Hematocrit (HCT) is a common blood test that measures the proportion of red blood cells within an individual’s total blood volume. This measurement provides a snapshot of the red blood cell component, which is crucial for oxygen transport. The HCT test is often included as part of a complete blood count (CBC), offering insights into overall health.
What Hematocrit Represents
Hematocrit quantifies the percentage of whole blood made up of red blood cells. For example, a hematocrit of 45% means 45 out of every 100 milliliters of blood volume consists of red blood cells. Red blood cells contain hemoglobin, a protein that binds to oxygen in the lungs and delivers it throughout the body.
The Centrifugation Method
The traditional, manual method for determining hematocrit involves centrifugation, known as the microhematocrit method. A small blood sample is drawn into a narrow glass capillary tube, which is then sealed. The sealed tube is placed into a centrifuge, spinning at high speeds (e.g., 11,000-12,000 rpm) for several minutes.
This rapid spinning generates centrifugal force, separating blood components by density. Denser red blood cells settle at the bottom, forming a packed column, while less dense plasma remains at the top, separated by a thin layer of white blood cells and platelets called the buffy coat. The height of the packed red blood cell column is then measured and compared to the total blood sample height, providing the direct hematocrit value as a percentage.
Automated Measurement and Derivation
Modern clinical laboratories primarily use automated hematology analyzers to determine hematocrit levels. These instruments often derive hematocrit mathematically from other measured parameters, such as the red blood cell count (RBC) and the mean corpuscular volume (MCV), which is the average size of individual red blood cells.
Analyzers use principles like electrical impedance or light scattering to count and size red blood cells. For instance, in the impedance method, cells passing through an aperture change electrical resistance, translating into cell counts and volumes. Hematocrit is then calculated using the formula: HCT = (RBC count × MCV) / 10. This automated approach allows for rapid and efficient processing of many samples.
Common Influences on Hematocrit Levels
Several non-disease factors can influence hematocrit levels. Hydration status is one factor; dehydration can lead to a falsely elevated hematocrit due to concentrated red blood cells, while overhydration can dilute the blood and lower the reading.
Environmental factors also play a role; individuals at high altitudes often have higher hematocrit levels as their bodies produce more red blood cells to compensate for lower oxygen. Recent blood loss can acutely lower hematocrit, while a transfusion would increase it. Smoking can also elevate hematocrit due to chronic oxygen deprivation.