Which of the following factors can differentiate primary polycythemia from secondary polycythemia

Answer is B (Aerial oxygen tension < 90% on room air) Aerial oxygen tension < 90% on room air indicate a secondary cause of polycythemia from either hea, i or lung disease Primary polycythemia (Polycythemia vera) is characteristically associated with a normal aerial oxygen saturation >92% on room air. Both primary and secondary polycythemia will have an elevated red cell mass. Pruritis and Hypeension are consequences of increased red cell mass and the resultant hyperviscocity, and may hence be seen in both forms of polycythemia. A summary of significant differences between Polycythemia Vera and Other Types of Polycythemia Findings Polycythemia vera Secondary polycythemia Red cell mass Increased Increased Erythropoietin Decreased or absent Normal or increased Aerial oxygen saturation Normal Decreased or normal Erythrocyte sedimentation rate Decreased Normal Leukocyte alkaline phosphate stain Increased Normal Vitamin B12 binding capacity Increased Normal Total leukocytes Increased (Leukocytosis) Normal Total Platelets Increased (Thrombocytosis) Normal Basophil count Increased Normal Blood histamine Increased Normal Serum iron Decreased Normal or increased Splenomegaly Present Absent Marrow Panhyperplasia Erythroid hyperplasia