Hematocrit Test
(Sample Lesson)
Introduction
Blood normally consists of plasma (about 55%) and red blood cells (about 45%)) by volume.
The plasma, which is mostly (92%) water, transports a variety of dissolved and suspended substances. These substances include proteins, electrolytes. gases, and nutrients.
The red blood cells (RBCs or erythrocytes) transport oxygen and carbon dioxide gases between the lungs and tissues.
Also found in the blood are white blood cells (WBCs or leukocytes) and platelets (thrombocytes). However, they normally comprise only 1% of the volume of whole blood. The WBCs are a part of the body’s immune system and the platelets participate in blood coagulation.
The hematocrit test measures the percentage of RBCs by volume in a blood sample. Because the purpose of red blood cells is to transfer oxygen from the lungs to body tissues, a blood sample’s hematocrit can become a point of reference for its capability of delivering oxygen. Hematocrit levels that are too high or too low can indicate a blood disorder, dehydration, or other medical conditions.
Conditions that lower the HCT value.
(A) Low numbers of red blood cells due to …
- Bleeding.
- Anemias.
- Iron-deficiency.
- Vitamin B12 deficiency.
- Folic acid deficiency.
- Leukemia or lymphoma.
- Kidney disease.
- Thyroid disease.
- Immune destruction of RBCs.
(B) A relatively high plasma volume caused by …
- Water intoxication.
- Circumstances that cause fluid retention …
- Pregnancy.
- Congestive heart failure.
- Kidney disease.
- High sodium intake.
- Catabolic steroids.
Conditions that increase the HCT value.
(A) High numbers of RBC (polycythemia) due to …
- Circumstances that induce hypoxia (low blood oxygen).
- Living at high altitudes.
- Chronic obstructive pulmonary disease (COPD).
- Emphysema and other lung diseases.
- Congenital heart disease.
- Testosterone supplement therapy.
- Blood doping or erythropoietin (EPO).
- Anabolic steroids.
- Polycythemia vera – bone marrow abnormality.
(B) A relatively low plasma volume caused by …
- Dehydration due to …
- Diarrhea.
- Vomiting.
- Sweating too much.
- Too much urination.
- Fever
- Capillary leak syndrome.
Test Process
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, a ‘spun hematocrit’ can be performed. Using this method, the HCT can be measured using a centrifugate to separate the blood plasma from the RBCs and WBCs (white blood cells).
Step 1
A lancing device is first loaded with a lancet. It is then adjusted so the subject’s skin will be penetrated to the proper depth. One of the subject’s fingers is selected, and the lancing device is positioned against the surface of the skin. The trigger button is pressed, and the skin is pierced. A small drop of blood will typically accumulate on the skin surface. If more blood is needed, squeeze the punctured area until the proper amount of blood appears.
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Step 2
One end of a microhematocrit tube is inserted into the blood droplet. Capillary attraction will draw the blood up the narrow glass tube. When it is about 3/4 full, remove the tubed. A lining of heparin, an anticoagulant, keeps the blood from clotting as it makes contact with the tube.
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Step 3
The blood end of the microhematocrit tube is inserted into a tray of sealing clay. The clay prevents the blood from draining out of the tube while it is being centrifuged.
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Step 4
After being collected, the blood-filled microhematocrit tubes are carefully placed in a centrifuge. The tubes are positioned so that their sealed ends face out. A lid is placed over the tubes to secure their position while the centrifuge rotor spins.
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Step 5
The lid of the centrifuge is closed, and the apparatus is typically set to spin at 10,000 to 12,000 rpm for approximately 4 to 6 minutes.
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Step 6
The rotor of the centrifuge is started. While the rotor is spinning, centrifugal force causes the blood in the tubes to separate into layers based on their density.
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Step 7
The blood in the microhematocrit tubes typically separates into three distinct 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% – 48%;
- 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%.
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Results
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%).
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OpenStax College, Anatomy and Physiology
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Reference: “An Overview of Blood“
