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Lesson 6-1 Hemocrit


The hematocrit or the packed-cell volume is the percentage of the total volume of red blood cells in relation to the total volume of whole blood. The term "hematocrit" is derived from two Greek words: Hemato, meaning blood; and Krites, meaning to judge (then reduced to crit, meaning to separate). The procedure is performed by filling a capillary tube with blood and centrifuging at a constant speed for a constant period of time. The packet cell volume is then measured. The hematocrit can also be determined by automated sequential analyzers but is usually a calculated value.

The hematocrit is the most useful single index for determining the degree of anemia or polycythemia. It can be the most accurate (2-4 percent error) of all hematological determinations. In contrast, the direct red blood cell chamber count has a percent error of 8-10 percent. The hematocrit is, therefore, preferable to the red blood cell count as a screening test for anemia. The values for the hematocrit closely parallel the values for the hemoglobin and red blood cell count.


A capillary tube is filled with whole blood by capillary action to within one to two cm of the end. The unfilled end is sealed and the tube is centrifuged. After centrifugation, the capillary tube is placed in a reading device and the hematocrit value determined.

b. Procedure.

(1) If anticoagulated venous blood is the specimen, fill a plain capillary tube with blood. If blood without anticoagulant is used, fill a heparinized capillary tube with the blood specimen. A heparinized capillary tube is identified by red line on the tube specimen. A heparinized capillary tube is identified by a red line on the tube.

(2) Allow blood to enter two capillary tubes until they are approximately 2/3 filled with blood. Air bubbles denote poor technique, but do not affect the results of the test.

(3) Seal the unfilled ends of the tubes with clay.

(4) Place the two hematocrit tubes in the radial grooves of the centrifuge head exactly opposite each other, with the sealed end away from the center of the centrifuge.

(5) Screw the flat centrifuge head cover in place.

(6) Centrifuge at 10,000 rpm for 5 minutes.

(7) Remove the hematocrit tubes as soon as the centrifuge has stopped spinning. Determine the hematocrit values with the aid of a microhematocrit reader. Results should agree within +1 percent. If they do not, repeat the procedure.

NOTE: Since there are a variety of readers available, it is necessary that the technician carefully follow the directions of the manufacturer for the particular device utilized.

c. Sources of Error.

(1) Inadequate mixing of the blood prior to sampling.

(2) Improper sealing of the capillary tube causes the blood to blow out of the capillary tube during centrifugation.

(3) Capillary tubes must be properly identified. Numbered holders for capillary tubes are available. Place the tubes in slots on the holder and record the numbers on the laboratory request slip.

(4) Improper centrifugation leads to varied results. For good quality control, maintain prescribed centrifuge speed and time.

(5) Misreading the red cell level by including the buffy coat causes elevated values.

d. Discussion.

(1) The microhematocrit technique is advantageous because of speed, and because only a small quantity of blood is necessary for the determination. An additional advantage is the ease with which this procedure is adapted to infants and small children. The microhematocrit technique requires only a simple capillary puncture whereas in the Wintrobe method venous blood must be used. Another advantage is the use of disposable capillary tubes.

(2) If the microhematocrits cannot be read promptly, the capillary tubes must be properly identified and placed in a vertical position. Slanting of the cell layer will occur if tubes are left in a horizontal position for more than 30 minutes.

(3) Hematocrit results may also be obtained or computed through use of an automated cell counter. Hematocrit calculated on the automated hematology analyzer is calculated from the determination of MCV and RBCs. The computation makes no allowance for the trapped plasma that always occurs to some degree in manual packing procedures.

e. Normal Values.

(1) Birth: 45 to 60 percent.

(2) Male: 40 to 52 percent.

(3) Female: 36 to 48 percent.

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