A patient with mild congestive hea failure is treated with high-dose furosemide and diuresis 3 liters of fluid.A complete blood count (CBC) taken before the diuresis shows an RBC count of 4 million/mm3; a CBC taken after diuresis shows a RBC count of 7 million/mm3. Which of the following is the most likely explanation?

This is an example of relative polycythemia, in which there is an increased hematocrit or RBC count without a true increase in the total number of body RBCs. What usually happens in these cases is a significant reduction in plasma volume due to processes such as dehydration, vomiting, diarrhea, or diuresis. Cyanotic hea disease, appropriate erythropoietin secretion, can cause secondary absolute polycythemia. Increased erythropoietin, whether appropriately or inappropriately secreted, can cause secondary absolute polycythemia. Polycythemia vera causes primary absolute polycythemia with usually low erythropoietin levels. Ref: Prchal J.T. (2010). Chapter 56. Primary and Secondary Polycythemias (Erythrocytosis). In J.T. Prchal, K. Kaushansky, M.A. Lichtman, T.J. Kipps, U. Seligsohn (Eds), Williams Hematology, 8e.