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Venipuncture is discussed in detail in Chapter 13, Venipuncture. The best CBC sample is venous blood drawn with a 22-gauge or larger needle. For a routine CBC, venous blood must be placed in a special hematology lab tube, usually a purple top tube, containing an anticoagulant (EDTA) with which the blood must be mixed gently. Blood for a CBC should be fresh, < 3 h old. Most samples for coagulation studies are submitted in a blue top (citrate) tube. (See Table 13–8 for detailed description of blood collection tubes.) If a capillary fingerstick or heelstick (see Heelstick and Fingerstick) is used, the hematocrit may be falsely low. If the finger has to be “milked,” sludging of the RBCs can create a falsely high hematocrit. Wright staining can also be done and viewed as outlined in the next section.

Most clinical labs perform automated cell counts. The formal blood smear and Wright stain can provide a manual differential leukocyte count for the evaluation of anemia and other conditions. The slide is usually available for review by students and house staff. The main benefit is to allow identification of abnormal cells and other subtleties that may not be detected with automated systems (Figure 5–1).

Figure 5–1.

Technique of preparing a blood smear for staining and distribution of white blood cells on the standard smear.

Viewing the Film: The Differential WBC

  • 1. Examine the smear in an area where the red cells approximate one another but do not overlap.
  • 2. If the film is too thin or if a rough-edged spreader is used, as many as 50% of the WBCs may accumulate in the edges and tail (see Figure 5–1).
  • 3. WBCs are not randomly dispersed even in a well-made smear. Polys and monos predominate at the margins and tail, and lymphs are prevalent in the middle of the film. To overcome this problem, use the “high dry” or oil immersion objective and count cells in a strip running the entire length of the film. Avoid the lateral edges of the film.
  • 4. If fewer than 200 cells are counted in a strip, count another strip until at least 200 are seen. The special white cell counter found in most labs is ideal for this purpose. In patients receiving chemotherapy, the total count may be so small that only a 25–50 cell differential is possible.
  • 5. In smears of blood from patients with very high white counts, such as those with leukemia, count the cells in any well-spread area where the different cell types are easy to identify. Table 5–1 shows the correlation between the number of cells in a smear and the estimated white cell count. Estimate the platelet count by averaging the number of platelets seen in 10 hpf (under oil immersion) and multiplying by 20,000.


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