Methods of
Hematology

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5-2. WHITE BLOOD CELL COUNT

a. Principle. A sample of whole blood is mixed with a weak acid solution that lyses nonnucleated red blood cells. Following adequate mixing, the specimen is introduced into a counting chamber where the white blood cells (leukocytes) in a diluted volume are counted.

b. Reagent. White-count diluting fluid. Either of the following diluting fluids may be used:

(1) Two percent acetic acid. Add 2 ml glacial acetic acid to a 100 ml volumetric flask. Dilute to the mark with distilled water.

(2) Onepercenthydrochloric acid. Add 1 ml hydrochloric acid to a 100 ml volumetric flask. Dilute to the mark with distilled water.

c. Procedure.

(1) Draw well-mixed capillary or venous blood exactly to the 0.5 mark in a white blood cell diluting pipet. This blood column must be free of air bubbles.

(2) Wipe the excess blood from the outside of the pipet to avoid transfer of cells to the diluting fluid. Take care not to touch the tip of the pipet with the gauze.

(3) Immediately draw diluting fluid to the "11" mark while rotating the pipet between the thumb and forefinger to mix the specimen and diluent. Hold the pipet upright to prevent air bubbles in the bulb.

(4) Mix the contents of the pipet for 3-5 minutes to ensure even distribution of cells. Expel unmixed and relatively cell-free fluid from the capillary portion of the pipet (usually 4 drops).

(5) Place the forefinger over the top (short end) of the pipet, hold the pipet at a 450 angle, and touch the pipet tip to the junction of the cover glass and the counting chamber.

(6) Allow the mixture to flow under the cover glass until the chamber is completely charged. Similarly, fill the opposite chamber of the hemacytometer.

NOTE: If the mixture overflows into the moat or air bubbles occur, clean and dry the chambers, remix the contents of the pipet, and refill both chambers.

(7) Allow the cells to settle for about 3 minutes. Under low-power magnification and reduced light, focus on the ruled area and observe for even distribution of cells.

(8) Count the white cells in the four 1 sq mm corner areas corresponding to those marked A, B, C, and D of Figure 5-1 in each of two chambers.

(9) Count all the white cells lying within the square and those touching the upper and right-hand center lines. The white cells that touch the left-hand and bottom lines are not to be counted. In each of the four areas, conduct the count as indicated by the "snake-like" line in figure 5-1. A variation of more than 10 cells between any of the four areas counted or a variation of more than 20 cells between sides of the hemacytometer indicate uneven distribution and require that the procedure be repeated.


Figure 5-1. Hemacytometer counting chamber (WBCs). Areas marked A, B, C, and D are used to count white blood cells.

d. Calculation.

(1) Routinely, blood is drawn to the 0.5 mark and diluted to the 11 mark with WBC diluting fluid. All the blood is washed into the bulb of the pipet (which has a volume of 10). Therefore, 0.5 volumes of blood are contained in 10 volumes of diluting fluid. The resulting dilution is 1:20. (These figures are arbitrary and refer strictly to dilution and not to specific volumetric measurements.)

(2) The depth of the counting chamber is 0.1 mm and the area counted is 4 sq mm (4 squares are counted, each with an area of 1.0 sq mm therefore, 4 x 1.0 sq mm = a total of 4 sq mm). The volume counted is: area x depth = volume. Four sq mm x 0.1 mm = 0.4 cu mm.

(3) The formula is as follows:

 
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(4) For example:

First Chamber
Cells counted in each square
Second Chamber
Cells counted in each square
35 45
40 37
44 36
39 44
158 WBCs counted 162 WBCs counted

Calculate the average number of WBCs per chamber:

equ0502.gif (1270 bytes)

Calculate the number of WBCs per cubic mm:

equ0503.gif (1256 bytes)

e. Sources of Error.

(1) Improper collection of blood specimens causes variable results.

(2) Wet or dirty pipets.

(3) Poor condition or inaccurate calibration of pipets. Pipets must be in good condition and calibrated to have maximum error of 1 percent.

(4) Poor pipetting technique causes high or low counts. Poor pipetting technique includes:

(a) Undershooting desired line with blood or diluting fluid.
(b) Overshooting desired line with blood or diluting fluid.
(c) Air bubbles in the column on bulb.
(d) Failure to wipe tip free of blood.
(e) Too slow manipulation following the withdrawal of the specimen thus, allowing some of the blood specimen to coagulate.
(f) Failure to mix the blood and diluent properly.

(5) Failure to expel 2 or 3 drops in the pipet tips before charging the hemacytometer.

(6) Overfilling the chamber of the hemacytometer, which causes erroneously high counts.

(7) Wet or dirty cover glasses and hemacytometers.

(8) Uneven distribution of cells in the counting chamber causes erroneous results.

(9) Inaccuracy or carelessness in marking counts.

(10) Diluent that which is cloudy or contains debris.

(11) Failure to mix anticoagulated blood thoroughly before use.

f. Discussion.

(1) The available error when four large squares are counted is +20 percent. Counting eight large squares decreases the error to +15 percent.

(2) The importance of clean, dry diluting pipets cannot be stressed too much as the greatest source of error in the counting of WBC is the use of wet and/or dirty pipets.

(3) The counting chamber must be scrupulously clean and free of debris that might be mistaken for cells.

(4) The minimum blood sample recommended for performing routine white blood cell counts is that obtained using one pipet and counting two chambers as previously outlined.

(5) In cases where the WBC count is exceptionally high, as in leukemia, the dilution should be made in the red blood cell diluting pipet. The blood is drawn to the “1.0” mark and the diluting fluid is drawn to the “101” mark. The resulting dilution is 1:100.

(6) In cases of leukopenia, the white pipet should be filled to the “1.0” mark and diluted to the “11” mark with 2 percent acetic acid. The resulting dilution is 1:10.

(7) If nucleated erythrocytes are present, the count is corrected by the following formula:

equ0504.gif (1533 bytes)

The percent nucleated erythrocyte is obtained from the differential count, which is discussed in another course.

g. Normal Values.

(1) Adults (both sexes): 4,500-11,500 WBCs per cu mm.

(2) Childhood: 6,000-14,000 WBCs per cu mm.

(3) Birth: 9,000-30,000 WBCs per cu mm.

h. Unopette Procedure for White Blood Cell Count.

(1) Follow procedure described in para 2-5 for blood dilution.

(2) Prepare diluted specimen for count.

(a) Mix diluted blood by inverting reservoir to resuspend cells.

(b) Convert to dropper assembly by withdrawing pipet from reservoir and reseating securely in reverse position.

(c) To clean capillary more, invert reservoir, gently squeeze sides, and discard first three to four drops.

(d) Carefully load hemacytometer with diluted blood by gently squeezing sides of reservoir to expel contents until chamber is properly filled.

(e) Place hemacytometer in moisture chamber, let stand for 3 to 5 minutes (10 minutes for platelets) to allow cells to settle.

(3) Counting and calculation of leukocytes.

(a) Under 100X (low power) magnification, count leukocytes in all nine large squares of the counting chamber.

(b) Add 10 percent of count to total number of cells counted. This step simplifies the calculation that actually entails dividing the number of cells by the number of squares counted and multiplying by 10 to correct for the depth of the chamber.

(c) Multiply this figure by 1000 to get total leukocyte count.


Curriculum design: David L. Heiserman
Publisher: SweetHaven Publishing Services

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