To perform a differential white cell count, you should follow the steps listed below:

1. Place the slide under the microscope. Switch the oil immersion objective (red) (100X) into position above the stage. Turn the coarse adjustment to raise the oil immersion objective about 1 inch above the opening in the stage. Open the condenser and switch on the microscope light.

2. Place a large drop of immersion oil on the thin area of the blood smear. See figure 7-19.

3. Hold the slide so the thin area is on your left. Then fix the slide firmly in the jaws of the mechanical (movable) stage. Move the mechanical stage so the drop of oil on the slide is directly over the bright light coming up from the condenser.

4. Using the coarse control knob, you should now slowly lower oil immersion objective into the drop of oil (on the slide). When the objective is in the drop of oil, continue turning the coarse adjustment until the objective is touching the glass slide.

5. Now, while continually looking through the eyepiece, VERY SLOWLY rotate the coarse adjustment toward you until you see some cells. After you have brought the cells into view with the coarse adjustment, bring the cells into perfect focus by rotating the fine adjustment. NOTE: Always rotate the fine adjustment back and forth when identifying cells. This step will help you see the various layers of the cell and thereby help you to identify the different types of white cells.

6. Count 100 consecutive white cells, pressing the correct key on the cell counter for each type of white cell identified. (If the cell counter is not available, record cell type and number of cells encountered on a piece of paper.) Follow path similar to one illustrated in figure 7-20 to count cells.

7. Total each type of white cell. If you count 20 lymphocytes among the 100 cells, the differential count for lymphocytes is 20%. Continue this process until your count totals 100%. This differential count is referred to as a relative count. Another differential count that may be requested is an absolute count. To perform an absolute count, multiply the total white cell count by the individual cell percentages. See the example below.

Figure 7-19.-Placement of immersion oil on blood smear.

Figure 7-20.-Counting path for differential count.

NOTE: When performing the white cell count, you may observe abnormal white cells such as distorted lymphocytes, smudge cells, and disintegrated cells.

Distorted lymphocytes, which appear squashed or distorted, are caused by excessive pressure on the cell during the process of making the smear. Distorted cells should be recorded as normal lymphocytes. Smudge cells are white cells that have ruptured and only the nucleus remains. Afew smudge cells may be found in a normal blood smear. Smudge cells should not be added to the count or recorded. Disintegrated cells are ruptured cells, but the nucleus and cytoplasm still remain. Disintegrated cells should not be counted as one of the 100 cells, but should be recorded on the report as "disintegrated cells."

8. Once the differential count is completed, proceed to step 4, reporting the count.

NOTE: If it is desirable to save a smear for reexamination, remove the immersion oil by placing a piece of lens tissue over the slide and moistening the tissue with xylene. Draw the damp tissue across the slide, and dry the smear with another piece of lens paper.