Definition of Hematocrit (Hct)
Hematocrit (Hct) is defined as the proportion or percentage of whole blood volume that is occupied by red blood cells. 1
Technical Definition and Measurement
Hematocrit is typically measured as part of a complete blood count (CBC) using automated cell counters in clinical settings. The measurement is calculated using the following relationship:
- Hct = (Red blood cell volume / Total blood volume) × 100%
In automated analyzers, hematocrit is calculated from:
- Hct = Mean Cell Volume (MCV) × Erythrocyte count 1
Normal Reference Ranges
Normal hematocrit values vary by age, sex, and physiological state:
| Age/Gender | Hematocrit (%) |
|---|---|
| Birth | 51 ± 9 |
| 1 month | 43 ± 6 |
| 2 to 6 months | 35 ± 7 |
| 6 months to 2 years | 36 ± 3 |
| 2 to 6 years | 37 ± 3 |
| 6 to 12 years | 40 ± 5 |
| 12 to 18 years (male) | 43 ± 6 |
| Menstruating female | 41 ± 5 |
| Adult male/post-menopausal female | 47 ± 6 |
Clinical Significance and Limitations
Limitations of Hematocrit as a Diagnostic Tool
Storage Sensitivity: Unlike hemoglobin, hematocrit is not stable when blood samples are stored at room temperature. MCV (from which Hct is calculated) is stable at room temperature for only 8 hours and for only 24 hours when refrigerated. When stored longer, MCV increases, resulting in falsely elevated calculated Hct by as much as 2-4%. 1
Hyperglycemia Effect: In the presence of hyperglycemia, MCV (and consequently Hct) can be falsely elevated, while hemoglobin remains unaffected. 1
Analyzer Variability: There is greater variability across automated analyzers in estimation of erythrocyte number and size (used to calculate Hct) than in measurement of hemoglobin. Data show that the coefficient of variation for hemoglobin measurement is one-half to one-third that of hematocrit. 1
Fluid Status Influence: Hematocrit reflects both red cell volume and plasma volume, making it susceptible to changes in hydration status. Dehydration can elevate hematocrit without any change in actual red cell mass. 1, 2
Limited Value in Trauma: Single hematocrit measurements have limited diagnostic value for detecting bleeding in trauma patients, with a low sensitivity (0.5) for detecting traumatic hemorrhage requiring surgical intervention. 1
Hemoglobin vs. Hematocrit
Many guidelines now recommend hemoglobin as the preferred measurement over hematocrit for monitoring anemia because:
- Hemoglobin is more stable in stored samples
- Hemoglobin is less affected by sample storage conditions
- Hemoglobin measurement has less variability between different analyzers 1
Clinical Applications
Anemia Detection: Hematocrit is used alongside hemoglobin to detect and monitor anemia, though it is considered a late indicator of iron deficiency. 1
Polycythemia Assessment: Elevated hematocrit is a key diagnostic criterion for polycythemia vera, with treatment targets of <45% in men and <42% in women. 3
Blood Loss Monitoring: Serial hematocrit measurements increase sensitivity for detecting blood loss in patients with severe injury. 1
Transfusion Decisions: During cardiopulmonary bypass, packed red blood cell transfusions are recommended when HCT is <18% (Hb 6.0 g/dL), with consideration between 18-24% based on tissue oxygenation assessment. 1
Important Considerations in Clinical Practice
Serial Measurements: Single hematocrit measurements may mask early-phase bleeding. Repeated measurements increase sensitivity for detecting blood loss. 1
True Red Cell Volume: Peripheral blood hematocrit may not accurately estimate true red blood cell volume in critically ill patients. 2
Complementary Tests: For comprehensive assessment of oxygen-carrying capacity and iron status, hematocrit should be evaluated alongside other parameters such as hemoglobin, red cell indices, and iron studies. 1
Trauma Management: In trauma patients, decreasing serial hematocrit measurements may reflect continued bleeding, but a patient with significant bleeding may maintain stable serial hematocrit values due to concurrent fluid shifts. 1
In summary, while hematocrit remains a widely used parameter in clinical practice, its limitations should be recognized, and it should be interpreted in conjunction with other laboratory and clinical parameters for optimal patient management.