Hematocrit is a measurement of the proportion of red blood cells in your blood. It is a common blood test, frequently performed as part of a complete blood count (CBC), that provides information about a person’s overall health. The results can help in the identification of various medical conditions.
What Hematocrit Represents
Hematocrit quantifies the volume percentage of red blood cells, also known as erythrocytes, within a total blood sample, which is why it is sometimes called packed cell volume (PCV). The measurement is determined by the number and size of the red blood cells. When a blood sample is spun in a centrifuge, its components separate into layers based on density. This results in a bottom layer of red blood cells, a thin middle layer (buffy coat) with white blood cells and platelets, and a top layer of plasma. The hematocrit value is the fraction of the total column occupied by the packed red blood cells, expressed as a percentage.
Calculating Hematocrit: Formulas and Steps
The calculation of hematocrit is done after a blood sample has been separated in a centrifuge. Using a microhematocrit tube, the heights of the layers are measured. The primary formula is: Hematocrit (%) = (Height of Red Blood Cell Column / Height of Total Blood Column) × 100. To perform the calculation, one measures the length of the packed red blood cell layer and the total height of the entire blood sample. The red blood cell height is then divided by the total height, and multiplying this ratio by 100 provides the final percentage. Modern automated analyzers can also calculate hematocrit by multiplying the red cell count by the mean cell volume.
Interpreting Hematocrit Levels: Normal and Abnormal
Interpreting hematocrit levels involves comparing the result to established normal ranges, which vary by age and sex. For adult males, the range is 40.7% to 50.3%, and for adult females, it is 36.1% to 44.3%. Newborns have higher levels that decrease over the first few months.
A low hematocrit level indicates anemia, a condition defined by a deficiency in red blood cells or hemoglobin. A high hematocrit level is known as polycythemia. Values falling outside the normal ranges may suggest an underlying medical issue requiring further investigation.
Practice Problems with Solutions
Understanding the calculation in a practical context can clarify how hematocrit is determined. Consider a scenario where a centrifuged blood sample has a total height of 40 millimeters (mm). The column of packed red blood cells at the bottom measures 18 mm. The calculation would be (18 mm / 40 mm) × 100, resulting in a hematocrit of 45%. This value would be considered normal for an adult male and slightly above the typical range for an adult female.
In another example, a blood sample shows a total column height of 50 mm, and the red blood cell layer measures 17 mm. The calculation would be (17 mm / 50 mm) × 100, which equals 34%. For an adult female, this level is below the normal range of 36.1% to 44.3%, suggesting a potential case of anemia.
Factors That Can Alter Hematocrit
Several physiological and environmental factors can influence a person’s hematocrit level.
- Dehydration: A common cause of elevated hematocrit, as the liquid plasma volume decreases, the concentration of red blood cells increases proportionally.
- High Altitudes: Living at high altitudes can naturally raise hematocrit, as the body produces more red blood cells to compensate for lower oxygen levels.
- Polycythemia Vera: This bone marrow disorder causes a marked increase in red blood cell production and, consequently, a high hematocrit.
- Pregnancy: During pregnancy, a woman’s plasma volume increases, which can lead to a lower hematocrit reading even with a normal red blood cell mass.
- Significant Blood Loss: This will directly decrease the volume of red blood cells, resulting in a lower hematocrit.
- Nutritional Deficiencies: Issues with red blood cell production in the bone marrow, sometimes from a lack of iron, can cause anemia.