The hematocrit (Hct) measures the percentage of red blood cells within the total volume of your blood. Red blood cells carry oxygen from the lungs to every tissue and organ throughout the body. This percentage is a measure of the blood’s capacity to deliver oxygen. Doctors routinely measure hematocrit as a general indicator of health, particularly when evaluating symptoms related to fatigue or weakness. The resulting value provides information about the balance between the solid components of blood and the liquid plasma.
Understanding the Hematocrit Test
The hematocrit test is most commonly performed as a component of a larger diagnostic panel known as a Complete Blood Count (CBC). A blood sample is collected, typically from a vein in the arm, and then sent to a laboratory for analysis. The test works by measuring the volume occupied by the red blood cells.
In the physical method, the blood sample is spun at high speeds in a centrifuge. This forces the heavier red blood cells to the bottom of the tube, separating the blood into distinct layers. The height of the packed red blood cell layer is then measured against the height of the total blood volume. The final result is expressed as a percentage, meaning a result of 40 indicates that 40% of the blood volume is composed of red blood cells.
Standard Reference Ranges for Adults
For adult females who are not pregnant, the standard reference range for hematocrit generally falls between 36% and 44%. This range is slightly lower than that for adult males, whose levels typically range from 41% to 50%. This physiological difference is largely attributed to the regular blood loss experienced by women during menstruation. The cyclical loss of blood can lead to lower iron stores and a reduced red blood cell volume compared to males.
Pregnancy introduces a specific physiological variation that influences hematocrit levels, often causing them to drop below the typical non-pregnant range. During gestation, the body significantly increases its plasma volume to support the growing fetus. This substantial increase in plasma volume happens more rapidly than the production of new red blood cells, which dilutes the blood. This dilution, known as hemodilution, results in a lower hematocrit percentage, even if the total number of red blood cells remains healthy.
Implications of Low Hematocrit Levels
A hematocrit level below the established reference range is often indicative of anemia, a condition characterized by an insufficient amount of healthy red blood cells to transport adequate oxygen. This lack of oxygen delivery to tissues causes the most common symptoms associated with low hematocrit, such as weakness and fatigue. Other signs can include pallor (paleness of the skin) and shortness of breath, particularly when exerting oneself.
Low hematocrit in females is frequently linked to chronic blood loss, with heavy or prolonged menstrual periods being a primary cause. This regular loss of blood depletes the body’s iron reserves over time, leading to iron-deficiency anemia, which is the most common nutritional deficiency worldwide. Nutritional factors like insufficient dietary intake or poor absorption of iron, Vitamin B12, or folate can also impair the bone marrow’s ability to produce new red blood cells.
Chronic diseases can also suppress the body’s red blood cell production, leading to lower hematocrit values. Conditions such as chronic kidney disease interfere with the production of erythropoietin, a hormone that signals the bone marrow to create red blood cells. Other underlying causes can include certain cancers, such as leukemia, or conditions that cause red blood cells to be destroyed prematurely. Identifying and addressing the specific underlying cause is the basis for treating a low hematocrit result.
Implications of High Hematocrit Levels
A hematocrit result above the standard range, a condition called erythrocytosis, means that the blood contains a higher percentage of red blood cells. This elevated concentration causes the blood to become thicker, increasing its viscosity. Higher viscosity forces the heart to work harder to pump the blood, and it raises the risk of complications like blood clots, stroke, and heart attack.
High hematocrit can be classified into two main categories: relative and absolute. Relative erythrocytosis is the most common cause and occurs when the total volume of blood plasma decreases, such as with dehydration. The red blood cell count itself may be normal, but the lack of fluid concentrates the remaining blood components, artificially inflating the percentage.
Absolute erythrocytosis involves a true increase in the number of red blood cells. One common reason is the body’s response to chronic low oxygen levels, a state known as hypoxia. People who live at high altitudes naturally develop higher hematocrit levels as their bodies attempt to compensate by increasing oxygen-carrying capacity. Chronic respiratory conditions like severe lung disease or obstructive sleep apnea can also trigger this compensatory mechanism. A less common cause is polycythemia vera, a bone marrow disorder that causes the uncontrolled overproduction of red blood cells. Symptoms of high hematocrit often include headaches, dizziness, and a flushed appearance to the skin.