What is Hematocrit?
Hematocrit is the percentage of blood volume occupied by red blood cells, calculated from the mean corpuscular volume (MCV) multiplied by the red blood cell count. 1
Definition and Measurement
Hematocrit represents the proportion of whole blood that consists of red blood cells, expressed as a percentage. 1 It is a calculated value derived from automated analyzer measurements of red blood cell count and mean corpuscular volume (MCV), using the formula: MCV × erythrocyte count = Hct. 1
Normal Reference Ranges
Normal hematocrit values vary significantly by age and sex: 2
- Adult males/post-menopausal females: 47 ± 6% 2
- Menstruating females: 41 ± 5% 2
- Children 12-18 years (male): 43 ± 6% 2
- Children 6-12 years: 40 ± 5% 2
- Children 2-6 years: 37 ± 3% 2
- Newborns: 51 ± 9% 1
Clinical Significance
Oxygen Transport Function
Hematocrit determines the oxygen-carrying capacity of blood, though it simultaneously increases blood viscosity and flow resistance. 3 This creates a dual role where optimal tissue oxygenation requires balancing adequate red cell mass against increased viscosity. 3
Thrombotic Risk
Elevated hematocrit is an independent risk factor for thrombosis. 4 Men with hematocrit ≥46% have a 1.5-fold increased risk of venous thromboembolism and a 2.4-fold increased risk of unprovoked venous thromboembolism compared to those with lower values. 5
Diagnostic Thresholds
- Absolute polycythemia is always present when hematocrit exceeds 60% in males or 55% in females. 4
- Elevated values requiring evaluation: >52% in males, >49.5% in women. 6
- Transfusion threshold in stable hospitalized patients: Hematocrit 20-24% (hemoglobin 7-8 g/dL). 3
Important Measurement Limitations
Why Hemoglobin is Preferred Over Hematocrit
Hemoglobin is a more accurate measure than hematocrit for clinical assessment because: 1
Storage instability: MCV (and thus calculated hematocrit) is stable at room temperature for only 8 hours and refrigerated for only 24 hours, whereas hemoglobin remains unchanged. 1 Prolonged storage can falsely increase hematocrit by 2-4%. 1
Hyperglycemia interference: In the presence of hyperglycemia, MCV is falsely elevated, resulting in falsely elevated calculated hematocrit, but hemoglobin is unaffected. 1
Greater measurement variability: The coefficient of variation for hematocrit measurements is 2-3 times higher than for hemoglobin across automated analyzers. 1
Natural Variation
Within-subject biological variation of hematocrit is approximately 3%, which combined with analytical variation can explain a relative change of approximately 12% between two successive measurements taken 1 day to 1-2 months apart in healthy adults. 7 Seasonal variation can cause population mean hematocrit values to be approximately 3% lower in summer than winter, with some studies showing differences up to 7%. 7
Clinical Applications
Anemia Management
In chronic kidney disease, target hematocrit is 33-36% (corresponding to hemoglobin 11-12 g/dL), with survival rates improving when hematocrit exceeds 32-33%. 2
Cyanotic Heart Disease
In patients with cyanotic congenital heart disease, elevated hematocrit represents a physiological compensatory response to improve oxygen transport in the setting of chronic hypoxemia. 1 Routine phlebotomy is not supported by data and should be avoided, as there is no clear correlation between hematocrit level and hyperviscosity symptoms. 1
Fetal Medicine
In fetal transfusion, target hematocrit is approximately 40-50%, with higher levels avoided due to increased blood viscosity and theoretical increased risk of complications. 1