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).
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
- 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