What is the definition of hematocrit (Hct)?

Definition of Hematocrit

Hematocrit (Hct) is the percentage of blood volume occupied by red blood cells, expressed as a percentage. 1

Technical Definition and Measurement

• Hematocrit represents the proportion of total blood volume that consists of red blood cells, as opposed to plasma volume 1
• The value is calculated using the formula: Hct = MCV × erythrocyte count, where MCV is mean corpuscular volume 1
• Normal hematocrit values vary by age and sex: adult males/post-menopausal females have a mean of 47 ± 6%, while menstruating females have a mean of 41 ± 5% 1, 2

Measurement Considerations and Limitations

• Hemoglobin is a more accurate measure than hematocrit for assessing red blood cell status because hematocrit is unstable when blood samples are stored at room temperature. 1
• MCV (and therefore calculated hematocrit) remains stable at room temperature for only 8 hours and for only 24 hours when refrigerated, whereas hemoglobin remains unchanged under the same storage conditions 1
• When blood samples are stored for longer periods, MCV increases, resulting in falsely elevated calculated hematocrit by as much as 2% to 4%, while hemoglobin measurements remain accurate 1, 2
• Hyperglycemia falsely elevates MCV and calculated hematocrit, but does not affect hemoglobin measurement 1, 2
• Automated analyzers show greater variability in hematocrit estimation compared to hemoglobin measurement, with within-run and between-run coefficients of variation for hemoglobin being one-half and one-third those for hematocrit, respectively 1

Clinical Context and Interpretation

• Hematocrit is influenced by both red blood cell mass and plasma volume, meaning it can be elevated due to increased red cells (true polycythemia) or decreased plasma volume (relative polycythemia/hemoconcentration) 3, 4
• In critically ill surgical patients, peripheral blood hematocrit may not accurately estimate true red blood cell volume due to alterations in plasma volume 3
• Single hematocrit measurements should not be used as an isolated laboratory marker for bleeding, as initial values in the normal range may mask early-phase bleeding; repeated measurements are necessary. 1
• The traditional assumption that hematocrit equals three times the hemoglobin value (Hct = 3 × Hgb) is not consistently accurate and varies with age and clinical conditions 5

Practical Implications

• For monitoring anemia in chronic kidney disease and other conditions, hemoglobin is strongly preferred over hematocrit due to superior reproducibility, stability, and accuracy 1
• Hematocrit measurements are particularly unreliable when blood samples are shipped to centralized laboratories under poorly controlled conditions, a common practice in consolidated healthcare systems 1
• Point-of-care hematocrit measurements may show systematic bias compared to traditional capillary centrifugation methods, particularly in specific populations 6