponding increases in the hemoglobin and hematocrit (though the hematocrit may be affected by plasma volume). Polycythemia may occur as a primary disease of unknown cause (polycythemia vera) or as a secondary manifestation of other illnesses. The diagnosis of PV is made only after other causes of secondary poly-cythemia have been excluded (Table 47.1).
The terms erythremia and erythrocytosis are often used to refer to primary and secondary polycythemia, respectively. Others use erythremia as a classification for patients whose only abnormality is an increased RBC volume. Both terms are superfluous and confusing, and therefore are not recommended. The term "relative" polycythemia is also a misnomer and its use should be discontinued. Similarly, "false" poly-cythemia is a confusing term; it is not polycythemia because the RBC volume per kilogram of body weight is normal. In this instance, the increased hematocrit is related to a decrease in plasma volume (whereas in true polycythemia, the plasma volume is usually increased, but it may also be decreased). Other terms referring to false or relative polycythemia include
Table 47.1 Clinical classification of the polycythemias
(1) Related to inadequate oxygen delivery to tissues with respect to need
(1.1) Due to decreased arterial oxygen tension
(1.1.1) With physiologic or anatomic cardiopulmonary abnormalities:
■ abnormalities of lungs, chest bellows, or ventilatory
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