Evaluation of a properly prepared blood smear by the trained human eye is an absolutely necessary compliment to machine evaluation
of peripheral blood in all species. From the first generation table top complete blood count (CBC) analyzers to machines that
fill half a room and cost hundreds of thousands of dollars, a machine generated CBC has numerous limitations. These limitations
produce holes in the data that could be derived from the peripheral blood that, if left unfilled, can lead to misdiagnosis
(including non-diagnosis), inappropriate therapy, or other preventable errors that result in inadequate patient care. This
lecture is meant to be a guideline that can be used to navigate your way through a blood smear and get all of the important
information on a routine basis. The role of the technician in this process is indispensable, as they often provide the initial,
rapid evaluation of samples that they obtained and prepared. Initial treatments and choice of advanced diagnostics can hinge
on the information provided. In depth pathophysiology, information treatment information, etc. can be found in any number
of excellent sources listed in the references at the end.
The first step to evaluation of the peripheral blood is to obtain a sample. Ethylenediaminetetraacetic acid (EDTA) is the
anticoagulant of choice for white blood cell (WBC) morphology. Citrate can be used if platelet or white blood cell (WBC) clumping
occurs. If possible collection directly into a vacuum tube is preferred, using the largest needle size possible to minimize
hemolysis. Use the largest free-flowing vein possible to minimize hemolysis. Fill tube to full, as dictated by the amount
of vacuum in the tubes.
Once a sample is obtained, several smears should be made from the fresh sample. Preparing a diagnostic blood smear is something
that takes practice, yet over time will become almost a reflex. A properly made blood smear is indispensable to a complete
examination. Place a new clean slide on a firm, level surface. Using a sample that is grossly free from clots, place 2-3mm
drop of well-mixed blood near one end of the slide or label. Keeping the spreader slide at a 30º angle, it is backed into
the drop of blood. Allow the drop to spread towards, but not TO, the sides of the slide. In one smooth, firm motion, move
the spreader slide directly toward the end of the slide and off the edge. The smear produced should be bullet shaped and not
reach the end of the slide
Figure 1- The spreader slide is backed into the drop of blood until the two touch. Capillary action will spread the drop towards the
edge of the slide.
Figure 2- Before the drop reaches the edge of the slide, the spreader slide is pushed forward in a single smooth motion, streaking out
the drop of blood into a bullet shaped smear that does not reach the end of the slide.
Inappropriate smears include those with chatter in them, smears that reach the edge(s) or end of the, smears that abruptly
end, smears that curve off to one side.
Microscopic evaluation begins with a low power (10x or 20x) scan (Figure 4). This initial scan allows rapid evaluation of
the whole smear that is an important part of the overall evaluation, even though this scan should take no more than 30 seconds
Figure 3- One pattern of low power scan that encompasses all major components of the smear