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Polycythemia.

Relative:

Reduced plasma volume (hemoconcentration)

Primary:

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).