What is Hematocrit
Hematocrit is the percentage of blood volume occupied by red blood cells, representing the ratio of red blood cell volume to total blood volume. 1
Definition and Measurement
Hematocrit measures the proportion of blood that consists of red blood cells compared to the total blood volume (red blood cells plus plasma). 1 This value is typically expressed as a percentage or decimal fraction. 2
Normal Reference Ranges
The physiological hematocrit values vary by sex and hormonal status:
- Adult males and post-menopausal females: 47 ± 6% (mean ± standard deviation) 1
- Menstruating females: 41 ± 5% 1
- Men (clinical threshold): Normal range approximately 39-50% 3
- Women (clinical threshold): Normal range approximately 35-40% 3
These gender differences emerge at puberty due to the effects of testosterone (which increases red blood cell production) and estrogen (which has the opposite effect). 1
Clinical Significance
Hematocrit serves as a critical indicator of oxygen-carrying capacity, as red blood cells contain hemoglobin that binds and transports oxygen to tissues. 3, 4 However, hematocrit also directly affects blood viscosity—higher hematocrit values increase blood thickness and flow resistance. 3, 4
Elevated hematocrit (>55% in men, >49.5% in women) indicates erythrocytosis and requires evaluation to distinguish between primary causes (such as polycythemia vera) and secondary causes (such as chronic hypoxia, smoking, or dehydration). 1, 5
Decreased hematocrit indicates anemia and may reflect blood loss, decreased red blood cell production, or increased red blood cell destruction. 2
Measurement Considerations
Hemoglobin is more reliable than hematocrit for clinical monitoring because hematocrit can falsely increase by 2-4% with prolonged sample storage due to changes in mean corpuscular volume, while hemoglobin remains stable. 1 Additionally, hyperglycemia can falsely elevate calculated hematocrit but does not affect hemoglobin measurement. 1
In critically ill surgical patients, peripheral blood hematocrit may not accurately reflect true red blood cell volume due to alterations in plasma volume, with differences of up to ±15 hematocrit percentage points between measured and normalized values. 6
Optimal Hematocrit Concept
The concept of "optimal hematocrit" balances oxygen-carrying capacity against blood viscosity. 3, 4 In healthy individuals, theoretical models suggest an optimal hematocrit of 50-60% for maximizing oxygen delivery. 3 However, in clinical practice with hospitalized patients, restrictive transfusion strategies targeting hemoglobin of 7-8 g/dL (hematocrit 20-24%) produce better outcomes in stable patients. 3 This discrepancy reflects factors such as decreased perfusion pressures, endothelial dysfunction, and altered vascular tone in disease states. 3
Human evolution has achieved hematocrit values of 0.3-0.5 in systemic circulation, representing an optimized balance between oxygen transport and blood flow. 4