The hematocrit (HCT) measures the percentage of red blood cells (RBCs; erythrocytes) in a blood sample and is an important clinical index that provides invaluable information on the patient’s health.
(A) A low RBC count due to …
- Vitamin B12 deficiency.
- Folic acid deficiency.
- Leukemia or lymphoma.
- Kidney disease.
(B) A high plasma level due to …
- Water intoxication.
- Retention of fluids.
- Congestive heart failure.
- Kidney disease.
- High sodium intake.
- Catabolic steroids.
(A) A high RBC count due to …
- Hypoxia (reduced oxygen) induced RBC production.
- Living at high altitudes.
- Chronic obstructive pulmonary disease (COPD).
- Emphysema and other lung diseases.
- Testosterone supplement therapy.
- Blood doping or erythropoietin (EPO).
- Anabolic steroids.
- Polycythemia vera – bone marrow abnormality.
(B) A low plasma level due to …
- Capillary leak syndrome.
In a well-equipped lab, an automated (computerized) analyzer is often used to calculate the hematocrit. The analyzer determines the value by multiplying the red cell count by the mean cell volume (MCV). If an automated analyzer is not available, the HCT can be measured directly using centrifugation that separates the plasma from the packed RBCs.
A blood sample is initially collected by lancing a finger and drawing blood into a small, heparinized glass tube as shown below.
After being collected, the blood sample is placed in a specialized centrifuge and spun for a short period (10,000 RPM for five minutes). The process causes the blood components to separate into layers based on their density.
The blood components typically separate into three layers.
- Red blood cells, the heaviest blood components, form a dark red column at the bottom of the tube. The volume of RBCs after centrifugation is also commonly referred to as packed cell volume (PCV).
- Female range: 36% – 44%;
- Male range: 40% – 50%.
- White blood cells (WBCs; leukocytes) and platelets (thrombocytes) form a thin layer (1% of blood) directly above the RBCs. This layer is referred to as the buffy coat because of its light color.
- Blood plasma, a pale, straw-colored fluid, fills the hematocrit tube above the buffy coat. The mean plasma percentage is the percent of the blood that is not RBCs, WBCs, or platelets.
- Females: it is approximately 60%;
- Males, it is approximately 55%.
A hematocrit reader card is used to determine the percentage of RBCs in the blood sample. First, the glass tube is positioned on the left side of the reader card, with the bottom of the RBC column on the 0% line. Then, the glass tube is moved across the reader card until the top of the plasma column reaches the 100% line. The percentage of RBCs is then determined from the markings on the reader card.
- Male normal value = about 45% (40% – 50%);
- Female normal value = about 40% (36% – 44%).
OpenStax College, Anatomy and Physiology
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Reference: “An Overview of Blood“
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