Hemoglobin and Hematocrit are Dependent on Red Blood Cell Mass and Plasma Volume
Erythropoietin Synthesis
Approximately 90% of Erythropoietin is Synthesized by the Kidneys in Response to Hypoxia
Mechanisms of Activation of Erythropoietin-Producing Cells
Decreased Serum Hemoglobin (Anemia): as in the case of anemia
Decreased Hemoglobin Saturation (Hypoxemia): as in the case of hypoxemia
Decreased Oxygen Release from Hemoglobin: as in the case of hemoglobinopathy
Decreased Oxygen Delivery to the Kidney: as in the case of renal artery stenosis, etc
Erythropoietin-Dependence of Polycythemic States
Erythropoietin-Independent Erythrocytosis (As Occurs in Polycythemia Vera) Inhibits Renal Erythropoietin Synthesis, Resulting in Decreased Serum Erythropoietin Levels
Erythropoietin-Dependent Erythropoiesis (As Occurs in Hypoxia or with an Erythropoietin-Secreting Tumor) Causes Secondary Erythrocytosis, Resulting in Normal-High Serum Erythropoietin Levels
Types of Polycythemia
Relative Polycythemia (Gaisbock’s Disease, Spurious Polycythemia, Stress Erythrocytosis, Apparent Polycythemia, and Pseudopolycythemia): defined as an isolated decrease in plasma volume
Elevated Hemoglobin
Elevated Hematocrit
Elevated Red Blood Cell Count
Absolute Polycythemia (Erythrocytosis): defined as an increase in red blood cell mass
Primary Polycythemia: due to an abnormality within red blood cell progenitors
Secondary Polycythemia: due to a circulating erythropoietic factor (usually erythropoietin)
Combined Polycythemia: due to both a decrease in plasma volume and an increase in red blood cell mass
This Combination is Most Commonly Observed in Smokers (“Smokers Polycythemia”)
Inapparent Polycythemia: defined as an equivalent increase in both the red blood cell mass and plasma volume
Hemoglobin, Hematocrit, and Red Blood Cell Count All Remain Normal: therefore, polycythemia can only be diagnosed by blood volume studies (using isotopic dilution or other methods)
Hemoglobin >16.5 g/dL: defined as polycythemia in males
Hemoglobin >16.0 g/dL: defined as polycythemia in females
Hematocrit
Hematocrit >49%: defined as polycythemia in males
Hematocrit >48%: defined as polycythemia in females
Red Blood Cell Count: not used as often as hemoglobin and hematocrit in the diagnosis of polycythemia (since diseases such as thalassemia minor may have an elevated red blood cell count with a decreased hemoglobin or hematocrit due to an increased number of small/microcytic, poorly hemoglobinized/hypochromic red blood cells)
