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Hematocrit Lab Test Simulation

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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) and carbon dioxide (CO2). The end products exit the mitochondria, and the energy stored in ATP drives cellular reactions.

The carbon dioxide diffuses out of the tissue cells and is transported back to the lungs by RBCs. The RBCs release the CO2 into the lungs, where it is expelled into the atmosphere. 

Because the hematocrit value estimates the number of circulating RBCs, it also indirectly indicates the tissue cells’ ability to store energy in the form of ATP.

The role of red blood cells or RBCs any why their number in the blood is relevant

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.

Blood component layers: plasma, red blood cells (RBCs), and white blood cells (WBCs)

Conditions that Can Decrease the HCT Value

Low Numbers of RBCs Caused By:High Plasma Volume Caused By:
BleedingWater intoxication
Iron-deficiency anemiaFluid retention due to pregnancy
Vitamin B12 deficiency anemiaFluid retention due to congestive heart failure
Folic acid deficiency anemiaFluid retention due to kidney disease
Leukemia or lymphomaFluid retention due to high sodium intake
Kidney diseaseFluid 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 altitudesDehydration due to diarrhea
Hypoxia due to chronic obstructive pulmonary disease (COPD)Dehydration due to vomiting
Hypoxia due to emphysema and other lung diseasesDehydration due to sweating
Hypoxia due to congenital heart diseaseDehydration due to increased urination
Testosterone supplement therapyCapillary leak syndrome
Blood doping or erythropoietin (EPO)
Anabolic steroids
Polycythemia vera – bone marrow abnormality

Subject Information

Personal and Medical History

The 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 often feels somewhat fatigued.

He wants some blood tests to determine if he’s OK.

Hematocrit test subject

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.

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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.

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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.

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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.

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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.

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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.

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Examine Blood Layers

Remove the microhematocrit tubes from the centrifuge and examine the quality of the blood component layers. The blood column should be divided into three distinct layers to produce the most accurate test results.

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. A thin white layer called the buffy coat should be directly above the RBCs. This area contains white blood cells (leukocytes; WBCs) and platelets (thrombocytes).

c. A pale, straw-colored fluid, blood plasma, should appear above the buffy coat.

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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.

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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.

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What is your subject’s hematocrit value?

Is your subject’s hematocrit value within the normal range for males?

Is there always a direct correlation between the HCT and the number of circulating RBCs?

Offer a possible explanation for why your subject feels fatigued.

What will likely happen to your subject’s hematocrit value over time as he continues to live in Denver?

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References and Attributions

OpenStax, Anatomy, and Physiology

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Reference page: “An Overview of Blood