Clean, high-quality glass microscope slides are essential to blood smear preparation. Glass slides should be free of chipped edges, glass fragments, dust, powder, fingerprints, and other debris, as these can affect blood spreading. Slides with a frosted end (Figure 1) can be labeled with the date and the patient’s identification information, which should be written with a pencil or water- and solvent-resistant laboratory marker. Traditional ballpoint pens and markers should be avoided, as the ink typically washes off during the staining process. Two clean glass slides should be used as the sample slide and the spreader slide. Slides should be handled by holding the frosted end; touching the short, nonfrosted end of the spreader slide should be avoided.
Blood smears should be prepared using well-mixed EDTA-anticoagulated blood. The sample can be mixed by gently inverting the tube several times before slide preparation. To achieve the appropriate sample volume for spreading (see Step 2), using a microhematocrit capillary tube rather than a pipette to place a drop of the sample on the glass slide is recommended.
Three factors can be altered to achieve the desired blood smear quality: size of the blood drop, angle of the spreader slide, and speed of spreading. If a drop is too small, the smear may not be long enough for adequate diagnostic evaluation; if the drop is too large, the smear may extend beyond the edge of the slide, which can cause clumps of cells to be lost and prevent formation of the feathered edge needed for microscopic evaluation.1 An angle greater than 30° for the spreader slide results in a thicker and shorter preparation, whereas a smaller angle results in a thinner and longer smear. Speed of the spreader slide can also affect smear length and thickness. Fast spreading can result in a shorter or thinner blood smear preparation, whereas slower spreading can result in a longer or thicker preparation.1,2
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Blood viscosity also affects slide preparation. For instance, a patient with an increased hematocrit will have increased blood viscosity that can result in a preparation that is too thick. Decreasing the angle of the spreader slide for such cases can help achieve a thinner preparation that is more suitable for microscopic evaluation. Similarly, an anemic patient will have decreased blood viscosity, resulting in a thin preparation. For such cases, increasing the angle of the spreader slide will yield a thicker preparation.1,3