PREPARATION OF BLOOD SMEAR (WEDGE METHOD)
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(2) A 'spreader' slide is placed at an angle of 30° in front of the drop and then drawn back to touch the drop of blood. Blood spreads across the line of contact of two slides.
(3) Smear is made by smooth, forward movement of the 'spreader' along the slide. The whole drop should be used up 1 cm before the end of the slide. The length of the smear should be about 3 cm. The 'spreader' should not be raised above the slide surface till whole drop of blood is spread out.
(5) Patient's name or laboratory number and date are written (with a lead pencil, a permanent marker pen, or a diamond pencil) on the thicker end of the smear.
(6) The smear is fixed immediately with absolute methyl alcohol (which should be moisture- and acetone-free) for 2-3 minutes in a covered jar (Absolute ethyl alcohol can also be used, but not methylated spirit as it contains water). Aim of fixation is to prevent washing off of the smear from the slide. Following this, color of the smear becomes light brown. This fixation is desirable even when Leishman stain is used which contains methyl alcohol. This is because Leishman stain may have absorbed moisture leading to poor fixation. If methanol is contaminated with water, sharpness of cell morphology is lost and there is vacuolation of red cells. Methanol should be acetone-free since acetone washes out nuclear stain. (In many laboratories, slide is stained immediately after air-drying without prior fixation, and the results are satisfactory; however, if delay of >4 hours is anticipated between air-drying and staining, the slide should be fixed. If not, a background gray-blue staining of plasma occurs).
(1) Making a 'spreader' slide—a glass slide with absolutely smooth edges should be selected and one or both corners at one end of the slide should be broken off. The 'spreader' slide should be narrower (width of about 15 mm) so that edges of the smear can be examined microscopically. The 'spreader' slide should be discarded after use. If the same is to be reused, its edge should be thoroughly cleaned and dried (otherwise carryover of cells or parasites can occur).
(2) A well-spread blood smear (a) is tongue-shaped with a smooth tail, (b) does not cover the entire area of the slide, (c) has both thick and thin areas with gradual transition, and (d) does not contain any lines or holes.
(3) By changing the angle of the 'spreader' and its speed, thickness of the blood smear can be controlled. In patients with anemia, a thicker smear can be obtained by increasing the angle and the speed of spreading. In patients with polycythemia, a thinner smear is obtained by decreasing the 'spreader' angle and the speed of spreading.
(5) It is recommended to stain blood films in reagent filled Coplin jars (rather than covering them with the staining solution) to avoid formation of stain precipitates due to evaporation.
(6) A drawback of this method is uneven distribution of leukocytes (i.e. monocytes, neutrophils, and abnormal cells are pushed towards the extreme tail end of the smear) and distortion of red cell morphology at the edges.
(7) Blood smear is covered with a coverslip and mounted in a mounting medium (e.g. DPX) for protection against mechanical damage and deterioration of staining with time on exposure to air.
STAINING OF BLOOD SMEAR
Blood smears are routinely stained by one of the Romanowsky stains. Romanowsky stains consist of a combination of acidic and basic dyes and after staining various intermediate shades are obtained between the two polar (red and blue) stains. Romanowsky stains include May-Grunwald-Giemsa, Jenner, Wright's, Leishman's, and Field's stains. Staining properties of the Romanowsky stains are dependent on two synthetic dyes: methylene blue and eosin. International Committee for Standardization in Haematology has recommended a highly purified standardized stain, which contains azure B and eosin Y; it, however, is very expensive. Romanowsky stains are insoluble in water but soluble in methyl alcohol. Methyl alcohol acts as a solvent as well as a fixative. Staining reaction is pH-dependent. These stains have a tendency towards precipitation and should be filtered before use.
Methylene blue and azure B are basic (cationic) dyes and have affinity for acidic components of the cells (like nucleic acids or basophil granules) and impart purpleviolet color to the nuclear chromatin, dark blue-violet color to the basophil granules, and deep blue color to the cytoplasm of lymphocytes. Eosin is an acidic (anionic) dye and has affinity for basic components like hemoglobin (stained pink-red), and granules in eosinophils (stained orange-red). Neutrophil granules are slightly basic and stain violet-pink or lilac.
Romanowsky stains impart more colours than just blue (from methylene blue or azure B) and red-orange (from eosin Y). Usefulness of the Romanowsky stains lies in their ability to differentially stain leucocyte granules.
Method of Leishman staining is given below:
(1) Leishman stain: William Boog Leishman, a British pathologist, modified the original Romanowsky method and devised a stain which is widely known as Leishman's stain. This consists of methylene blue and eosin dissolved in absolute methyl alcohol. Commercially available Leishman stain powder (0.6 gram) is mixed with water-free absolute methyl alcohol (400 ml). Prepared stain should be kept tightly stoppered in a brown bottle and stored in a cool, dark place at room temperature. Exposure to direct sunlight causes deterioration of the stain. After preparation, stain should be kept for 3-5 days before using since it improves the quality of the stain.
(2) Buffered water (pH 6.8).
(1) Air-dry the smear and fix with methanol for 2-3 minutes.
(2) Cover the smear with Leishman stain for 2 minutes.
(3) After 2 minutes, add twice the volume of buffered water and leave for 5-7 minutes. A scum of metallic sheen forms on the surface.
(4) Wash the stain away in a stream of buffered water. Tap water can also be used for washing if it is not highly alkaline or highly acid.
(5) Wipe the back of the slide clean and set it upright in the draining rack to dry.
(6) Mount the slide in a suitable mounting medium (e.g. DPX) with a clean and dry 25 × 25 mm coverslip.
• Red cells: pink-red or deep pink
• Polychromatic cells (Reticulocyt-es): Gray-blue
• Neutrophils: Pale pink cytopla-sm; mauve-purple granules
• Eosinophils: Pale-pink cytoplasm; orange-red granules
• Basophils: Blue cytoplasm; dark blue-violet granules
• Monocytes: Gray-blue cytoplasm; fine reddish (azurophil) granules
• Small lymphocytes: Dark blue cy-toplasm
• Platelets: Purple
• Nuclei of all cells: Purple-violet