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BLOOD SMEAR BASICS
JENNIFER A. NEEL, DVM, DACVP (CLINICAL)
ASSOCIATE PROFESSOR, CLINICAL PATHOLOGY
NC STATE COLLEGE OF VETERINARY MEDICINE
RALEIGH, NC, 27607
Introduction Although tremendous advances have been made in the field of point-of-care hematology analyzers, examination of a well prepared, well stained blood smear remains the cornerstone of veterinary diagnostic hematology. Even the most sophisticated hematology instruments are unable to consistently provide accurate differential cell counts, and no analyzer is capable of accurately identifying morphology changes, hemoparasites, neoplastic cells, etc. This review will cover the basics of how to approach blood smear evaluation in a consistent and systematic manner and will focus on recognition of clinically significant findings.
Making a quality blood smear Although there are several techniques described for making blood smears, most people use the wedge or push technique. Always start with room temperature, well-mixed, clot free, EDTA anticoagulated blood (heparin is used for some exotic species). If the blood has been refrigerated, allow it to return to room temperature. The best slides to use for making blood smears are the premium, pre-cleaned kind; those with a frosted end also facilitate easy labeling. If you are not using premium pre-cleaned slides, you will need to wipe off each slide you plan to use for making the blood smear, including spreader slides, in order to eliminate glass grit and dust which can ruin your smear. Always mix the specimen immediately before making the smear by gently rolling the tube several times to ensure good cellular distribution, don’t shake or invert. To transfer blood from the tube to the slide, fill a plain microhematocrit tube with blood by capillary action and place a finger over the end to prevent the blood from running out. Do NOT try to dab blood from the cap onto the slide or use a 1 ml plastic pipette; the microhematocrit tube will give you better control and, if you have a microhematocrit centrifuge, you can simply plug the clean end after making your slide and spin the tube for your hematocrit and total protein. Place an approximately 4mm diameter drop of blood on one end of the slide. If there isn’t enough blood in the microhematocrit tube to produce the required droplet of blood, DO NOT repeatedly tap the tube onto the glass to try and get enough blood out; go back and get some more from the EDTA tube. Holding the spreader slide at an approximately 30-45º angle, back into the drop of blood and, as soon as you see the drop spreading along the edge of the spreader slide, push the spreader slide forward in a smooth, moderately fast motion; apply only enough
A well made blood smear. The classic blood smear has a thumb print appearance. A well developed feathered edge is apparent. The dense body of the smear takes up most of the slide and there is a thin counting area which blends with the feathered edge and the dense body.
pressure to keep the spreader slide on the glass, excessive pressure will push too much blood forward without allowing development of a good body and monolayer; you should be pulling the blood along the slide, not pushing it. If the blood is very thin (severe anemia) you may need to increase the angle of the spreader slide to avoid going off the end of the slide.
A good quality smear has the following features; A dense body; this should take up about 2/3 of the entire smear and should blend smoothly into the monolayer area. A well-developed feathered edge. This edge should have a fine, feathery appearance; if there is a thick line of blood where the slide stopped, it’s an indication of a poorly made smear. A monolayer area just behind the feathered edge. This region should be noticeably thinner than the body, but should blend in with the body of the smear. Often this area is only about ½ cm wide. Prior to staining, if the slide is held up to the light, there is a rainbow effect seen just behind the feathered edge on well made smears.
Poor quality slides are typically cause by excessive downward pressure, a slow spreading motion, or wobbling of the spreader slide on the surface of the smear. Slow spreading motion tends to cause long, thin smears that lack a dense body, thin monolayer and well-developed feathered edge. The leading edge may have a line of blood instead of a thin, feathered appearance from pushing the blood instead of pulling it. Often there is streaking in the smear. Linear lines arranged horizontally to the leading edge are called hesitation marks and they indicate hesitation in the forward motion. Excessive downward pressure will produce short slides with hesitation marks and a poorly developed feather edge and monolayer region. Wobbling is often due to inexperience or trying to exert pressure rather than letting the spreader slide rest on the surface of the smear. If you are having trouble making quality slides, it can be helpful to have an experienced individual spot your technique
Slide staining Romanowsky-type stains give good nuclear and cytoplasmic detail. Red blood cells stain red-orange, nuclei stain blue-purple and cytoplasm stains blue to pink. Most commercial laboratories use some form of Romanowsky-type stain (e.g. Wright - Geimsa) and these stains give excellent result but tend to be fussy. Quick Romanowsky type stains (such as Diff-Quik) are modified versions that are inexpensive, robust, fast and easy to use and generally give very good results. However, note that mast cell and basophil granules may not stain. I recommend the standard 6/6/6 dipping protocol for blood smears; dips should be slow (1-2 seconds each) and smooth. Some people prefer to add 2-3 extra dips when examining a smear for red cell parasites as this makes the organisms stand out more, but this shouldn’t be done routinely because it imparts an excessive blueness to the slide making toxic change and polychromasia difficult to recognize.
Staining problems. The most common problems arising from use of rapid stain are 1) poor sample fixation 2) under-staining of the specimen 3) poor staining from use of weakened or exhausted stain or fixative and 4) stain contamination 5) excessive stain precipitation. Poor sample fixation. You can’t over fix a slide, but you can definitely under fix one, and under-fixed cells will lyse and fail to stain adequately. Once you move from the fixative solution to the stain solution, an under-fixed slide can’t be improved. For very thin cytology smears and for blood smears, 5 or 6 dips in the fixative solution should be adequate, but for thick cytology smears, up to 120 seconds may be required for adequate fixation. When in doubt, fix them a little longer. Under-staining of the specimen. This occurs when there is inadequate contact with the stain solutions. It can also occur if you attempt to stain several slides at once and the specimen
to 100x without worrying about oiling the 40x. It also magnifies everything more than the 40x which is helpful for morphology and cell evaluation.
Always adjust the microscope for Köhler illumination when you get ready to look at blood smears or cytology slides. It is done to ensure optimal lighting of the slide. Examining a slide with a microscope adjusted for urine sediments or fecal wet-mounts is a waste of time. The procedure is as follows;
- Focus on a slide at 10x
- Close down the field diaphragm; this is located at the base of the microscope where light comes out. Look into the microscope while you close it down, you should see the light reduced to the center of the field as you close it. If your microscope is really off, instead of seeing a central circle of light, everything may just get really dim. Inexpensive microscopes may not have a field diaphragm. If you are trying to adjust a microscope that doesn’t have one, skip the next step and simply raise the condenser almost all of the way up.
- Adjust the condenser vertically via the condenser focus knob until the circle of light has a sharply defined edge. The condenser is portion of the microscope underneath the stage and there is a knob located on one side that moves the entire unit up and down. Usually the condenser must be moved up because someone has lowered it to look at urine sediments or fecal wet mount preps.
- Center the ring of light using the silver centering screws on the condenser
- Open the field diaphragm so that the entire field is flooded with light.
- Open the condenser diaphragm about ¾ of the way. NOTE: This is actually a small fib, but it is easy to remember and works pretty well. If you want to learn more about microscopy including the technically correct last step in Köhler illumination, visit http://microscopy.fsu.edu/primer/index.html for more information.
Systematic smear evaluation Be systematic and thorough when evaluating blood smears. It actually makes the procedure more efficient and you are less likely to overlook a major finding. You might find it’s beneficial to start a small slide collection. Include a few examples of normal smears from the species you work with and examples of anemia, leukopenia and leukocytosis (especially useful if you have the actual counts to correlate with the appearance on the slide) and good examples of atypical/abnormal cells and morphology changes. These slides can be used as references or to train new individuals.
- Examine the entire feathered edge. You are looking for large things because very large items often end up on the feathered edge. Common things include platelet clumps and microfilaria, less common things include blast cells, mast cells, and macrophages, rare things include megakaryocytes or schizont laden macrophages in Cytauxzoon infections. You also want to see if a disproportionate number of the WBCs have ended up on the feathered edge – this happens with poor smear technique and is important if you are estimating cellularity vs. using a machine count.
- Estimate/confirm cell counts. If you don’t have a hematology instrument, you will need to estimate WBC density and the platelet count, and even if you do have an analyzer, you should still correlate the machine count with the slide. Estimations are crude, but are useful for detecting significant changes. For WBCs, examine the slide on 10x in the monolayer (see # for how to find the monolayer) region. Normal animals will have anywhere from 18-50 cells per 10x field in health on a well made blood smear. Each WBC corresponds to about 330 cells/μl. While typically assessed via a PCV or with an analyzer, if needed, the RBC density can also be estimated. Using the 10x objective, start at the middle of the feathered edge and move back in a straight line through the monolayer counting area and into the body of the slide. In a normal animal, the RBCs should quickly become thickly piled up after leaving the monolayer region, but in anemic animals, the cells will never pile up as well, and the counting
area will be extended. Anemic animals will also have a paler blood smear grossly, so it’s beneficial to look at the smear against a white background before putting it on the microscope.
- Find the monolayer. Start at the very edge of the feathered edge on 10x. Move back towards the body of the slide. At the leading edge of the smear, you’ll notice the blood cells break into little pools of cells with white spaces between them giving a reticulated pattern. As you move away from the feathered edge and into the monolayer, the white spaces disappear and the red cells form a smooth monolayer. Generally, it takes about 2 - 4 10x fields to get from the feathered edge into the monolayer. When you are doing your WBC and RBC morphology, you need to stay in the monolayer region. If you wander too close to the feathered edge, you’ll see the reticulated pattern forming and if you wander too deep into the smear, you’ll see the red cells start piling up on each other.
- Examine the WBCs. Classify 100 cells for a differential count. It’s often useful to just spend a few minutes looking around at what cells are present before you do a differential so you can decide who’s who. Note any major morphology changes (toxic changes, reactive lymphocytes) and any abnormal cell populations (blast cells, immature neutrophils).
- Examine the RBCs. Give an estimation (rare, mild, moderate, marked) of anisocytosis, poikilocytosis and polychromasia. Note that a well stained blood smear is needed to clearly see polychromasia. For poikilocytosis, look to see if there’s anything specific such as acanthocytes or spherocytes.
- Estimate or confirm the platelet count. Remember to check the feathered edge for platelet clumps as these will invalidate a low count. Using the 100x objective, average the number of platelet over several fields in the monolayer. Multiply this number by 20,000 to give a rough estimate of the platelet count.
Red cells Color changes. Polychromasia is recognized as larger, slightly bluer-staining red cells and is an excellent indicator of a regenerative response. Note: you will have a hard time identifying polychromasia if your smear is over-stained. Polychromasia is typically semi-quantified as rare, slight, mild, moderate or marked (can use similar criteria as with morphology quantification, see below).
The transition zone between the feathered edge on the right and the monolayer on the left. Near the feathered edge, the red cells and white spaces for a reticulated pattern. As you move deeper into the smear, white spaces disappear as the monolayer forms
Schistocytes. These are small, irregular fragments of cells and are most commonly seen in hemangiosarcoma, microangiopathies, DIC and iron deficiency.
Spherocytes. These are red cells that are round instead of flat and most commonly indicate IMHA, but can also form after removal of Heinz bodies by the spleen and low numbers are noted in fragmentation anemia. Because they are rounded, they appear smaller and darker than normal red cells and there is a lack of central pallor. Spherocytes are best appreciated in dogs due to the prominent central pallor in the normal canine RBC, and are more difficult to detect in other species. Be careful not to interpret RBCs along the feathered edge as spherocytes; cells in these areas (where there are large open spaces) tend to round up and lose their central pallor normally.
Many acanthocytes and schistocytes. On the left, several acanthocytes are indicated by the arrows. On the right , two schistocytes are indicated. Acanthocytes have irregularly spaced cytoplasmic projections that are typically blunted and may have a knob-like end. Schistocytes are irregular, torn pieces of RBC cytoplasm formed by sheering of cells.
Right; an example of spherocytes in a dog. Spherocytes classically are small, round cells that lack central pallor and appear darker red than surrounding blood cells. Sometimes, when they have all the features of spherocytes but do not appear darker than surrounding cells, they are referred to as ‘pre-spherocytes’ or ‘imperfect’ spherocytes and may also indicate immune mediated targeting of RBCs. Left; RBCs from the feathered edge of a canine blood smear. Note that the cells appear to have lost their central pallor – don’t mistake RBCs without central pallor on the feathered edge for spherocytes, this is normal. This dog also has a large platelet and, in the RBC in the center, a Babesia canis piroplasm.
Heinz bodies. These indicate oxidative damage to hemoglobin, and large numbers are seen with oxidative hemolytic anemia. They appear as small, roundish structures can protrude from the margin of the cell as a pale structure, or appear as a small pale dot near the edge of the cell. Cats often develop large Heinz bodies when it’s the result of an oxidative drug or plant; Heinz bodies seen in sick cats (renal disease, lymphoma, hyperthyroidism) tend to be smaller and present in lesser numbers.
Blister cells and keratocytes. Blister cells are red cells with a round, clear blister at the edge of the cell. When the blister ruptures, one or two thin remnants of cytoplasm are seen and it’s then called a keratocyte. These shape changes indicate oxidative damage and are most common in iron deficiency, but can also be seen in any cause of oxidative damage including toxicities (e.g. onions) or drugs. Eccentrocytes are a related shape change also indicating oxidative damage. They form the RBC cell membranes stick together and are identified by seeing cells with a pale region where membranes are fused and a darker region where all the cytoplasm has been pushed to one side. These can look like spherocytes, but careful examination will ensure you see the pale area of fused membranes. Echinocytes. These are cells that are covered with small, blunted to sharp, evenly distributed spicules; they look sort of like a sea urchin. This can be a drying artifact, in which case they will be unevenly distributed on the slide and are called crenated cells. When real, they are associated with renal disease, severe electrolyte abnormalities and snake-bite envenomation. With snake bites, virtually all cells are affected and the spicules are small and very sharp.
Heinz bodies. Left; on Romanowsky stained slides, Heinz bodies appear either as small projections from the margin of the cell, or pale stained regions on the interior of the cell. Right; with new methylene blue staining, Heinz bodies are basophilic blue dots.
Red cell parasites Mycoplasma. M. haemocanis and M. haemofelis appear as small, epicellular bacterial organisms on RBCs. They can form small rings, dots, and linear chains. Cats can also be infected with M. haemominutim which is less pathogenic and also smaller and more difficult to identify. When Mycoplsma is seen on the very periphery of the cells, it’s possible to see a slight indentation of the cell membrane. The organism can be associated with agglutination and, in cats with M. haemofelis, there is typically a strongly regenerative hemolytic anemia present unless there is a concurrent immunosuppressive disease or condition (FeLV for example). There are a few things to keep in mind with this parasite. The first is that cytologic detection has low sensitivity compared with PCR, so not seeing the organism is no reason to exclude it. Second, appearance of the organism in peripheral blood, especially in cats, can be
episodic, so it can be missed. Third; once the infected RBCs sit in EDTA for awhile, the organisms start to fall off the cells, so a freshly made smear soon after collection is best. Fourth; if your stain has a lot of precipitate or your fixative is contaminated with water causing refractile water artifact, this organism can be very difficult to distinguish from debris, so use clean stain.
Left; canine blood smear showing agglutination. Cells are present in small clusters that are easiest to appreciate on low power. Right; normal equine blood smear showing the ‘stacked coin’ appearance of rouleaux.
Mycoplasma. Left; M. haemofelis ; note the extremely small size of the organisms. Left; M. haemocanis. Note the organisms in the background that have fallen off the RBCs. Clean stain is essential when looking for this organism
Babesia canis. B. canis is a protozoal organism that forms large, piriform (tear drop) shaped merozoites in RBCs and these may be paired. They generally are about 3-5 μm in size with a dot-like nucleus and pale blue cytoplasm with a distinct cell wall. There are subspecies of B. canis and, in the US, the most common is vogeli which also happens to be the least pathogenic. The same is not true of the other subspecies which can cause more severe disease. Most cases in the US represent subclinical carrier states, although this organism can cause more severe disease in young animals, animals that are heavily infected and in those with underlying conditions. Babesia can be detected by PCR. Examination of peripheral capillary blood from the ear or nail bed may concentrate the organisms making it easier to identify them. Although B. canis is the most common large form Babesia seen in dogs, it’s not the only one; other species have been detected. Babesia gibsoni. B gibsoni is a small form of Babesia and is much more pathogenic; acute infection can cause severe hemolytic anemia, thrombocytopenia and a shock-like syndrome. It’s also been linked to dog fighting or bites from dogs involved in fighting. Dogs surviving the acute phase can show non-specific hematologic findings but often have mild anemia and thrombocytopenia. Although this organism is also detectable by PCR, making a timely and accurate diagnosis can be more urgent, thus identification of infection on a blood smear is of significant value. These organisms are much smaller than B. canis ; they are typically 1-2.5 μm in size with a very small dot like nucleus and a ring shaped or safety pin shaped membrane. The membrane is thinner and more delicate and the nucleus smaller compared with B. canis making them more difficult to see. As with B. canis , capillary blood from the ear or nail may concentrate infected RBCs. There does appear to be different isolates of this organism, and there may be different organisms with similar cytologic appearance, but all are associated with more severe disease. Staining slides with quick stains such as Diff Quik and staining them more intensely with the second stain solution (6- 6 - 9 or 6- 6 - 12) can facilitate identification. Clean stain is essential.
Cytauxzoon felis. C. felis is a devastating protozoal disease of cats that is often, but not always, fatal; timely identification with aggressive treatment is successful in a significant number of cats. Although PCR is available, it is not really of much diagnostic utility due to the rapid progression of the disease. Identification of organisms by light microscopy remains the best means of diagnosis in the clinical setting. Parasitemia is variable necessitating a thorough
Babesia. Left; B. canis showing the classic piriform (tear) shaped organism. Here they are paired and you can clearly see the nucleus, cell margin and the pale cytoplasm. Note that they are relatively large and easy to see. Right; B. gibsoni. Notice these organisms are very small and difficult to see. They are ring shaped with a dot like nucleus. This slide was stained with a quick stain (Diff Quik) with extra dips in solution II (dark blue) in order to enhance detection of the parasites.
when present without intense regeneration, underlying diseases or conditions affecting the bone marrow or spleen should be considered. Nucleated RBCs have grayish blue cytoplasm and a nucleus that varies from small, eccentric, deep blue and condensed (metarubricytes) to large with coarse chromatin and a thinner rim of more basophilic cytoplasm (rubricytes). Cats with myleoproliferative disease can have less mature stages; this is rare in dogs. Less mature nRBCs can be mistaken for lymphocytes, but compared with a lymphocyte, they have coarser chromatin, more cytoplasm and the cytoplasm is more grayish blue; lymphocytes typically have only a small amount of cytoplasm visible on one side of the nucleus.
White cell changes
Toxic change Toxic change indicates acute, systemic inflammation and, when moderate or marked, is most closely associated with toxemia from bacterial infections. Slight toxic change is common in cats and slight to mild toxic change can be seen in a variety of disease processes that are not necessarily infectious (immune mediated disease, neoplasia, drugs) or bacterial (fungal disease, protozoal infection, viruses). Toxic change consists of 4 potential components; Cytoplasmic basophilia due to retained ribosomes. This is often the last change to resolve. Döhle bodies. These are small, cytoplasmic, angular, grey-blue aggregates of retained rough ER. Cats develop these commonly and a rare Döhle body can be seen in healthy animals. Cytoplasmic vacuolation due to granule dissolution, it’s identified as cytoplasm with a frothy appearance. This is often seen in conjunction with increased basophilia but is a more serious manifestation of toxic change. Toxic granulation from retained primary granules. This is a severe change and is rarely seen in small animals (I’ve yet to see a convincing case in a dog or cat), is seen occasionally in large animals, and is most commonly seen in avian and reptile species. Toxic granules are pink- purple in color.
Nucleated RBCs from a canine blood smear. Left; lower power view of one metarubricyte (right), one late stage rubricyte (left) and a lymphocyte (bottom right). Metarubricytes have smooth, condensed nuclei that may be partially extruded from the cell. Late stage rubricytes have slightly larger nuclei that have not undergone condensation yet. Nucleated RBCs have a darker nucleus and more abundant gray-blue cytoplasm compared with lymphocytes which have lighter nuclei and a scant amount of paler cytoplasm. Note there are also many spherocytes present, a few polychromatophilic cells and one HJ body. Right; same dog showing one late stage rubricyte and two earier stages of rubricytes; earlier stages are larger with larger nuclei and less cytoplasm, but the chromatin pattern and cytoplasm color are similar. There is also a band neutrophil present.
Grading of toxic change is subjective and depends on the features seen, the strength of the feature and the number of cells affected. In general, the level of importance is Döhle bodies < basophilia < vacuolization < toxic granulation, but, remember, toxic granulation is almost never seen in small animals. It’s also important to remember when judging basophilia that 1) immature neutrophils (bands or earlier) are more basophilic normally so they are not the best cells to judge by and 2) over-stained slides make basophilia difficult to assess.
Left shifting Left shifting is an important indicator of acute inflammation. Some hematology analyzers attempt to detect immature neutrophils, but none do it reliably. Band neutrophils are the most common stage identified and they are characterized by a horse-show shaped nucleus lacking the normal lobulation seen in mature cells. Note that there are differences between laboratories regarding the distinction between mature neutrophils and bands. Some labs are conservative and will only call a cell a band if it lacks any indentation at all (must be smooth horse-shoe shape with parallel cell walls), while others are more liberal and will call bands even if there is some indentation as long as it is mostly horseshoe shaped as lon. Less mature forms than bands (metamyelocyte stages and earlier) are characterized by a more bean-shaped nucleus that, in very immature stages, may totally lack any indentation.
Canine toxic change. Left; a neutrophil shows increased basophilia, vacuolization and many small blue gray Döhle bodies (image courtesy of Ms. Tillie Laws). Right; similar changes but fewer, smaller Döhle bodies.
Left shifting. Right; a band is present at the top, in the middle is a mature segmented cell and at the bottom there is a non- filamentous segmented cell meaning a cell that is midway between being a mature segmented cell and a band. Middle; a band cell characterized by a smooth, evenly sized horse-shoe shaped nucleus compared with a mature cell. Left; on the bottom is a band cell while on top there is an early metamyelocyte, the stage before bands.
Blast cells in circulation can be an important indicator of underlying neoplasia, recurring neoplasia or stage of lymphoma. Note that with a severe inflammatory leukocytosis with a significant left shift, a few myeloblasts may be seen as part of the bone marrow response, but in the absence of these findings, or when blast cells are present in disproportionate numbers compared with other neutrophil stages, underlying neoplasia is a concern. Blasts are characterized by cells that are medium to large in size with pale chromatin, a visible nucleolus or nucleolar ring and, typically, a scant rim of cytoplasm. Other features may be present depending on the origin of the cell. Often small cytoplasmic fragments are also found in circulation. They are small platelet sized fragments of cytoplasm – they can be distinguished from platelets because they lack normal platelet granulation.
In small animals, neoplastic blast cells in peripheral blood are mostly lymphoid in origin, but acute myeloid leukemias do occur. Acute lymphocytic leukemia arises in the bone marrow and is typically associated with very high blast cell counts, but aleukemic and subleukemic leukemias with few or no blast cells in circulation can happen. Lymphomas arise in peripheral tissues and are associated with variable, but typically much lower, blast cell counts. Chronic lymphocytic leukemia is characterized by small to medium sized lymphocytes that lack overt blastic features. Sometimes ALL and end-stage lymphoma are difficult to distinguish.
Sometimes finding blast cells can be tricky. They are often very fragile cells so it’s not uncommon for them to all rupture in the monolayer. Sometimes your first clue that they may be present is seeing very large ruptured cells in the monolayer and on the feathered edge of the smear. If you see this and can’t find intact cells in the monolayer, try moving deeper into the slide along the lateral edges of the smear; often these areas are thin enough to see the cells but thick enough to protect them from getting broken. Another good place to look is the spot where you first put a drop of blood onto the slide to make a smear; often this area is also thinner but protected.
Mast cells Mast cells can be found in circulation secondary to mast cell neoplasia or severe inflammation, especially of the GI tract. Often they are found on the feathered edge of the smear, but may be present in the monolayer when in high numbers. These are large cells with round nuclei that are filled with purple granules; in dogs, there is often a smudged purple ‘halo’ around the cells from partial degranulation, cats don’t tend to do this. Don’t mistake these cells for basophils. Basophils have band-
Blast cells. Left; the blast cells are large and variably shaped; they are seen molding to each other and to adjacent RBCs causing distortion of the cell shape. The chromatin is very pale and delicate. Nucleoli were faintly visible on higher power. Right; these blast cells are also large but exhibit less molding, have darker, coarser chromatin and more robustly visible nucleoli and nucleolar rings.
like nuclei and few dark blue granules (dogs) or many pale gray-blue granules (cats). Remember mast cell granules may not stain with quick stains.
Mast cells. Left; low power appearance of mast cells on the feathered edge of a blood smear; cells appear as dark purple spots. Right; a high power image of a mast cell. These cells are characterized by a round nucleus and purple granules. Note the smudged purple ‘halo’ streaming away from the cell to the left; this is typical of canine mast cells seen on blood smears; cats do not tend to have a hale.
Platelets and miscellaneous
Confirm/estimate count This is an important step, especially in animals with low platelet counts from a hematology analyzer. Most analyzers can’t reliably detect platelet clumps and will give a falsely low number when clumps are present. Although this is more common in cats, it also happens in dogs. Clumps are easiest to identify on the feathered edge of the smear. They have a pale pink/blue stippled appearance. Platelet clumps always invalidate hematology analyzer or hemocytometer counts. If clumps are not present, you should perform an estimate in the monolayer region by averaging the number of platelets present in several 100x fields and multiplying that by 20,000. In general, if there is a large discrepancy between an analyzer count and your estimate (as long as there are no clumps present!), most people trust
Left; low power view of platelet clumps which appear as pale blue material stippled with pink granules. Platelet clumps invalidate platelet counts and estimates. Right; a giant platelet is present above the band neutrophil. Giant platelets may not be counted correctly; they suggest a regenerative response to thrombocytopenia.
