Packed Cell Volume (PCV), often referred to as hematocrit (Hct), is a fundamental measurement in blood testing. It represents the proportion of blood volume occupied by red blood cells (RBCs). Since RBCs transport oxygen from the lungs to the body’s tissues, the PCV is a direct indicator of the blood’s oxygen-carrying capacity. This test is a routine part of a complete blood count (CBC), offering initial insights into overall blood health.
How Packed Cell Volume is Determined
The determination of Packed Cell Volume relies on centrifugation. A small blood sample, treated with an anticoagulant, is placed into a narrow microhematocrit tube. The tube is then spun at a high speed, often between 10,000 and 15,000 revolutions per minute, for a few minutes.
This rapid spinning forces the blood components to separate based on density. The heavier red blood cells settle tightly at the bottom, forming the “packed” cell layer. Above this layer is the “buffy coat,” a thin, grayish-white layer containing white blood cells and platelets. The least dense component, the plasma, remains as a clear or yellowish fluid at the top.
The PCV is calculated by measuring the height of the packed red cell column and dividing it by the total height of the blood column. This ratio is multiplied by 100 to express the result as a percentage. For example, a PCV of 45% means that 45 milliliters of red blood cells are present in every 100 milliliters of whole blood.
Understanding Normal Reference Ranges
Normal PCV values vary based on physiological factors, with gender being the most significant determinant in adults. For adult males, the typical reference range falls between 40% and 54%. Adult females usually have a slightly lower range, often between 36% and 48%, due to hormonal differences and menstrual blood loss.
These ranges represent the expected percentage of red blood cells in a healthy, well-hydrated person. Ranges also differ for infants and children, who have distinct red cell requirements. Laboratories may have slightly adjusted reference intervals based on the specific equipment used and the demographics of the local population.
What Abnormal Results Indicate
Results outside the normal PCV range signal an imbalance in the number of red blood cells relative to the total blood volume. This imbalance requires further investigation to determine the underlying cause.
Low PCV (Reduced Red Cell Mass)
A low PCV indicates anemia, a condition where the body lacks sufficient red blood cells to transport adequate oxygen. Common causes include nutritional deficiencies, such as insufficient iron, vitamin B12, or folate, which are necessary for red blood cell production. Chronic blood loss, often subtle and long-term from sources like gastrointestinal ulcers or heavy menstrual bleeding, also results in a reduced red cell mass.
Chronic diseases can suppress red blood cell formation, leading to a low PCV. This includes chronic kidney disease, which reduces the production of erythropoietin, the hormone that stimulates red cell creation. Bone marrow disorders, where the blood-forming tissue is compromised, can also decrease red cell output. Additionally, plasma volume expansion, such as during pregnancy, can dilute the blood and cause a relatively low PCV reading.
High PCV (Increased Red Cell Mass)
An elevated PCV suggests an increased proportion of red blood cells, a condition referred to as erythrocytosis or polycythemia. The most frequent cause is dehydration, where fluid loss concentrates the blood, leading to a high PCV even if the actual red blood cell count is normal. Healthcare providers often re-test a patient’s PCV after ensuring proper hydration to rule out this “relative” polycythemia.
A true increase in red cell mass is caused by conditions that stimulate red blood cell production. Examples include living at high altitudes, where the body compensates for lower oxygen levels by making more red cells. Chronic lung or heart disease can also lead to persistently low blood oxygen, triggering the body to produce more red cells in response. In some cases, a high PCV is due to a bone marrow disorder like polycythemia vera, which involves the overproduction of red blood cells, increasing the risk of blood clots and stroke.