What does an elevated Red Blood Cell (RBC) count and hematocrit indicate?

Elevated RBC and Hematocrit: Clinical Significance and Implications

Elevated red blood cell (RBC) count and hematocrit primarily indicate polycythemia, which significantly increases the risk of thrombosis and represents a serious clinical concern requiring prompt evaluation and management. 1

Understanding Elevated RBC and Hematocrit

Elevated RBC count and hematocrit represent an increase in the proportion of blood volume occupied by red blood cells. This condition can be categorized as:

1. True Polycythemia: Actual increase in red cell mass

   • Primary: Polycythemia vera (PV) - a clonal myeloproliferative disorder
   • Secondary: Non-clonal increase often mediated by erythropoietin (EPO)

2. Apparent Polycythemia: Normal red cell mass with reduced plasma volume (relative polycythemia)

   • Caused by dehydration, diuretics, severe burns, etc.

Clinical Significance and Risks

The most significant concern with elevated RBC and hematocrit is the increased risk of thrombosis:

• Thrombotic Risk: During the pre-phlebotomy era, thrombosis was the major cause of death in patients with PV, with median life expectancy less than 2 years 1
• Blood Viscosity: Hematocrit is the major determinant of whole blood viscosity, especially at low shear rates 1
• Blood Flow: Increased hematocrit is associated with decreased cerebral blood flow and altered flow dynamics 1
• Platelet Function: Elevated hematocrit enhances platelet accumulation following vascular injury 2
• Thrombin Generation: RBCs contribute to thrombin generation and platelet activation 3

Diagnostic Approach

When elevated RBC and hematocrit are detected:

1. Rule out relative polycythemia by assessing hydration status and other causes of plasma volume contraction

2. Measure serum EPO levels:

   • Low EPO: Highly suggestive of polycythemia vera (specificity >90%)
   • Normal EPO: May still be PV (sensitivity of low EPO for PV is <70%)
   • High EPO: Suggests secondary polycythemia 1

3. Consider bone marrow examination with cytogenetic studies if primary polycythemia is suspected

4. Evaluate for secondary causes if EPO is elevated:

   • Chronic hypoxemia (lung disease, sleep apnea)
   • High altitude
   • Smoking (carbon monoxide exposure)
   • EPO-producing tumors
   • Certain congenital disorders

Management Considerations

The management approach depends on the underlying cause:

• Polycythemia Vera: Phlebotomy to maintain hematocrit <45% significantly reduces thrombotic risk 1

• Secondary Polycythemia: Address underlying cause (e.g., smoking cessation, CPAP for sleep apnea)

• Relative Polycythemia: Correct dehydration or other causes of plasma volume contraction

Clinical Pitfalls to Avoid

1. Don't rely solely on hemoglobin/hematocrit values: The diagnostic possibility of polycythemia should be considered if:

   • Hemoglobin/hematocrit is higher than the 95th percentile adjusted for sex and race
   • There is a documented increase from the patient's baseline
   • PV-related features accompany borderline-high hematocrit 1

2. Don't overlook the thrombotic risk: Elevated hematocrit significantly increases thrombosis risk through multiple mechanisms 1, 2

3. Don't assume all polycythemia is the same: Different types of erythrocytosis have different thrombotic risks 4

4. Don't miss secondary causes: Always evaluate for underlying conditions that may cause secondary polycythemia

In conclusion, elevated RBC and hematocrit represent a significant clinical concern primarily due to increased thrombotic risk. Prompt evaluation to determine the underlying cause and appropriate management are essential to reduce morbidity and mortality.