To perform a differential count, the following materials are required:

Four plain glass microscope slides, clean and dry
Wright-Giemsa stain solution (follow manufacturer's directions for use and storage)

Staining containers
Deionized or distilled water
Microscope with light source
Immersion oil
Blood cell counter

Differential Count Procedure
The procedure for the differential white cell count is done in 4 steps:

Step 1: Making the blood smear
Step 2: Staining the cells
Step 3: Counting the cells
Step 4: Reporting the count

Each step of this procedure will be discussed in the following sections.

MAKING THE BLOOD SMEAR.-The simplest way to count the different types of white cells is to spread them out on a glass slide. The preparation is called a blood smear. There are two methods of making a blood smear: the slide method (covered in this chapter) and the cover glass method.

It is very important to make a good blood smear. If it is made poorly, the cells may be so distorted that it will be impossible to recognize them. You should make at least two smears for each patient, as the additional smear should be examined to verify any abnormal findings.

To prepare a blood smear for a differential count, follow the steps below:

1. Using a capillary tube, collect anticoagulated blood from a venous blood sample.

2. Deposit a drop of blood from capillary tube onto a clean, grease-free slide. Then place the slide on a flat surface, blood side up.

3. Hold a second slide between your thumb and forefinger and place the edge at a 23-degree angle against the top of the slide that holds the drop of blood (see figure 7-18, view A). Back the second slide down until it touches the drop of blood. The blood will distribute itself along the edge of the slide in a formed angle (see figure 7-18, view B).

4. Push the second slide along the surface of the other slide, drawing the blood across the surface in a thin, even smear (see figure 7-18, view C). If this is done in a smooth, uniform manner, a gradual tapering effect (or "feathering") of the blood will occur on the slide. This "feathering" of the blood is essential to the counting process and is the principal characteristic of a good blood smear (see figure 7-18, view D). When you are making the smear, prevent blood from reaching the extreme edges of the slides. Allowing the smear to reach the edges of the slide will aggravate the tendency of large cells to stack up on the perimeter of the smear. Asmear with wavy lines or blanks spots should be discarded, and a new smear made.

5. Once the blood smear is made, let it dry (it will take a few minutes). Then write the patient's name in pencil on the bottom edge of the slide, as illustrated in figure 7-18, view D). Proceed to step 2, staining the cells.