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What does a hematocrit test measure in a blood sample?
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Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).
A hematocrit test is a common diagnostic procedure used to analyze a sample of blood. During the test, a blood sample is placed in a calibrated tube and centrifuged to separate its components into distinct layers: plasma at the top, a layer of white blood cells and platelets in the middle, and packed red blood cells at the bottom.1
The hematocrit specifically measures the percentage of the total blood volume that is occupied by these packed red blood cells. Because the proportion of red blood cells can indicate various health conditions—such as polycythemia when the layer is too thick, or iron-deficiency anemia when it is too thin—this test serves as a vital early indicator of disease.12