Hematocrit Drop After 500 mL Venesection
A 500 mL venesection typically decreases hematocrit by approximately 3-4 percentage points in adults, though individual variation is substantial.
Expected Hematocrit Change
- The standard clinical estimate is approximately 3% hematocrit decrease per 500 mL of blood removed 1
- Research in transfusion medicine demonstrates that 300 mL of packed red blood cells (approximately one standard unit) increases hematocrit by 1.9% ± 1.2%, which when extrapolated to 500 mL whole blood removal suggests a 3-4% decrease 1
- However, the variability is substantial (standard deviation of ±1.2% per 300 mL), meaning individual patients may experience anywhere from 2-6% hematocrit drop after a 500 mL venesection 1
Physiologic Basis
- The relationship between blood volume and hematocrit is influenced by multiple factors including baseline hematocrit, fluid resuscitation status, and individual patient characteristics 1
- In polycythemia patients undergoing therapeutic venesection, studies document mean hematocrit reductions from 59.6% to 43.9% (approximately 16 percentage points) with larger volume removals, demonstrating the dose-dependent relationship 2
- Another study in polycythemia patients showed hematocrit reduction from mean 53.7% to 44.4% (approximately 9 percentage points) after venesection 3
Clinical Context for Therapeutic Venesection
- In polycythemia vera, aggressive phlebotomy should target hematocrit <45% in men and <42% in women to reduce thrombotic risk 4
- Maintaining hematocrit below 44% significantly reduces vascular occlusive episodes in polycythemia patients 4
- Each venesection should be performed with careful fluid replacement to avoid hypotension, particularly in patients with cardiovascular disease 4
Important Clinical Caveats
- The 3% per 500 mL rule is an approximation only - actual changes depend on patient size, baseline hematocrit, concurrent fluid administration, and timing of measurement 1
- Hematocrit measurements should be obtained 24 hours post-venesection for accurate assessment, as immediate post-procedure values may not reflect equilibration 1
- In hemochromatosis patients, hemoglobin and hematocrit should be monitored at each venesection, with phlebotomy postponed if anemia develops 4