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Hematocrit Lab Test


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Background Information Background information icon

Normally, blood consists mostly of plasma (about 55%) and red blood cells (about 45%)) by volume.

The plasma, which is primarily (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% 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
Blood Component Layers

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.

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.

Subject InformationSubject information icon

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 to have some blood tests performed to determine if he’s OK.

Hematocrit test subject

Test Procedures Test procedures icon

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 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.

Plug 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 Tubes in Centrifuge

Place the blood-filled microhematocrit tubes 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 is composed of red blood cells (erythrocytes or RBCs), the heaviest blood components. 
b. Directly above the RBCs should be a thin white layer, called the buffy coat. This area is composed of white blood cells (leukocytes or WBCs;) and platelets (thrombocytes).
c. A pale, straw-colored fluid, called blood plasma, should appear above the buffy coat.

Test ResultsTest results icon

Determine Hematocrit Value

Use the hematocrit reader card to ascertain the percentage of RBCs in the subject’s blood sample (packed cell volume; PCV).

a. Position the glass tube on the left side of the reader card, and align the bottom of the RBC column on the 0% line.
b. Drag the tube across the reader card until the top of the plasma column reaches the 100% line.
c. Locate the percentage markings closest to the top of the RBC column. Follow the lines to left or right to determine the subject’s hematocrit value.

AssessmentAssessment icon

Approximately 46%

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.

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.

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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Reference page: “An Overview of Blood

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