when it becomes dark blue or when film forms on the surface.) NOTE: Rinse time is critical and must be shorter than the stain time. 5. Drain excess water and wipe the back of the slide to reduce background color. 6. Place slide in horizontal position on table and allow to air dry. NOTE: Do not accelerate drying time by placing slide on a warmer or in front of a fan. The film of water on the slide is important for the color development. 7. Once the slide is dry, proceed to step 3, counting the cells. COUNTING THE CELLS.—Once the blood smear has been stained, it is placed under a microscope, and the differential count is conducted. 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. 7-23 DROP OF IMMERSION OIL HM3f0719 THIS THIN AREA OF THE BLOOD SMEAR IS FOR IDENTIFYING THE CELLS Figure 7-19.—Placement of immersion oil on blood smear. HM3f0720 Figure 7-20.—Counting path for differential count. Example: Patient has a total white cell count of 8,000. Differential count shows 20% leukocytes. Multiply: 8,000 x 0.20 (20%) = 1,600 Patient has 1,600 lymphocytes/mm³