Blood Smear Evaluation

Jessica Seid, DVM
angell.org/emergency
emergency@angell.org
781-902-8400

June 2024

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Introduction

A blood smear is an invaluable diagnostic tool as it provides crucial information about the hematologic status of a patient. Ideally, a blood smear should be done on every patient receiving a Complete Blood Count (CBC) to verify results and evaluate for any abnormalities that may not have been accounted for by the machine; however, it may not always be done due to various circumstances. A blood smear is especially important in situations such as when a CBC cannot be done, when results are delayed, or in situations where patients are critically ill and automated machine counts may be inaccurate and will not give information on important morphologic changes of the cells.

Making a Blood Smear

Making a good quality blood smear is the first step in the process and can make all the difference in the ability to perform accurate cell counts and blood smear evaluation. Ideally, blood smears should be made as quickly as possible after obtaining the blood sample to minimize any artifactual changes to the cells.

Steps to Making a Blood Smear

1. Place a small drop of well-mixed blood at the frosted end of a glass slide using a microhematocrit tube or needle. Ensure the slide is on a flat surface.
2. Place a 2nd spreader slide in front of the drop of blood. Held at approximately a 30-degree angle, it is drawn back into the drop of blood, resulting in a capillary spread of the blood drop across the edge of the spreader slide.
3. Once the blood drop is dispersed, the spreader slide is pushed quickly and smoothly to create the blood smear. Care should be taken not to apply pressure when advancing the spreader slide.
4. A good quality blood smear slide will have a “thumbprint” shape, be approximately ¾ the length of the slide, and have a distinct feathered edge.
5. By altering the angle of the spreader slide, the smear can be made slightly thicker or thinner. In very anemic patients, where the blood is thinner, increasing the angle of the spreader slide can aid in obtaining a good smear.
6. Let the blood smear air dry and stain. Most practices use a quick stain kit such as DiffQuik.
7. It is always a good idea to make multiple blood smear slides so you can choose which one you prefer or have the option to look at numerous slides if needed to obtain a thorough evaluation.
8. As the old adage says, “Practice makes perfect!” To get better at making blood smears, make MORE blood smears!

Types of Cells Evaluated on a Blood Smear

• Erythrocytes, Red blood cells (RBC)
• Platelets
• Leukocytes, White Blood Cells (WBC) including neutrophils, monocytes, lymphocytes,basophils, eosinophils
• Other interesting blood smear findings could include heartworms, parasite inclusions,neoplastic cells, etc.

Steps to Blood Smear Evaluation

1. Scanning the blood smear: Scan the whole blood smear using a microscope on low power (10X). Make sure there is an optimal monolayer/good area for counting cells. Evaluate RBC density and presence of any agglutination, note platelet clumps, and for WBCs, confirm that mature neutrophils are the most common cell type and generally evaluate for abnormal WBC distribution or immature cells. You can also look for large blood parasites, like heartworms. Remember to assess the feathered edge, as this is a common place for platelet clumps, parasites, and neoplastic/atypical cells.
2. Erythrocyte Evaluation: RBCs in dogs and cats are biconcave discs, with the dog’s RBC usually having central pallor (paleness). On 100X, evaluate the RBC size, shape, and color. Look for common abnormalities that can give clues to disease processes, including anisocytosis (variation in size), polychromasia (variation in color), hypochromia (increase in central pallor), spherocytes, Heinz bodies, etc., as well as for hemoparasites, clumping, or nucleated RBCs. Remember that RBCs are usually very uniform and have minimal size, shape, and color variation.
3. Platelet Evaluation: On 100X, estimate platelet numbers by counting the number of platelets in five to ten fields, calculate the average, and multiply by 10,000 to 15,000 to obtain the platelet estimate. (A reminder that platelet clumping will artifactually decrease the platelet estimate.) The presence of large platelets or mega platelets may indicate platelet regeneration secondary to destruction or consumption.
4. Leukocyte Evaluation: On 50 to 100X, perform a differential count. Look at 100 WBCs in the monolayer (using a zig-zag approach can help so you don’t count the same cells twice) and categorize/count them based on their morphology as neutrophils, lymphocytes, monocytes, basophils, eosinophils (a manual cell counter or cell phone App can be used to aid in counting). Make note of bands or toxic changes to neutrophils, reactive/immature lymphocytes, or any other cells or organisms that would not usually belong in the peripheral blood (i.e., mast cell, histiocyte.) Common leukocyte patterns may be identified, including left, regenerative, and degenerative left shifts.

Interesting Blood Smear Findings

• Heartworm microfilaria in circulation.
• Anaplasma or Ehrlichia WBC parasitic inclusions: In dogs in the acute phase of the disease, a diagnosis can potentially be made prior to the dog coming up positive on routine 4DX SNAP screening test (and while still awaiting Tick PCR testing) based on identification of characteristic WBC inclusion bodies.
• Babesia (dogs) or Mycoplasma (cats) RBC parasitic inclusions.
• Neoplastic cells in circulation, usually hematopoietic in origin
• Nucleated RBCs (nRBC) are immature RBCs released from the bone marrow early, usually seen in regenerative anemias.
• Heinz bodies, pale round RBC inclusion bodies, which, when seen in large numbers in cats, can potentially indicate certain conditions like diabetes mellitus, onion toxicity, hyperthyroidism, lymphoma, or acetaminophen toxicity.
• RBC morphology is associated with immuno-mediated hemolytic anemia (IMHA), which includes autoagglutination, spherocytes, and decreased RBC density.
• Pancytopenia or a significant global decrease in all cell lines (RBC, WBC, and platelets) can indicate bone marrow disease or a peripheral destruction of cells.

Conclusion

Blod smear evaluation can feel daunting, but it is vital as it provides valuable information about our patients. An excellent resource with color photographs of the findings discussed above can be invaluable. Hopefully, this summary helps you feel more comfortable with the process, but ultimately, the more blood smear evaluations you do, the better you will be at it!