Reduced plasma volume (hemoconcentration)
Abnormal proliferation of myeloid stem cells, normal or low erythropoietin levels (polycythemia vera);
inherited activating mutations in the erythropoietin receptor (rare)
Secondary: Increased erythropoietin levels
Appropriate: lung disease, high-altitude living, cyanotic heart disease
Inappropriate: erythropoietin-secreting tumors (e.g., renal cell carcinoma, hepatoma, cerebellar hemangioblastoma);
surreptitious erythropoietin use (e.g., in endurance athletes).