Hematocrit (packed cell volume) determination

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The hematocrit or packed cell volume (PCV) determines the percentage of red blood cells (RBCs) in whole blood.

The normal hematocrit value for men is 42% to 52%; for women, 37% to 47%; and for newborns, 53% to 65%. When hematocrit determinations are below normal, medical conditions such as anemia and leukemia may be present. Above-normal hematocrit determinations indicate medical conditions like dehydration, such as occur in severe burn cases.

Currently, automated hematology analyzers supply most hematocrits. However, when hematology analyzers are not available, hematocrit determinations can be manually performed by the microhematocrit method or macrohematocrit method. Both methods call for the blood to be centrifuged, and the percentage of packed red cells is found by calculation.

The microhematocrit method is the most accurate manual method of determining blood volume and should be used whenever feasible. Material requirements and the step-by-step procedures for performing the microhematocrit method will be covered in the following sections.

Materials Required for Microhematocrit Procedure

To perform a hematocrit using the micro- hematocrit method, the following materials are required.

Capillary tubes, plain or heparinized
Modeling clay sealant
Microhematocrit centrifuge
Microhematocrit reader
Microhematocrit Procedure
To perform the microhematocrit method, you should follow the steps listed below:

1. Fill the capillary tube two-thirds to three-quarters full with well-mixed, oxalated venous blood or fingertip blood. (For fingertip blood use heparinized tubes, and invert several times to mix.)

2. Seal one end of the tube with clay.
3. Place the filled tube in the microhematocrit centrifuge, with the plugged end away from the center of the centrifuge.

4. Centrifuge at a preset speed of 10,000 to 12,000 rpm for 5 minutes. If the hematocrit exceeds 50 percent, centrifuge for an additional 3 minutes.

5. Place the tube in the microhematocrit reader. Read the hematocrit by following the manufacturer's instructions on the microhematocrit reading device.

The total white cell (leukocyte) count determines the number of white cells per cubic millimeter of blood. Agreat deal of information can be derived from white cell studies. The white blood cell count (WBC) and the differential count are common laboratory tests, and they are almost a necessity in determining the nature and severity of systemic infections. Normal WBCvalues in adults range from 4,500 to 11,000 cells per cubic millimeter; in children the range is from 5,000 to 15,000 cells per cubic millimeter; and in newborns the range is from 10,000 to 30,000 cells per cubic millimeter.

White blood cell counts are performed either manually or with automated hematology analyzers. Only the manual method will be covered in this chapter. After a brief discussion on abnormal white blood cell counts, we will cover the Unopette method for manually counting white blood cells.

Abnormal White Cell Counts
When white cell counts rise above normal values, the condition is referred to as leukocytosis. Leukocytosis frequently occurs when systemic or local infections (usually due to bacteria) are present. Counts for infections are highly variable. Examples of some infections and their representative white cell counts are as follows:

Dyscrasia (the diseased condition) of blood-forming tissues, such as occurs in leukemia (due to a malfunctioning of lymph and marrow tissues) also results in leukocytosis, with extremely high white cell counts. These white cell counts sometimes exceed 1,000,000/mm 3 .

Other physiological conditions that can cause leukocytosis and a white cell count as high as 15,000/mm 3 may occur as follows:

Shortly after birth
During late pregnancy
During labor
Accompanying severe pain
After exercise or meals
After cold baths
During severe emotional upset
An abnormally low count, known as leukopenia, may be caused by the following conditions:

Severe or advanced bacterial infections (such as typhoid, paratyphoid, and sometimes tularemia), or when the bacterial infection has been undetected for a period of time (as with chronic beta streptococcal infections of the throat).

Infections caused by viruses and rickettsiae, such as measles, rubella, smallpox (until the 4th day), infectious hepatitis, psittacosis, dengue, tsutsugamushi fever, and influenza (when it may fall to 1,500/mm 3 , or shift to leukocytosis if complications develop).

Protozoal infections (such as malaria) and helminthic infections (such as trichinosis). (For example, with victims of malaria, slight leukocytosis may develop for a short time during paroxysm (the sudden intensification of symptoms). Shortly thereafter, however, leukopenia ensues.)

Overwhelming infections when the body's defense mechanisms break down.

Anaphylactic shock


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