Hematocrit Lab Test Simulation
Test Background
Introduction
The hematocrit test (HCT) 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.
Blood Components
Typically, blood consists primarily of plasma (about 55%) and red blood cells (about 45%) by volume.
Red blood cells (RBCs or erythrocytes) transport oxygen (O2) to tissue cells from the lungs. In the cells, mitochondria use oxygen to convert fuel molecules into energy to make ATP (adenosine triphosphate). The process, called aerobic respiration, also produces water (H2O) carbon dioxide (CO2). The end products exit the mitochondria, and the energy stored in ATP drives cellular reactions.
The CO2, a waste product, diffuses out of the tissue cells and is transported back to the lungs by RBCs, where it is expelled.
Because the hematocrit value estimates the number of circulating RBCs, it also indirectly indicates the tissue cells’ ability to store in the form of ATP.
The plasma portion of the blood is primarily (92%) water and transports several types of dissolved and suspended substances. These substances include proteins, electrolytes, gases, and nutrients.
Also found in the blood are white blood cells (WBCs; leukocytes) and platelets (thrombocytes).
However, they usually comprise only 1% or less of the volume of whole blood.
The WBCs are a part of the body’s immune system, and the platelets participate in blood coagulation.
Conditions that Can Decrease the HCT Value
| Low Numbers of RBCs Caused By: | High Plasma Volume Caused By: |
|---|---|
| Bleeding | Water intoxication |
| Iron-deficiency anemia | Fluid retention due to pregnancy |
| Vitamin B12 deficiency anemia | Fluid retention due to congestive heart failure |
| Folic acid deficiency anemia | Fluid retention due to kidney disease |
| Leukemia or lymphoma | Fluid retention due to high sodium intake |
| Kidney disease | Fluid retention due to catabolic steroids |
| Thyroid disease | |
| Immune destruction of RBCs |
Conditions that Can Increase the HCT Value
| High Numbers of RBCs Caused By: | Low Plasma Volume Caused By: |
|---|---|
| Hypoxia (low blood oxygen) due to living at high altitudes | Dehydration due to diarrhea |
| Hypoxia due to chronic obstructive pulmonary disease (COPD) | Dehydration due to vomiting |
| Hypoxia due to emphysema and other lung diseases | Dehydration due to sweating |
| Hypoxia due to congenital heart disease | Dehydration due to increased urination |
| Testosterone supplement therapy | Capillary leak syndrome |
| Blood doping or erythropoietin (EPO) | |
| Anabolic steroids | |
| Polycythemia vera – bone marrow abnormality |
Subject Information
Personal and Medical History
Your subject is 27 years old and has recently moved to Denver, Colorado from Missouri.
He is in good health, eats well, and exercises regularly.
Since arriving in Denver, however, he has felt somewhat fatigued.
He wants some blood tests to determine if he’s OK.
Test Procedures
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).
Unfortunately, an automated analyzer is not available. Instead, a ‘spun hematocrit’ will be performed. A blood sample will be drawn from the subject’s finger and placed in a centrifugate. The radial (gravitational) force generated by the spinning centrifuge will separate the blood sample components into layers based on their density. Finally, the percentage of RBCs will be determined using the grid system printed on a reader card.
Lance Finger
Use a lancet device to pierce the skin of the index finger.
Allow a large drop of blood to accumulate on the finger surface.
Draw Blood into a Tube
Place one end of the glass tube into the drop of blood on the index finger surface.
The blood will flow into the tube by capillary attraction. A lining of heparin, an anticoagulant, keeps the blood from clotting after making contact with the glass tube.
Draw blood into the tube until it is about 3/4 full.
Plug End of Tube
Insert the blood-filled end of the microhematocrit tube into the clay sealer.
The clay will prevent the blood from draining out of the microhematocrit tube while it is being centrifuged.
Place Tube in Centrifuge
Place the blood-filled microhematocrit tube in a centrifuge.
Position the tubes so that their sealed ends face the outer rim of the rotor tray.
Next, cover tubes with a lid to secure their position while the centrifuge rotor spins.
Adjust Centrifuge Settings
After closing the lid, turn on the centrifuge.
Adjust the spin rate indicator to 10,000 rpm and the duration (time) indicator to 5 minutes.
Start Centrifuge
Start the centrifuge and allow the rotor to spin for the designated time.
While the tubes spin, watch the blood components separate into layers.
When the spin time is complete, push the button to stop the rotor.
Examine Blood Layers
Remove the microhematocrit tubes from the centrifuge and examine the quality of the blood component layers. To produce the most accurate test results, the blood column should be divided into three distinct layers.
a. A dark red column should fill the bottom portion of the tube. This area contains red blood cells (erythrocytes; RBCs), the heaviest blood components.
b. Directly above the RBCs should be a thin white layer, called the buffy coat. This area contains white blood cells (leukocytes; WBCs) and platelets (thrombocytes).
c. A pale, straw-colored fluid, called blood plasma, should appear above the buffy coat.
Test Results
Move Tube to Reader
Use a hematocrit reader card to ascertain the percentage of RBCs in the subject’s blood sample (packed cell volume; PCV).
Position the glass tube on the left side of the reader card, and align the bottom of the RBC column on the 0% line.
Drag the tube across the reader card until the top of the plasma column reaches the 100% line.
Determine HCT Value
Drag your subject’s spun hematocrit tube to the proper position on the reader card.
Locate the percentage markings closest to the top of the RBC column.
Follow the lines to the left or right to determine the subject’s hematocrit value.
Assessment
What is your subject’s hematocrit value?
Approximately 46%
Is your subject’s hematocrit value within the normal range for males?
Yes
Is there always a direct correlation between the HCT and the number of circulating RBCs?
No. The HCT measures the volume percentage of RBCs in the blood. A change in plasma volume, as caused by dehydration, will alter the relative volume of RBCs in the blood while the number of circulating RBCs remains unchanged.
Offer a possible explanation for why your subject feels fatigued.
Denver is located at a higher altitude than most other parts of the United States and, therefore, has a lower oxygen concentration. Although your subject has a normal number of RBCs, they may be undersaturated with oxygen, causing hypoxia.
What will likely happen to your subject’s hematocrit value over time as he continues to live in Denver?
His hematocrit value will probably increase somewhat due to his relative hypoxia. The hormone, erythropoietin (EPO), will be released from the kidney and stimulate increased RBC production in the bone marrow.
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References and Attributions
OpenStax, Anatomy, and Physiology
Access for free at – https://openstax.org/books/anatomy-and-physiology/pages/1-introduction
Reference page: “An Overview of Blood“