By Susan Magestro, DVM
Angell Animal Medical Center
Of all the diagnostic tests available to veterinarians, blood smear evaluation is one of the single most valuable tools. Each day in the clinic, many patients have a complete blood count (CBC) performed in order to evaluate their blood for abnormalities. The CBC begins with the quantitative evaluation of red and white blood cells and platelets. It ends with the microscopic evaluation, the blood film, to detect morphological abnormalities that can provide valuable insight to various diseases.
Examination of the peripheral blood smear is an inexpensive but powerful diagnostic tool. In some ways it is becoming a “lost art,” but when used it often provides rapid, reliable information about a variety of hematologic disorders. The smear offers a window into the functional status of the bone marrow, the factory producing all blood elements.
Figure 1: Pair of large Babesia dog blood 1000x WG
This test takes only a few minutes to perform and does not have the lag time of a reference lab, so results are instantaneous. With this evaluation, the veterinarian can often obtain clues to the patient’s underlying disease which will then help guide recommendations for additional testing, such as a bone marrow aspirate, infectious disease testing, or abdominal imaging.
Preparation of blood smears requires practice but is a technique easily perfected. Ideally, smears should be made at the time of blood collection, and no later than an hour after collection to avoid artifacts. Slide preparation requires clean slides and a capillary tube or pipet. The slide or coverslip method is used to make the smear. Once made and dried, slides should be stained with Romanosky-type stain such as modified Wright’s stain which has been reserved for hematology and cytology only.
Evaluation of the blood slide starts with a scan of the feathered edge to look for platelet clumping, abnormal cells, and microfilaria followed by review at the monolayer. This is the area where the red blood cells will occasionally touch without overlap. Once a good area is found, evaluation includes examination of the erythrocytes, leukocytes, and platelets.
Erythrocytes, or red blood cells, should have their size, shape, and color evaluated. Important aspects to note include regenerative changes such as polychromasia or reticulocytes which can be indicators of immature red blood cells, as can the presence of nucleated red blood cells. Cells such as spherocytes, acanthocytes, and schistocytes can be observed when hemolysis is present. Auto-agglutination, or clumping of the red blood cells, indicates an immune mediated response. Inclusions in red blood cells can often be clues to toxicities such as lead or acetaminophen toxicosis. Additionally, some infectious diseases, such as Babesia, mycoplasma, and distemper, can often create inclusions in red blood cells.
Figure 2: Normal dog blood smear with neut 1000x WG
Leukocyte, or white blood cell, evaluation includes assessing the total number of cells, or performing a differential. The appearance of the cells also should be reviewed for evidence of inflammation such as when there are increased band neutrophils (a left shift) or toxic neutrophils present. Reactive lymphocytes will indicate stimulation of the immune system while atypical lymphocytes could indicate neoplasia or an infectious disease. An increased number of eosinophils may indicate a hypersensitivity reaction such as heartworm disease, feline asthma, or parasites. Similar to erythrocytes, leukocytes can also contain inclusions consistent with infectious disease such as Anaplasmosis or distemper. 
Platelet counts are another important part of blood smear evaluation. As platelets often aggregate at the feathered edge, it is important to review the feathered edge for clumping to get the best assessment of whether the platelet numbers are appropriate, decreased, or increased. Most often platelet counts are performed to look for thrombocytopenia, or a decreased platelet count. There are multiple reasons that platelet numbers can be decreased including increased use from DIC, decreased production from the bone marrow, or destruction from an immune mediated disease. Enlarged platelets may also be noted, indicating an increased rate of thrombopoeisis secondary to increased peripheral demand.
Blood smears are an excellent diagnostic tool which should be used routinely when assessing a patient’s blood work. They can be performed at any time of the day, can be done quickly and at low cost, and provide a large amount of information to help create a more complete clinical picture.
- Stockholm SL, Scott, MA. 2002. Fundamentals of Veterinary Clinical Pathology. Ames (IA): Iowa State Press. 40-41.
- Metzger Jr FL, Rebar A. Symposium on a three-minute peripheral blood film evaluation. Veterinary Medicine. December 2004; 1(9):4-5.
- Sharkey L, Heinrich D. In-clinic hematology: The blood film review. Today’s Veterinary Practice. July/August 2015; 5(4):43-47.
- Metzger Jr FL, Rebar A. Symposium on a three-minute peripheral blood film evaluation. Veterinary Medicine. December 2004; 1(9): 14-16.
- Metzger Jr FL, Rebar A. Symposium on a three-minute peripheral blood film evaluation. Veterinary Medicine. December 2004; 1(9): 12-13.