Why a blood smear evaluation should be performed with every CBC

dvm360dvm360 May 2022
Volume 53
Issue 5
Pages: 16

Blood smear evaluations, in concert with automated cell counts, play a vital role in confirming results and potentially guiding diagnosis and treatment

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Image provided by Zoetis

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Hematology is the study of numbers and morphology of blood cells and encompasses some of the most important and commonly performed tests in veterinary diagnostics.1 The complete blood count (CBC) is the main diagnostic test for hematology and defines the number of red blood cells (RBC), white blood cells (WBC), and platelets (PLT) in a sample. The CBC also provides information about some specific physical parameters of the RBC population. These are known as RBC indices.

Each of these types of blood cellular elements performs vital physiologic functions, so determining their numbers and physical characteristics can provide important and meaningful health information on the patient being evaluated. Hematology testing consists of 2 main components: a quantitative and a qualitative evaluation of the blood.2

The quantitative evaluation of the blood includes a CBC, which is generally performed on an automated hematology analyzer or using manual techniques. However, it is typically and preferably performed on an automated cell analyzer.2-4 The complete blood count classifies, enumerates, and differentiates the types of cells present in the peripheral blood.2 This quantitative evaluation provides the different cell population counts and associated indices as well as graphical representations of the different results when performed on an automated analyzer.2 Typical CBC parameters provided in a report are shown in Table 1.

Table 1. CBC Parameters

Evaluating the WBC parameters serves to assess whether there is evidence of stress, excitement, inflammation/infection, hypersensitivity, lymphoid/myeloid neoplasia, or bone marrow conditions. Evaluating the RBC parameters serves to detect the presence of anemia and erythrocytosis, and in some situations, evidence of the etiology may be detected. Evaluating the PLT parameters serves to determine if the platelet count is lower than normal (thrombocytopenia) or higher than normal (thrombocytosis). This is often a sign of underlying medical conditions or may be a side effect from chronic administration of medication, giving the clinician indications that additional tests to diagnose the cause may be necessary.

The CBC can be complemented with the manual measurement of the packed red blood cell volume, (PCV) which is the percentage of blood volume that is made up of erythrocytes and is comparable to the hematocrit calculated from the automated CBC results.2 Ideally, a CBC should be part of a minimum database, along with a serum/plasma chemistry analysis and a complete urinalysis, so that results can be interpreted together to deliver a comprehensive laboratory diagnosis.5

The qualitative evaluation of the blood, also known as a blood smear or blood film evaluation, is the other component of a complete hematological evaluation. Blood smears are made using the EDTA-anticoagulated blood and stained using water-based Romanowsky-type stains, following the stain manufacturer’s protocol. The stained blood smear is then examined for estimated counts and morphological characteristics of the blood cells.

Why perform a blood smear evaluation?

Microscopic evaluation of a well-prepared blood film is a vital diagnostic procedure that is used to verify the automated analyzer’s results, such as total and differential white blood cell count and platelet count. It is also used to identify very important diagnostic information that automated analyzers cannot evaluate, such as diagnostically essential changes in cellular morphology, the presence of abnormal cells, or cellular inclusions.2,6 These morphologic abnormalities and cellular inclusions can be present even in patients with quantitatively normal results for all hematologic parameters. Ideally, a blood film should be examined every time a CBC is performed.2

A blood smear evaluation complements both point-of-care and reference laboratory automated hematology counts and should be performed regularly with every CBC. It is especially important for sick animals and those with hematologic abnormalities. A blood smear enables the veterinarian to confirm results and ensure quality and may provide additional insights to guide diagnosis and treatment.6-9 It provides the ability to confirm automated CBC results by verifying RBC, WBC, and platelet counts.4 In addition, a blood smear evaluation evaluates cell morphology that is not reported by automated CBC analyzers.

Ideally, a blood smear evaluation should always be performed as part of every CBC, but it is vital in the following clinical situations2:

  • anemia (low red blood cell count);
  • thrombocytopenia (low platelet count);
  • neutrophilia or neutropenia
    (verify count and examine cells);
  • lymphocytosis;
  • severe illness;
  • suspicion of hemoparasites; and
  • when certain warning flags are present
    on the automated CBC report.

In 2 surveys at Michigan State University, the frequency (%) of anemia was shown to be 23% to 29% in dogs and 10% to 20% in cats.7 In the same surveys, 5% to 15% of dogs and 7% to 60% of cats were thrombocytopenic.7 These are examples of cases in which a blood smear should always be performed to confirm anemia and/or low platelet counts. In addition, for anemic patients, cell morphology can be equally important for patient diagnosis as confirming the number and presence of reticulocytes.

Alternatively, a blood smear evaluation should never be utilized as a replacement for an automated cell count, as automated analyzers, if properly maintained, are more precise and accurate than manual counting of cells. Automatic analyzers will count several thousand cells in the liquid phase of blood, whereas only a few hundred cells are analyzed on the slide for blood smear evaluation.4 Some examples of the morphological changes that can be appreciated on a blood smear evaluation are shown in Table 2.2-4

Examples of Morphological Changes

The practice of a blood smear evaluation is routinely performed in reference laboratories to supplement high-quality, “gold-standard” automated CBC analyzers.2 In general veterinary practices, gold-standard CBC analyzers are impractical because of cost, challenge of operation, maintenance, and footprint. Therefore, the blood smear evaluation is even more critical in these situations. Failure to perform a blood smear evaluation can result in potential incomplete and potentially misleading diagnostic information, complicating clinical decisions.2

In some instances, blood smears are not performed in veterinary practices because of the technical challenges of making and reliably interpreting the blood smear and the general lack of easily available, timely expert evaluation. Blood smear evaluation success is dependent on a number of components, including the following3:

  • quality of the blood smear preparation;
  • staining solution quality maintenance;
  • ability to evaluate correct areas of a blood smear;
  • ability to differentiate artifacts from legitimate morphologic abnormalities; and
  • user experience with preparing and interpreting blood smears.

Closing thoughts

Evaluations of blood smears will continue to play a critical role in confirming the presence of abnormal cell populations that automated CBC analysis has identified as suspect and flagged for the operator’s attention. When there is a lack of confidence in dealing with these challenges, blood smears should be referred to an expert veterinary clinical pathologist.3

Using the VETSCAN IMAGYST AI Blood Smear application, challenges regarding workflow, time, and interpretation can be alleviated for veterinary practitioners. Additionally, there is easy and timely access to clinical pathologists for add-on expert review of the blood smear by digital image transfer when there is a potential need to evaluate cellular information not reported by the AI, such as toxic changes to WBC and morphology changes to RBCs. With the VETSCAN IMAGYST, a blood smear evaluation can complement each and every CBC easily and efficiently at the point of care.


  1. Kahn C, Line S, eds. The Merck Veterinary Manual. 10th ed. Merck & Co, Inc.; 2010:1487-1492.
  2. Villiers E. Introduction to Haematology. In: Villiers E, Ristic J, eds. BSAVA Manual of Canine and Feline Clinical Pathology. 3rd ed. British Small Animal Veterinary Association; 2016:27-33.
  3. Weiser G. Laboratory Technology for Veterinary Medicine. In: Thrall, MA, Weiser G, Allison RW, Campbell TW, eds. Veterinary Hematology and Clinical Chemistry, 2nd ed. John Wiley & Sons, Inc; 2012:3-18.
  4. Harvey JW. Hematology Procedures. In: Harvey JW. Veterinary Hematology: A Diagnostic Guide and Color Atlas. Saunders; 2012:16-18.
  5. Barger AM. The complete blood cell count: a powerful diagnostic tool. Vet Clin North Am Small Anim Pract. 2003;33(6):1207-1222. doi:10.1016/s0195-5616(03)00100-1
  6. Zabolotzky SM, Walker DB. Peripheral blood smears. In: Valenciano SA, Cowell R. Cowell and Tyler’s Diagnostic Cytology and Hematology of the Dog and Cat, 5th ed. Elsevier; 2019:438-467.
  7. Weiss DJ, et al. The Complete Blood Count, Bone Marrow Examination, and Blood Banking. In: Willard MD, Tvedten H. Small Animal Clinical Diagnosis by Laboratory Methods, 5e. 5th ed. Saunders; 2012:12-37
  8. Stirn M, Moritz A, Bauer N. Rate of manual leukocyte differentials in dog, cat and horse blood samples using ADVIA 120 cytograms. BMC Vet Res. 2014;10:125. doi:10.1186/1746-6148-10-125
  9. Sharkey L, Heinrich D. In-clinic hematology: the blood film review. Today’s Veterinary Practice website. July/August 2015. Accessed November 16, 2021. https://todaysveterinarypractice.com/in-clinic-hematology-the-blood-film-review
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